Vibrating element liquid discharging apparatus having gas pressure sensing

BACKGROUND OF THE INVENTION

P-0001[0001] The present invention relates to an improved apparatus, such as a nebulizer apparatus, for discharging fluids, and to a method of operating same. Nebulizers, or atomizers, are devices that generate a fine spray or aerosol, usually of a liquid. A particularly useful application for nebulizers is to provide a fine spray containing a dissolved or suspended particulate or colloidal pharmaceutical agent for administration to a subject by inhalation. Such inhalation treatment is highly effective for conditions affecting the subject's respiratory organs. Further, since the lungs are close to the heart and the circulatory system of the body, drug administration by inhalation provides an effective and rapid delivery system for a drug to all organs of the body. In other applications, nebulizers provide a fine spray of water for humidification.

P-0002[0002] When used to dispense a pharmaceutical agent to a subject, a nebulizer in the form of an inhaler may be placed directly in the mouth or nose of the subject so that the spray can be entrained in the respiratory gases which are inhaled during normal, spontaneous breathing of the subject.

P-0003[0003] In other applications, the subject breathes with the aid of a respiratory ventilator. A typical ventilator has a breathing circuit comprising an inhalation limb and an exhalation limb connected to two arms of a Y-connector. The third arm of the Y-connector is connected, via a patient limb, to a mouthpiece, mask, or endotracheal tube for the subject. The ventilator provides a complete or partial supply of breathing gas to the subject through the inhalation limb during inhalation. The contraction of the subject's lungs discharges gases through the exhalation limb during exhalation. When a nebulizer is employed in conjunction with a ventilator, it is typically placed in the patient limb to discharge into the breathing gases inhaled by the subject but it can also be placed in the inhalation limb of the breathing circuit.

P-0004[0004] Nebulizers are currently in use that generate the spray either pneumatically or by means of ultrasonic vibrations. Pneumatic nebulizers are typically used with a liquid, such as an aqueous drug solution. High pressure driving gas is conducted through a nozzle to draw the drug from a drug supply to the nebulizer. The drug is discharged against a baffle or other similar separating means in a gas space of the nebulizer, breaking the liquid into a fine spray. The gas space is in fluid communication with the inhaled gas pathway for the subject so that the gas flow expelled from the nozzle along with the nebulized drug is conducted from the gas space to the pathway and ultimately to the subject.

P-0005[0005] Disadvantages in the use of pneumatic nebulizers include the following. If the nebulizer adds a significant quantity of gas, for example, up to five (5) liters/minute, into the breathing gases, the overall breathing gas composition to the subject may be significantly altered. Further, due to passage of the driving gas through the nozzle, the impingement of the drug on the baffle, etc., pneumatic nebulizers tend to be noisy. And, controlling the commencing and stopping of a drug spray is difficult and is not very accurate. This may result in wastage of the drug.

P-0006[0006] The foregoing shortcomings of pneumatic nebulizers have led to the use of ultrasonic nebulizers in which a fine spray is produced by ultrasonic vibration of the liquid containing the drug, as through the use of a piezoelectric crystal. The breathing gas composition and the on-off operation are easier to control with ultrasonic nebulizers than with a pneumatic nebulizer. However, ultrasonic devices require a large, bulky electrical power supply to power the crystal and may not be able to nebulizer colloidal or particulate suspensions.

P-0007[0007] In one type of ultrasonic nebulizer, the fine spray is produced by dropping the liquid on, or otherwise applying it to, the vibrating element. See Koeh et al. U.S. Pat. No. 5,443,059. Michaels et al., U.S. Pat. No. 3,812,854, describe another type of nebulizer for inhalation therapy in which the spray is generated on the front surface of a vibrating, porous body. The pores in the body form a network of passages that enable the liquid to flow through the body. The liquid to be nebulized is supplied under pressure from a liquid supply through a liquid conduit to the pores, and forced through the pores to the front surface of the porous body where it is discharged as a spray. Robertson et al., U.S. Pat. No. 5,487,378, describe a nebulizer in which the aerosol is formed using a mesh plate instead of a porous solid body. The mesh plate has a plurality of orifices through which the liquid can pass. Either the liquid or the mesh plate is vibrated ultrasonically by a piezoelectric element to nebulizer the liquid as it passes through the mesh plate.

P-0008[0008] A general shortcoming of current nebulizers is the efficiency with which the aerosol is transported into the subject's lungs. To increase the efficiency, the nebulizer may be operated so as to function in phase with the ventilator so that the aerosol is produced by the nebulizer either during, or partly during, inspiration by the subject. The proper timing can be achieved by switching the nebulizer on at the beginning of inspiration in response to a signal coming from the ventilator, or in response to information coming from a flow sensor as described in U.S. Pat. No. 5,964,219 to Pekka Merilainen.

P-0009[0009] However, a disadvantage currently exists in that to ensure that the nebulizer functions properly to achieve the best efficiency, a separate device is required to generate the necessary signal or information for optimal timing of the nebulization. This adds to the cost and complexity of the nebulizer and/or ventilator. For example, Ivri et al., U.S. Pat. No. 6,085,740, describe a nebulizer of the inhaler type in which the inhalation flow is detected from an audible signal produced during inhalation, which signal is then used to control the nebulization. In addition to cost and complexity, this approach may exhibit a sensitivity to external noise.

BRIEF SUMMARY OF THE INVENTION

P-0010[0010] An object of the present invention is to provide an improved apparatus and method for discharging fluids into a receiving gas flow and detecting changes in the pressure of the receiving gas. In a typical application, nebulized liquid is discharged into the breathing gas flow of a subject. The pressure detection so obtained may be used to sense the commencement of inspiration by the subject, or some other phase in the subject's respiratory cycle, to control operation of the nebulizer to provide nebulized liquid with a desired timing with respect to the subject's respiratory cycle. Typically, the commencement of inspiration would be determined and the supply of nebulized liquid would occur during, or partly during, the subject's inspiration as, for example, to administer a drug into the lungs of a subject.

P-0011[0011] The present invention thus avoids the need for the separate devices heretofore used in controlling the timing of the nebulization and introduction of the nebulized liquid into the receiving flow.

P-0012[0012] Another object of the present invention is to provide an improved apparatus and method for discharging fluid into a receiving gas flow and measuring the pressure of the receiving gas.

P-0013[0013] A further object of the present invention is to provide an improved apparatus and method for discharging fluid into a receiving gas flow and for determining the flow direction of the receiving gas.

P-0014[0014] In the present invention, the fluid discharging apparatus is placed in fluid communication with a receiving gas so that a member in the apparatus is subjected to pressure exerted by the receiving gas. When used to supply a drug, a liquid discharging apparatus in the form of a nebulizer may be placed in communication with the breathing gases pathway for a subject so that the member is subjected to the pressure of the breathing gases. The member of the apparatus that is subjected to pressure is coupled to a bi-directional mechanical-electrical conversion element, such as a piezoelectric element, to form a gas pressure transducer. The member exerts a mechanical loading on the element responsive to the gas pressure to which the member is subjected.

P-0015[0015] In an active embodiment of the invention, alternating electrical energization is applied to the element at a selected frequency. The element mechanically vibrates responsive to the application of the alternating electrical energization to the element to discharge the atomized liquid from the liquid discharging apparatus into the receiving gas. The energized element exhibits an electrical admittance. Admittance is the inverse of electrical impedance. The admittance of the element at the selected energization frequency is altered when the element is mechanically loaded by the member that is subjected to the pressure of the receiving gas.

P-0016[0016] The admittance exhibited by the element in an unloaded condition when energized by the electrical energy of the selected frequency is measured. When the member is subjected to the pressure of the receiving gas during operation of the fluid discharging apparatus to load the element, the admittance of the element is again measured. The difference between the admittances measured in the unloaded and loaded conditions is an indication of the pressure of the receiving gas.

P-0017[0017] The selected frequency used to energize the element may be the resonance frequency of the gas pressure transducer formed from the element and member or a frequency other than the resonance frequency.

P-0018[0018] By observing whether the magnitude of the admittance in the loaded state is greater or less than that in the unloaded state, a flow direction of the receiving gas may also be determined. Or, the flow direction may be determined by observing the changes in admittance as the frequency of the electrical energization is varied.

P-0019[0019] In another, passive, embodiment of the invention, no electrical energization is applied to the piezoelectric element of the pressure transducer. The voltage appearing at output terminals of the transducer is proportional to the mechanical loading applied to the element by the member which is subjected to the pressure of the receiving gas. The output voltage of the transducer is thus indicative of the receiving gas pressure.

P-0020[0020] As with the active embodiment of the invention described above, the transducer may have a plurality of mechanical resonance frequencies and a plurality of anti-resonance frequencies. The transducer may be constructed so that it is mechanically “tuned” to the frequency or dynamic properties of the receiving gas pressures to which it is subjected, thereby to maximize the voltage output of the transducer. This can be accomplished by appropriately establishing the dimensions and composition of the piezoelectric element and/or member subjected to the pressure of the receiving gas

P-0021[0021] Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

P-0022[0022] The present invention will be further understood by reference to the following detailed description and accompanying drawings, in which

P-0023[0023]FIG. 1 is a somewhat schematic, cross-sectional view of a fluid discharging apparatus of the present invention in the form of a nebulizer connected to a breathing circuit which, in turn, is connected to a ventilator;

P-0024[0024]FIG. 2 is an exploded, cross-sectional view of the apparatus of FIG. 1;

P-0025[0025]FIGS. 3<i>a</i>, <b>3</b><i>b </i>and <b>3</b><i>c </i>are schematic views showing the operation of vibrating and atomizing components of the apparatus of FIG. 1;

P-0026[0026]FIGS. 4<i>a </i>and <b>4</b><i>b </i>are schematic cross-sectional views of the apparatus of FIG. 1 showing operation under conditions in which a subject is breathing with the aid of a ventilator;

P-0027[0027]FIGS. 5<i>a </i>and <b>5</b><i>b </i>are schematic cross-sectional views of the apparatus of FIG. 1 similar to FIGS. 4 and 4<i>a </i>but showing operation under conditions in which a subject is breathing spontaneously;

P-0028[0028]FIG. 6<i>a </i>is an equivalent electrical circuit diagram of a structure incorporating a piezoelectric element which may be used as a transducer in the apparatus of FIG. 1;

P-0029[0029]FIG. 6<i>b </i>graphically illustrates a resonance curve of a piezoelectric element;

P-0030[0030]FIG. 6<i>c </i>graphically illustrates a resonance curve for a structure incorporating a piezoelectric element;

P-0031[0031]FIGS. 7<i>a</i>, <b>7</b><i>b</i>, and <b>7</b><i>c </i>are schematic circuit diagrams showing three different techniques for measuring electrical properties of a piezoelectric element to produce data of the type shown in FIGS. 6<i>b </i>and <b>6</b><i>c; </i>

P-0032[0032]FIGS. 8<i>a </i>and <b>8</b><i>b </i>are graphs illustrating electrical properties of the transducer of FIG. 6<i>a </i>that can be used to measure gas pressure magnitude and gas flow direction;

P-0033[0033]FIG. 8<i>c </i>is a graph illustrating another technique for such measurements;

P-0034[0034]FIG. 9 is a schematic diagram of one embodiment of a circuit suitable for use in the fluid discharging apparatus of the present invention;

P-0035[0035]FIG. 10 is a further schematic diagram of circuitry suitable for use in the fluid discharging apparatus of the present invention;

P-0036[0036]FIGS. 11<i>a </i>and <b>11</b><i>b </i>are graphs showing operation of the circuitry of FIG. 10;

P-0037[0037]FIGS. 12<i>a </i>and <b>12</b><i>b </i>are a schematic diagram and a graph, respectively, illustrating a further technique for measuring gas pressure magnitude and gas flow direction;

P-0038[0038]FIGS. 13<i>a </i>and <b>13</b><i>b </i>are schematic views showing a further embodiment of a structure suitable for use in the present invention and incorporating a piezoelectric element;

P-0039[0039]FIG. 14 is a cross sectional view of another embodiment of a fluid discharging apparatus of the present invention; and

P-0040[0040]FIGS. 15<i>a</i>, <b>15</b><i>b</i>, and <b>15</b><i>c </i>are schematic diagrams of simple circuits suitable for use in the fluid discharging apparatus of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

P-0041[0041] In the drawing figures, in which like reference numerals designate like parts throughout the disclosure, a fluid discharging and pressure sensing apparatus constructed according to the present invention is indicated generally by the reference numeral <b>1</b> in FIG. 1. In the application shown in FIG. 1, apparatus <b>1</b> is a nebulizer operatively connected to a breathing circuit <b>2</b> and a control unit <b>4</b>. Control unit <b>4</b> is typically located separately from nebulizer <b>1</b> but may be incorporated in ventilator <b>3</b>, if desired.

P-0042[0042] The substance to be nebulized typically comprises a solution, or a particulate or colloidal suspension, of a product but could comprise other substances, such as a dry fluid material. The substance may comprise water for humidification. For purposes of explanation, the fluid substance undergoing nebulization or atomization is hereinafter generally described as a liquid.

P-0043[0043] Nebulizer <b>1</b> atomizes the liquid for delivery in a breathing gas flow to a subject, for example, as a drug treatment for a patient. Breathing circuit <b>2</b> includes an inhalation limb <b>5</b>, one end of which is coupled to ventilator <b>3</b> at inhalation limb connector <b>6</b>, and an exhalation limb <b>7</b>, one end of which is connected to ventilator <b>3</b> at exhalation limb connector <b>8</b>. Inhalation limb <b>5</b> and exhalation limb <b>7</b> are connected at their opposite ends to two adjacent arms <b>9</b><i>a </i>and <b>9</b><i>b</i>, respectively, of a Y-connector <b>9</b>. A third arm <b>9</b><i>c </i>of the Y-connector <b>9</b> is connected to one end of patient limb <b>10</b>. The other end of patient limb <b>10</b> is directed to a gas administration structure (not shown) for the subject, such as a mouthpiece, a facemask, or an endotracheal tube.

P-0044[0044] When in use, ventilator <b>3</b> can selectively provide all or any predetermined portion of the breathing gases required by the subject by supplying breathing gases through the inhalation limb <b>5</b>. The breathing gases pass from the ventilator <b>3</b>, into inhalation limb <b>5</b>, through Y-connector <b>9</b> and into patient limb <b>10</b> and are inhaled by the subject. On exhalation, the exhaled breathing gases pass from the subject through patient limb <b>10</b> and Y-connector <b>9</b>, and into exhalation limb <b>7</b> back to the ventilator <b>3</b>. The subject may also breathe spontaneously through patient limb <b>10</b>.

P-0045[0045] As shown in FIG. 1, nebulizer <b>1</b> is preferably positioned in the breathing circuit <b>2</b> as near as possible to the subject to minimize the length of the aerosol transport path from the nebulizer <b>1</b> to the subject, and to minimize the deposition of the atomized liquid on the interior walls of the breathing circuit <b>2</b>. To this end, nebulizer <b>1</b> is preferably attached in the breathing circuit <b>2</b> between the Y-connector <b>9</b> and the patient limb <b>10</b>. The third arm <b>9</b><i>c </i>of the Y-connector <b>9</b> has a socket <b>11</b> sized for patient limb <b>10</b> which can alternatively receive a tubular projection <b>12</b> of an adapter <b>13</b> for mounting nebulizer <b>1</b>. The adapter <b>13</b> includes a first tubular socket <b>14</b>, disposed opposite the Y-connector <b>9</b>, and formed similarly to the socket <b>11</b>, that receives the patient limb <b>10</b>. The adapter <b>13</b> also has a second tubular socket <b>15</b> in which nebulizer <b>1</b> is positioned. To secure nebulizer <b>1</b> to the adapter <b>13</b>, the nebulizer includes a downwardly extending tubular projection <b>16</b> that is insertable into the second socket <b>15</b>. When nebulizer <b>1</b> is not needed, or when the nebulizer is removed for cleaning or maintenance, a removable cap (not shown) may be fitted into or over the second socket <b>15</b> to allow breathing circuit <b>2</b> to function in a normal manner. Alternatively, the entire adapter <b>13</b> may be removed from the breathing circuit <b>2</b> so that the patient limb <b>10</b> can be connected directly to the Y-connector <b>9</b>.

P-0046[0046] Nebulizer <b>1</b> also includes a liquid reservoir <b>17</b> for holding the liquid to be nebulized. In the embodiment shown in FIG. 1, reservoir <b>17</b> is removably mounted on the top of the nebulizer <b>1</b> opposite the projection <b>16</b>. Alternatively, it may be separate from the nebulizer <b>1</b> and connected to the nebulizer <b>1</b> by a suitable conduit.

P-0047[0047] Referring now to the detailed, exploded view of FIG. 2, the lower portion of the nebulizer <b>1</b> is formed by an annular housing <b>22</b> that includes the tubular projection <b>16</b> which extends downwardly from the housing <b>22</b>. Housing <b>22</b> may be formed of a generally rigid material such as a hard plastic or similar material and defines a cavity <b>28</b> therein. An O-ring <b>24</b>, made of a resilient material, such as rubber, is positioned around the periphery of a planar base member <b>23</b> which forms the bottom of the housing <b>22</b>. The housing <b>22</b> also includes a number of inwardly extending protrusions <b>25</b> disposed on a circular sidewall <b>22</b><i>a </i>of the housing that serve to engage and retain the O-ring <b>24</b> within the housing <b>22</b>.

P-0048[0048] Housing <b>22</b> receives and retains an internal plug member <b>30</b> within the cavity <b>28</b>. Plug member <b>30</b> is releasably retained within the cavity by a spiral or bayonet fastening means formed, in part, by openings <b>26</b> and <b>27</b> located on the side wall <b>22</b><i>a </i>of the housing <b>22</b>. Associated projections <b>31</b> and <b>32</b> are situated symmetrically on opposite sides of plug member <b>30</b> and fit into the openings <b>26</b> and <b>27</b> formed in the housing <b>22</b>. Plug member <b>30</b> may be separated from, or joined to, housing <b>22</b> by turning and pulling or pushing and turning the plug member <b>30</b> with respect to housing <b>22</b>. This allows the plug member <b>30</b> to be removed at the end of therapy, for replacement or for cleaning when a different pharmaceutical agent is to be subsequently administered to the subject.

P-0049[0049] Disc-like plate <b>50</b> is disposed between the base member <b>23</b> and the plug member <b>30</b>. The plate <b>50</b> may be positioned inside the cavity <b>28</b> of housing <b>22</b> between the upper surface of O-ring <b>24</b> and the lower ends of protrusions <b>25</b>. Plate <b>50</b> is spaced from base member <b>23</b> in order to prevent the base member and the adjacent face of the plate <b>50</b> from touching. Plate <b>50</b> is made of a conductive material, such as a conductive metal, and contains central opening <b>51</b>. Preferably, the plate <b>50</b> is made of brass. A mesh plate <b>52</b> is attached to plate <b>50</b> within the central opening <b>51</b>. Mesh plate <b>52</b> may be mounted to the plate <b>50</b> by any suitable technique, including, for example, brazing or welding. Plate <b>50</b> and mesh plate <b>52</b> may also be unitarily formed from a single sheet of material, if desired.

P-0050[0050] The mesh plate <b>52</b> is a relatively thin plate having a plurality of holes or pores <b>53</b> extending through the mesh plate <b>52</b> to discharge liquid, as hereinafter described. Mesh plate <b>52</b> may be between about 0.01 and 0.04 mm thick, and preferably is about 0.02 mm thick. The diameter of the holes <b>53</b> at the lower surface <b>54</b> of the mesh plate <b>52</b> is about 1 to 25 μm, and is preferably approximately 2-10 μm. The holes <b>53</b> may be formed in the mesh plate <b>52</b> by an electroforming process, which produces the holes <b>53</b> such that they have a diameter that decreases in a direction from the rear surface <b>55</b> to the front surface <b>54</b> of mesh plate <b>52</b>. However, holes <b>53</b> will function equally well if their diameter is uniform along their length as the primary criterion for the holes <b>53</b> is that the exit diameter in lower surface <b>54</b> of mesh plate <b>52</b> be sufficient to form liquid droplets of a desired size.

P-0051[0051] A ring-like vibrating element <b>56</b>, is mounted to the upper surface of plate <b>50</b>. An element formed of piezoelectric material may be used as vibrating element <b>56</b>. A piezoelectric material possesses the property of changing its dimensions when subjected to an electrical energization. The reverse is also true. That is, when the dimensions of a piezoelectric material are changed, as by compressing or stretching it, electrical energy is generated in the material. Thus, a piezoelectric element can convert electrical energy into mechanical energy, and vice versa. As hereinafter noted, a unique feature of the present invention is to employ both these conversion characteristics in the operation of the liquid discharge apparatus.

P-0052[0052] The piezoelectric element <b>56</b> is positioned above the plate <b>50</b> by a small distance to define a gap <b>57</b>, shown most clearly in FIG. 3<i>a</i>, between the element <b>56</b> and the plate <b>50</b>. The piezoelectric element <b>56</b> is secured above the plate <b>50</b> by a positioning means <b>58</b> which can retain the element <b>56</b> in a position spaced the small distance above the plate <b>50</b>. This positioning means <b>58</b> can be a brazed connection, a weld, a conductive glue or other suitable composition capable of holding the element <b>56</b> above the plate <b>50</b>. Piezoelectric element <b>56</b> includes a central opening. Plate <b>50</b> and piezoelectric element <b>56</b> form a pressure transducer, as hereinafter described in detail.

P-0053[0053] The plug member <b>30</b> is formed from a non-conductive, generally rigid material, such as a hard plastic, and includes a first terminal <b>35</b> and a second terminal <b>36</b> made of conductive material that are disposed on the bottom of the plug member <b>30</b>. Both terminals <b>35</b> and <b>36</b> are preferably in the form of ring-shaped conductors that are resilient or resiliently mounted in plug member <b>30</b> and that extend concentrically around the bottom of the plug member <b>30</b>. However, each terminal <b>35</b> and <b>36</b> can have any form capable of electrically engaging the piezoelectric element <b>56</b> and plate <b>50</b>, respectively. The first terminal <b>35</b> and second terminal <b>36</b> are connected to cable <b>100</b> within the plug member <b>30</b> by a pair of branch connectors <b>100</b><i>a </i>and <b>100</b><i>b</i>, respectively. Cable <b>100</b> is connected to cable <b>20</b> which is connected to control <b>4</b>. Via the branch connectors <b>100</b><i>a </i>and <b>100</b><i>b</i>, power can be supplied to, and signals can be received from, the terminals <b>35</b> and <b>36</b>, respectively. When the plug member <b>30</b> is placed on top of plate <b>50</b> so that the plate <b>50</b> is positioned between the plug member <b>30</b> and the O-ring <b>24</b>, the first terminal <b>35</b>, contacts piezoelectric element <b>56</b> and second terminal <b>36</b> contacts conductive plate <b>50</b>. Terminal <b>36</b> may be electrically grounded for purposes of applying a desired voltage to piezoelectric element <b>56</b> in conjunction with terminal <b>35</b>.

P-0054[0054] The plug member <b>30</b> also encloses liquid flow controlling valve <b>18</b> which is disposed concentrically within the plug member <b>30</b>. Valve <b>18</b> is connected to the control unit <b>4</b> by cable <b>100</b>, <b>20</b> and is used for controlling the supply of the liquid to vibrating mesh plate <b>52</b> from the liquid reservoir <b>17</b>. Valve <b>18</b> may comprise a spring loaded ferromagnetic valve member that closes a valve seat. Valve member is lifted off the valve seat when a surrounding magnetic coil is energized through cable <b>100</b>, <b>20</b> to supply liquid from reservoir <b>17</b> to mesh plate <b>52</b>.

P-0055[0055] Reservoir <b>17</b> comprises a liquid chamber <b>60</b> attached to the top surface of plug member <b>30</b> by spiral or bayonet fastening openings <b>61</b> and <b>62</b> located on opposite sides of the chamber <b>60</b>. A pair of projections <b>33</b> and <b>34</b> are situated symmetrically on opposite sides of plug member <b>30</b> and fit into the openings <b>61</b> and <b>62</b> formed in the chamber <b>60</b>. The chamber <b>60</b> may be fastened to, or unfastened from, plug member <b>30</b> by pushing and turning or turning and pulling the chamber <b>60</b> with respect to plug member <b>30</b>. This allows the chamber <b>60</b> to be removed at the end of therapy for replacement, or for the subsequent administration of a different drug to the subject.

P-0056[0056] The chamber <b>60</b> includes an outlet opening <b>66</b> through which the liquid to be nebulized passes from the chamber <b>60</b>. When chamber <b>60</b> is connected to the plug member <b>30</b>, the outlet opening <b>66</b> engages a depression <b>67</b> in the valve <b>18</b> so that, when the valve <b>18</b> is opened, liquid can flow from the chamber <b>60</b>, through the outlet opening <b>66</b> and through valve <b>18</b>. Opening <b>66</b> may also be used to fill chamber <b>60</b> with liquid, if desired. Chamber <b>60</b> is typically pressurized, as be a flexible membrane or external pressure source, to assist in the discharge of liquid from the reservoir.

P-0057[0057] Plug member <b>30</b> further includes a tubular sensing electrode <b>38</b> that is located adjacent the upper surface <b>55</b> of mesh plate <b>52</b> when the plug member <b>30</b> is placed in the housing <b>22</b> over the plate <b>50</b>. As shown in FIG. 2, tubular sensing electrode <b>38</b> may surround the outlet from valve <b>18</b>. The electrode <b>38</b>, in conjunction with mesh plate <b>52</b> which is spaced below sensing electrode <b>38</b>, is used to measure the impedance created by the presence of a column of liquid between the outlet of valve <b>18</b> and mesh plate <b>52</b>. Branch connector <b>100</b><i>c </i>connects electrode <b>38</b> to cables <b>100</b>, <b>20</b> and to the control unit <b>4</b> in the course of performing an impedance measurement. A small domed cavity <b>37</b> is disposed in the lower surface of plug member <b>30</b> and surrounds the sensing electrode <b>38</b> to facilitate impedance measurement.

P-0058[0058] To operate nebulizer <b>1</b>, valve <b>18</b>, which is used for supplying liquid to vibrating mesh plate <b>52</b>, is initially opened in response to a signal from the control unit <b>4</b> sent through the cable <b>20</b>, <b>100</b>. Liquid flows from the opening <b>66</b> of chamber <b>60</b> through the open valve <b>18</b> toward the mesh plate <b>52</b>. With continued supply of liquid, the cohesive forces in, and surface tension of, the liquid create a column of liquid that extends between the lower end of the valve and sensing electrode <b>38</b> and the mesh plate <b>52</b>. The sensing electrode <b>38</b> and mesh plate <b>52</b> detect the presence and magnitude of the amount of liquid between the sensing electrode <b>38</b> and the rear surface <b>55</b> of mesh plate <b>52</b> by an alteration of the impedance between the two elements due to the presence or absence of liquid between the electrode <b>38</b> and the mesh plate <b>52</b>.

P-0059[0059] A signal from mesh plate <b>52</b> is obtained via terminal <b>36</b> and a signal from sensing electrode <b>38</b> is obtained via cable branch <b>100</b><i>c</i>. The signals from the mesh plate <b>52</b> and the electrode <b>38</b> are transmitted through the cable <b>20</b> to an impedance sensor (not shown) disposed in the control unit <b>4</b>. When the signal indicates a liquid volume in the column that equals or exceeds a desired value, the control unit <b>4</b> operates valve <b>18</b> to close the valve and terminate the supply of liquid to the column. As liquid is discharged by nebulizer <b>1</b>, the size of the column of liquid is reduced. When the impedance signal obtained by the electrode <b>38</b> and mesh plate <b>52</b> indicates that the volume of liquid is below the desired value, due to the discharge of liquid through the mesh plate <b>52</b> during operation of nebulizer <b>1</b>, the control unit <b>4</b> reopens the valve <b>18</b> to allow more liquid to be supplied through the valve.

P-0060[0060]FIG. 3<i>a </i>is a simplified showing of piezoelectric element <b>56</b> and plate <b>50</b> in a state in which no voltage or energization is applied to the former. When high frequency alternating electrical energization is supplied to piezoelectric element <b>56</b> from a power source (not shown) inside control unit <b>4</b> through cable <b>20</b>, <b>100</b> and terminals <b>35</b> and <b>36</b> the element will vibrate. The electrical energization causes the piezoelectric element <b>56</b> to alternately contract from the unenergized equilibrium state, shown in FIG. 3<i>a</i>, to a radially decreased state shown in FIG. 3<i>b </i>and then expand to a radially increased state shown in FIG. 3<i>c</i>. Due to the fact that the piezoelectric element <b>56</b> is joined to plate <b>50</b> about the periphery of the element <b>56</b>, the radial reduction of piezoelectric element <b>56</b> causes plate <b>50</b> to bow in a downward direction as shown in FIG. 3<i>b</i>. Radial expansion of piezoelectric element <b>56</b> causes the element and plate <b>50</b> to bow upwardly as shown in FIG. 3<i>c </i>as a result of unsymmetrical forces occurring in these components from their peripheral joinder. When the plate <b>50</b> and element <b>56</b> alternatingly shift from the radially decreased condition shown in <b>3</b><i>b </i>to the radially increased condition shown in FIG. 3<i>c </i>and vice versa, the plate <b>50</b> moves through a flat or normal condition corresponding to that shown in FIG. 3<i>a. </i>

P-0061[0061] Due to the motion of plate <b>50</b> shown in FIGS. 3<i>a</i>, <b>3</b><i>b </i>and <b>3</b><i>c</i>, nebulized liquid is discharged from holes <b>53</b> in mesh plate <b>52</b>. At the front surface <b>54</b> of the vibrating mesh plate <b>52</b>, the discharged liquid will grow into drops at the front end of each hole <b>53</b> due to the surface tension of the liquid. The drops will increase in size until the expelling forces arising from the oscillating movement of mesh plate <b>52</b> and the mass of each drop exceeds the holding force determined by the size of the holes <b>53</b> in mesh plate <b>52</b> and the surface tension of the liquid. The drops expelled from plate <b>52</b> then pass through tubular projection <b>16</b> of housing <b>22</b> into the patient limb <b>10</b> of the breathing circuit <b>2</b> and are inhaled with the breathing gases by the subject as a nebulized or atomized liquid.

P-0062[0062] In the operation described immediately above, piezoelectric element <b>56</b> converts electrical energy into mechanical energy to cause the plate <b>50</b> to oscillate and discharge liquid from holes <b>53</b> in mesh plate <b>52</b> as a result of the mechanical deflection shown in FIGS. 3<i>a</i>-<i>c</i>. However, as noted above, a piezoelectric element can also convert mechanical energy to electrical energy. This characteristic is used in the present invention to obtain information about the breathing gases in the breathing circuit <b>2</b>, and particularly the pressure of such gases. When the housing <b>22</b> is secured to the adapter <b>13</b>, lower surface of plate <b>50</b> and the front surface <b>54</b> of mesh plate <b>52</b> are exposed to the pressure of the breathing gases flowing through the breathing circuit <b>2</b>. This pressure will vary significantly during inhalation and exhalation conditions in breathing circuit <b>2</b>. For example, when the ventilator <b>3</b> is producing entirely artificial ventilation for the subject, breathing gas pressures in the breathing circuit <b>2</b> may increase up to one hundred (100) mbar during inspiration and thereafter decrease during expiration. When the patient breathes spontaneously, the effect will be just the opposite, i.e., breathing circuit pressure will decrease during inspiration and increase during expiration.

P-0063[0063] The O-ring <b>24</b> separating the plate <b>50</b> from the base member <b>23</b> effectively seals the space between the plate <b>50</b> and base member <b>23</b> of housing <b>22</b>. As a result, the effects of the changing breathing gas pressures in breathing circuit <b>2</b> are present within the space between the plate <b>50</b> and the base member <b>23</b>. This is volume <b>65</b> shown in FIGS. 4 and 5.

P-0064[0064]FIG. 4<i>a </i>shows conditions in nebulizer <b>1</b> when breathing gases are being provided to a subject by ventilator <b>3</b>. The gas pressure in breathing circuit <b>2</b> is increased by ventilator <b>3</b> during inspiration. The pressure in volume <b>65</b>, formed between plate <b>50</b> and base member <b>23</b> and sealed by O-ring <b>24</b> is similarly increased, due to the communication of the volume <b>65</b> with the breathing circuit <b>2</b> through the opening of tubular projection <b>16</b>. When liquid is fed to the mesh plate <b>52</b> from valve <b>18</b>, the liquid column extending between the lower end of valve <b>18</b> and mesh plate <b>52</b> blocks the holes <b>53</b> in mesh plate <b>52</b>. This prevents the gas pressure created in the volume <b>65</b> from passing through the holes <b>53</b> of mesh plate <b>52</b>. Therefore, the increasing pressure in the volume <b>65</b>, illustrated as the upward arrow P<b>1</b> in FIG. 4<i>a</i>, acts to bend plate <b>50</b> upwardly and away from adapter <b>13</b>, as shown by the dotted line in FIG. 4<i>a</i>. The pressure on the upper side of plate <b>50</b>, illustrated as pressure P<b>2</b> in FIG. 4<i>a</i>, remains constant at the atmospheric pressure provided through bleed passage <b>57</b> in housing <b>22</b>. The atmospheric pressure is lower than the increased pressure P<b>1</b> provided by ventilator <b>3</b>. When the breathing gas pressure in breathing circuit <b>2</b> decreases during expiration, the plate <b>50</b> is restored to the state shown in FIG. 4<i>b. </i>

P-0065[0065]FIGS. 5<i>a </i>and <b>5</b><i>b </i>are similar to FIGS. 4<i>a </i>and <b>4</b><i>b </i>and show circumstances occurring when the subject is breathing spontaneously. The breathing gas pressure inside adapter <b>13</b> and volume <b>65</b> decreases during inspiration by the subject as the diaphragm of the patient falls to draw breathing gases into the lungs. This reduction in pressure in volume <b>65</b>, shown as the downward arrow P<b>1</b> in FIG. 5<i>a</i>, causes the plate <b>50</b> to bend towards the adapter <b>13</b>. When the breathing gas pressure increases during expiration by the subject, as shown by the upwardly extending arrow P<b>1</b> in FIG. 5<i>b</i>, plate <b>50</b> bends upwardly in a direction away from adapter <b>13</b>, also as shown in FIG. 5<i>b. </i>

P-0066[0066] The deflections of the piezoelectric element <b>56</b> shown in FIG. 3 result in a change in the dipole moment of the piezoelectric material forming the element <b>56</b>. This causes a voltage to be generated in the element <b>56</b> and a signal to appear at terminals <b>35</b> and <b>36</b> of nebulizer <b>1</b>.

P-0067[0067] The bi-directional electromechanical conversion properties of piezoelectric element <b>56</b> enable the element <b>56</b> to be used in two ways. First, they can be used to create mechanical vibrations in a piezoelectric element <b>56</b> and plate <b>50</b> for atomization of liquid. This is electrical to mechanical conversion. Second, they can be used to electrically measure the mechanical strains in element <b>56</b> caused by the external forces applied to the piezoelectric element <b>56</b> resulting from breathing gas pressures in breathing circuit <b>2</b> acting on plate <b>50</b> in the manner described in FIGS. 4 and 5. This is mechanical to electrical conversion. The effects of these conversions are most pronounced when they occur along the poling axis of the piezoelectric element established during manufacture and piezoelectric element is formed and mounted to plate <b>50</b> such that this will occur. The poling axis will be generally parallel to the plane of ring-like piezoelectric element <b>56</b>.

P-0068[0068] When a mechanical force or electrical energization is applied to a piezoelectric element that does not thereafter change or changes only very slowly, the conversion occurring in the piezoelectric element is somewhat ineffective. For example, the dimensional change in a piezoelectric element resulting from the application of a DC electrical energization is usually measured in nanometers. The conversion from mechanical energy to electrical energy is somewhat more effective and becomes more effective if the mechanical force is rapidly applied. For example, delivering a sharp blow to a piezoelectric element results in an output voltage spike.

P-0069[0069] When electrical energization that alternates in polarity is applied to a piezoelectric element, the piezoelectric element undergoes mechanical vibration at a frequency corresponding to that of the alternating electrical energization. A piezoelectric element, like other mechanical objects and structures, will have a natural frequency of vibration. When vibrating at the natural frequency, physical displacements in an object are at a maximum amplitude. When the frequency of the alternating electrical energization is that of the natural frequency of the piezoelectrical element, the condition is one of mechanical resonance. When converting electrical energy into mechanical energy at the frequency of mechanical resonance of the piezoelectric element, the maximum amplitude of mechanical displacement induced in the piezoelectric element by the alternating electrical energization is much greater, for example, 10-100 times greater, than the maximum displacement that can be obtained from the application of electrical energization that does not change or changes very slowly after application.

P-0070[0070] To consider the conversion of mechanical energy to electrical energy when a piezoelectric element is driven at the frequency of mechanical resonance, the electrical characteristics of the piezoelectric element may be illustrated by the simple equivalent circuit <b>68</b> shown in FIG. 6<i>a</i>. In the equivalent circuit, capacitance <b>70</b> is the capacitance of the piezoelectric element and resistance <b>72</b> is the dielectric loss of the piezoelectric element. Resistor <b>74</b> represents the mechanical loss in the piezoelectric element and resistance <b>76</b> represents the mechanical load on the transducer, such as that imposed by plate <b>50</b>. Capacitor <b>78</b> and inductor <b>80</b> represent the rigidity and mass of the material of the piezoelectric element, respectively.

P-0071[0071] The series and parallel connections of the capacitive and inductive components in the equivalent circuit shown in FIG. 6<i>a </i>cause the overall circuit impedance characteristics to vary with frequency. When a piezoelectric element is vibrated at the frequency of mechanical resonance, the impedance of the piezoelectric element is at its lowest value. The inverse expression of impedance is “admittance,” which quantity is used herein for ease of explanation. The admittance of piezoelectric element will be at its greatest value at the frequency of mechanical resonance of the piezoelectric element. Conditions at this frequency resemble the characteristic of a series connected, inductive-capacitance alternating current circuit and are sometimes called that of electrical “resonance.”

P-0072[0072] In addition to the high admittance characteristics appearing at the frequency of mechanical resonance, there will also be a vibration frequency at which the admittance of the piezoelectric element will be at a minimum value. Conditions at this frequency resemble those of a parallel inductive-capacitance alternating current circuit and this point is sometimes called that of electrical “anti-resonance.”

P-0073[0073] In FIG. 6<i>b</i>, the ordinate is scaled in the electrical admittance Y of the piezoelectric element. The abscissa is scaled in the frequency. The graph <b>200</b> of FIG. 6<i>b </i>shows the electrical admittance Y of a piezoelectric element with respect to the mechanical frequency of the element. The frequency, <b>202</b> at which the admittance Y is at a maximum value, is the mechanical resonance frequency of the piezoelectric element. The minimum value of admittance Y is found at frequency <b>204</b> which is characterized as the anti-resonance frequency.

P-0074[0074] The frequency of mechanical resonance of a piezoelectric element is established by the external dimensions of the element and/or the composition of the piezoelectric material forming the element and can be changed by changing these aspects of the element.

P-0075[0075] When the piezoelectric element is attached to another mechanical element, for example, plate <b>50</b> as shown in FIG. 3 to form a pressure transducer, the attachment affects the mechanical resonance frequency of the piezoelectric element. The attached element functions as the mechanical load to the piezoelectric element. The attached mechanical element will not necessarily vibrate at the same frequencies as the piezoelectric element so that the overall composite transducer construction may have a plurality of mechanically resonant frequencies at which the admittance Y is at high values and a plurality of anti-resonant frequencies at which the admittance has low values.

P-0076[0076]FIG. 6<i>c </i>shows, in a manner similar to FIG. 6<i>b</i>, a graph <b>206</b> of admittance Y versus frequency for a composite structure, such as that described above. Vibration of the composite structure at frequencies <b>208</b>, <b>210</b>, and <b>212</b> produce high values of admittance. Vibration at frequencies <b>214</b> and <b>216</b> produce low values for admittance Y.

P-0077[0077] As generally indicated in FIGS. 6<i>b </i>and <b>6</b><i>c</i>, the admittance Y values at the peaks of the resonance frequencies are from several to 100 times higher than those values found in the lower portions of the graph. The width of the peaking portions of the admittance frequency graphs, in terms of frequency at −3 dB admittance level, is usually from tens of hertz to several kilohertz, depending on the structure of the piezoelectric element and/or composite structure.

P-0078[0078] The graphs shown in FIG. 6 may be obtained by measuring the current through a piezoelectric element against the frequency of the electrical signal applied to the piezoelectric element when an alternating electrical energization of constant peak voltage magnitude is applied to the piezoelectric element. The measured current is used to compute the admittance of the piezoelectric element. The frequency that produces the highest current, and hence highest admittance, is the mechanical resonance frequency of the element. FIG. 7<i>a </i>shows a circuit that may be used to determine the admittance of a composite construction containing a piezoelectric element. Piezoelectric element <b>56</b> is connected in series with ammeter <b>220</b> across constant peak voltage magnitude, variable frequency AC voltage source <b>222</b>. As the frequency of voltage source <b>222</b> is varied, the current through piezoelectric element <b>56</b> is measured and the admittance determined as Y=I/V.

P-0079[0079] Or a current that alternates between fixed magnitudes may be applied to the piezoelectric element as shown in FIG. 7<i>b</i>. The current source <b>224</b> is of adjustable frequency. The voltage across the piezoelectric element is measured by voltmeter <b>226</b> as the frequency of the applied current is varied. With the current magnitude so fixed, the voltage across the piezoelectric element will decrease as the admittance of the piezoelectric element increases at the frequency of mechanical resonance. The same formula, Y=I/V, is used to determine admittance.

P-0080[0080] A third way to establish the data shown in FIG. 6 is to measure electrical phase differences occurring in the circuit containing piezoelectric element <b>56</b>. At the frequency of mechanical resonance, there will be a minimum phase difference, or no phase difference, between the voltage and current in the circuit. See FIG. 7<i>c </i>in which the phase difference may be determined by voltage and load current measurements carried out in connection with resistor <b>228</b> and voltage source <b>230</b>.

P-0081[0081] In the operation of nebulizer <b>1</b>, piezoelectric element <b>56</b> is preferably electrically energized at the frequency <b>208</b> shown in FIG. 6<i>c </i>to vibrate the composite pressure transducer structure with maximum magnitude mechanical deflections occurring in plate <b>50</b>, thereby to provide highly desirable atomization to the liquid being nebulized. However, the piezoelectric element may also be energized at frequencies <b>210</b> or <b>212</b> to vibrate plate <b>50</b> and produce atomization if the maximum resonant frequency <b>208</b> is out of a desired operating frequency range.

P-0082[0082] An external compressive or tensile load applied to the vibrating piezoelectric structure as when plate <b>50</b> is subjected to the breathing gas pressures in breathing circuit <b>2</b> shifts the series resonance frequency or frequencies, such as <b>202</b>, <b>208</b>, <b>210</b> and <b>212</b> and the parallel or anti-resonant frequency or frequencies, such as <b>204</b>, <b>214</b>, and <b>216</b>. The shift in resonance and anti-resonance frequencies will be related to the magnitude of the applied load. Furthermore, the shift in resonance and anti-resonance frequencies for a given applied load is greater when the effect of external force is directed along the poling axis of the piezoelectric element.

P-0083[0083] The characteristics described above are used to detect pressure changes and to measure pressures in the breathing circuit in the following manner. For explanatory purposes, FIG. 8<i>a </i>shows a simple admittance-frequency curve <b>300</b>, similar to that shown in FIG. 6<i>a</i>. It will be appreciated that the actual admittance-frequency curve for a nebulizer <b>1</b> will more generally resemble that of FIG. 6<i>b </i>since piezoelectric element <b>56</b> is coupled to plate <b>50</b> to form a composite pressure transducer structure. Piezoelectric element <b>56</b> is energized at resonance frequency <b>202</b>.

P-0084[0084]FIG. 9 shows a simple circuit that can be incorporated in control <b>4</b> and used to energize piezoelectric element <b>56</b> at resonance frequency <b>202</b>. Other suitable circuits are shown in more detail in FIGS. 10 and 12. The circuitry includes an adjustable frequency voltage source <b>700</b>, such as a voltage controlled oscillator. The output of source <b>700</b> may be provided through amplifier <b>702</b> to piezoelectric element <b>56</b> to energize the element. The admittance of piezoelectric element is measured, as by one of the techniques shown above in connection with FIG. 7, as schematically shown by circuit <b>704</b> in FIG. 9.

P-0085[0085] For example, a current signal may be supplied to circuit <b>704</b> for use in conjunction with a voltage signal from source <b>700</b> to determine admittance. In FIG. 8<i>a</i>, the measured admittance with the composite pressure transducer structure in the unloaded state is shown as level <b>240</b>. This level is used to establish a reference signal from reference signal source <b>708</b> to comparator <b>706</b>.

P-0086[0086] As shown in FIGS. 4 and 5, as the subject is ventilated by ventilator <b>3</b> or breathes spontaneously, pressure will be applied to plate <b>50</b> of nebulizer <b>1</b> by the breathing gases in breathing circuit <b>2</b>. These pressures will, in turn, be applied to piezoelectric element <b>56</b>. The mechanical loading applied to piezoelectric element <b>56</b> will cause the resonance frequency of the piezoelectric pressure transducer structure to shift from frequency <b>202</b> to frequency <b>242</b>, as shown on in FIG. 8<i>a </i>by graph <b>244</b>. The direction of the shift will depend on the construction of the piezoelectric pressure transducer structure and on whether the mechanical load applied to piezoelectric element <b>56</b> is tensile or compressive. With the admittance curve shifted to that shown by graph <b>244</b>, the admittance Y of the pressure transducer structure measured at the energization frequency <b>202</b> and determined by circuit <b>704</b> will fall to the level <b>246</b>. The difference in admittance between level <b>240</b> and level <b>246</b> as determined by comparator <b>706</b> is an indication of the pressure change and a measure of the breathing gas pressure in breathing circuit <b>2</b> which can be indicated by gas pressure readout circuit <b>710</b>.

P-0087[0087] The peaking nature of the graph shown in FIG. 8<i>a </i>at the resonance frequency is useful in providing difference values of a magnitude that assists in accurately determining breathing gas pressures. The output of comparator <b>706</b> may also be used to operate valve <b>18</b> to control the supply of liquid to mesh plate <b>52</b> for nebulization in synchronization with specific pressure conditions, such as those indicative of inhalation by the subject. Valve <b>18</b> is opened to commence nebulization and thereafter closed. Nebulization stops when the liquid supplied to mesh plate <b>52</b> is exhausted.

P-0088[0088] It will be appreciated that gas pressures in breathing gas circuit <b>2</b> continuously vary over most of the respiratory cycle as the subject inspires and expires. Thus, comparisons can also be made in which the fall/rise time and/or the duration of fall/rise of the breathing gas pressure signal are compared to one or several previously measured values and the change or the difference between the values is compared to some predetermined value. In that case, the comparison of the breathing gas pressure signal is based on a differential signal and, for example, drift or slow disturbances have a limited effect on the action used to initiate activation of value <b>18</b> or initiate some other operation in nebulizer <b>1</b>.

P-0089[0089] Further, the frequency of the electrical energization supplied to the piezoelectric pressure transducer structure by adjustable frequency source can be varied to determine whether the resonance frequency has shifted to a value higher or lower than frequency <b>202</b>. In the example shown in FIG. 8<i>a</i>, as the frequency of the electrical energization is lowered toward frequency <b>242</b>, the measured admittance value will increase along the right hand slope of graph <b>244</b> from the level <b>246</b> to a peak value corresponding to level <b>240</b>, indicating that the resonance frequency has shifted to a lower value. The construction of the pressure transducer containing piezoelectric element <b>56</b> will determine whether this frequency shift to a lower value indicates that the loading on the piezoelectric pressure transducer structure is that produced by a breathing gas pressure increase or that produced by a breathing gas pressure decrease.

P-0090[0090] Or, if the frequency of the electrical energization is increased, the measured admittance value will decrease since the frequency has been moved away from the resonance frequency. This also indicates that the resonance frequency has shifted to a lower value.

P-0091[0091]FIG. 8<i>b </i>shows the situation in which the pressure applied to plate <b>50</b> by the breathing gases in breathing circuit <b>2</b> results in a loading of piezoelectric element <b>56</b> that causes the resonance frequency to increase, as shown in the figure by frequency <b>248</b> and curve <b>250</b>. The admittance value Y measured at frequency <b>202</b> falls to a level <b>252</b> lower than level <b>240</b> to provide a difference value from comparator <b>706</b> that may be used to determine breathing gas pressure in breathing circuit <b>2</b>. By altering the frequency of the electric energization for piezoelectric element <b>56</b>, the direction of the shift can be determined by the change in admittance values in the manner described above to confirm the nature of the mechanical loading on the piezoelectric pressure transducer structure.

P-0092[0092] While FIGS. 8<i>a </i>and <b>8</b><i>b </i>have described operation of nebulizer <b>1</b> using resonance frequency <b>202</b>, it will be appreciated that the breathing gas pressure measuring technique described above will also work should nebulizer <b>1</b> be operated at a frequency other than the resonance frequency. The difference in admittance Y values between the unloaded and loaded states of the piezoelectric transducer structure will tend be less than those obtained through the use of the resonance frequency <b>202</b> and shown in FIGS. 8<i>a </i>and <b>8</b><i>b. </i>

P-0093[0093] Also, it will be appreciated that the flow of liquid onto the vibrating mesh plate <b>52</b> from valve <b>18</b> also temporarily affects the piezoelectric pressure transducer structure as a mechanical load and thus also acts to shift the resonance frequency, such as frequency <b>202</b>. However, in a simple circuit such as shown in FIG. 9, appropriate compensation can be provided in the determination of admittance Y, as by a bias or offset value circuit <b>712</b>, to accommodate the effect of the column of liquid between valve <b>18</b> and mesh plate <b>52</b>. Or, in a preferred embodiment of the invention, by periodically adjusting the magnitude of the alternating energization to piezoelectric element <b>56</b>, atomization may occur at certain times and pressure measurement may take place at other times, as shown and described in more detail in connection with FIGS. 10 and 11.

P-0094[0094] Another technique to measure gas pressure magnitude and flow direction of breathing gas flow is shown in FIG. 8<i>c</i>. In this technique, the characteristics of admittance versus frequency, shown graphically in FIG. 8<i>c </i>as curve <b>260</b>, are determined for a state in which the piezoelectric pressure transducer structure is not subject to any mechanical loading. The graph will exhibit a resonance frequency <b>262</b>.

P-0095[0095] Nebulizer <b>1</b> is then operated to supply electrical energization to piezoelectric element <b>56</b> at a frequency <b>264</b>, different from frequency <b>262</b> and the admittance Y for the unloaded state is measured, as level <b>266</b> which value is used by reference signal source <b>708</b> to provide a reference input to comparator <b>706</b>.

P-0096[0096] Thereafter, the piezoelectric pressure transducer structure is subjected to the breathing gas pressures in breathing circuit <b>2</b>. The mechanical loading applied to piezoelectric element <b>56</b> by the breathing gas pressure will shift the admittance-frequency curve, as shown in FIG. 8<i>c </i>by graph <b>268</b>. This shift will cause the admittance of the piezoelectric pressure transducer structure measured at frequency <b>264</b> to change to the value indicated by level <b>270</b>. The change in admittance value can be used by comparator <b>706</b> to determine the pressure of the breathing gas in breathing circuit <b>2</b>.

P-0097[0097] The fact that the level <b>270</b> is greater than admittance level <b>266</b> indicates that the resonant frequency has shifted to a lower value. That is, the resonance frequency has shifted from that indicated by frequency <b>262</b> for graph <b>260</b> to that indicated by frequency <b>272</b> for graph <b>268</b>. This fact can then be used to indicate whether the loading on piezoelectric element <b>56</b> applied by plate <b>50</b> is tensile or compressive. As noted in FIGS. 4<i>a </i>and <i>b </i>and <b>5</b><i>a </i>and b whether the loading is tensile or compressive depends on whether the patient is being mechanically ventilated or is breathing spontaneously and the direction of gas flow in patient limb <b>10</b>. Knowing the breathing mode for the patient and the type of loading exerted on piezoelectric element <b>56</b>, the direction of gas flow in patient limb <b>10</b> can be determined.

P-0098[0098] The frequency <b>264</b> used for measuring purposes can be chosen in accordance with the construction of the piezoelectric pressure transducer structure and the minimum and maximum gas pressures to be measured. It is usually spaced tens or hundreds of hertz greater or lower than the resonance frequency <b>262</b>. Also, it is desirable to select a frequency <b>264</b> that lies in a generally linear portion of graph <b>260</b> for the range of gas pressures to be measured. This provides linearity in the measurement of gas pressure within the pressure range. A linear portion of curve <b>260</b> is shown by line <b>274</b> and dots <b>276</b><i>a </i>and <b>276</b><i>b. </i>

P-0099[0099] When the mechanical loading applied to piezoelectric element <b>56</b> by the breathing gas pressure on plate <b>52</b> is opposite to that described above, the admittance versus frequency curve will shift in the opposite direction from that described above. This is shown by the partial curve <b>278</b> in FIG. 8<i>c</i>. In this circumstance, the admittance value Y measured at frequency <b>264</b> will decrease to level <b>280</b>. The difference between the admittance value <b>266</b> and the admittance value <b>280</b> may be used by comparator <b>706</b> to determine the pressure of the gas in breathing circuit <b>2</b>. The fact that the admittance value <b>280</b> is decreased from admittance value <b>266</b> indicates that the loading on piezoelectric element <b>56</b> is opposite that which produces admittance level <b>272</b>. As noted above, this information can be used to determine the direction of gas flow in breathing circuit <b>2</b>.

P-0100[0100] While FIG. 8<i>c </i>shows operation of nebulizer <b>1</b> at a frequency <b>264</b> less than resonance frequency <b>262</b>, it will be appreciated that nebulizer <b>1</b> may be operated in an analogous manner for a frequency greater than frequency <b>262</b>. The changes in admittance caused by a compressive loading of piezoelectric element <b>56</b> and a tensile loading of the piezoelectric element will be opposite to that described above in connection with FIG. 8<i>c. </i>

P-0101[0101] A benefit achieved in measuring the breathing gas pressure at a frequency point aside from the natural resonant frequency point is lower power consumption. However, to ensure that the admittance measurements are sufficient to measure pressure changes with the desired degree of accuracy, the amplitude of alternating voltage supplied to the piezoelectric element <b>56</b> must be sufficiently high to provide the desired signal to noise the ratio in the signals used for measurement.

P-0102[0102] By detecting the direction and pressure of the breathing gas flow in breathing circuit <b>2</b> using the electric characteristics of the element <b>56</b> as described above, and providing this information to control unit <b>4</b> it is possible for the control unit <b>4</b> to initiate and halt the atomization of the drug to coincide with the breathing of the subject in the manner described generally and schematically above in connection with FIG. 9. This allows the drug to be introduced into the breathing circuit <b>2</b> at those moments when the drug can be delivered most effectively through breathing circuit <b>2</b> to the subject. Typically, the discharge of liquid into the breathing gas is preferably carried out either during, or partly during, inspiration. If the atomized liquid is dispensed into the breathing circuit <b>2</b> only within certain periods during the time the subject is inspiring, the pressure measurement may be carried out only at the beginning of inspiration and during or partly during expiration to find the leading edge of the breathing gas pressure change that characterizes inspiration. This is utilized as the trigger for the start of atomization of the liquid by nebulizer <b>1</b>. Such a drug delivery technique can provide both an effective drug administration and conserve power in the operation of nebulizer <b>1</b>. If atomized liquid is dispensed at all times during inspiration, the pressure measurement may be carried out throughout the nebulization to find the leading edge of the breathing gas pressure change that characterizes inspiration to start the atomization and trailing edge of the breathing gas pressure change to stop the atomization.

P-0103[0103] While FIGS. 8<i>a</i>, <b>8</b><i>b</i>, and <b>8</b><i>c </i>show comparison of the admittance at the shifted resonance frequency to the admittance at the original resonance frequency, the former admittance value can be compared to a predetermined reference value, if desired, in a manner analogous to that hereinafter described in connection with FIG. 15 to control the operation of nebulizer <b>1</b>. Or, a given measured admittance value can be compared with a previously measured admittance value, or to an average of previously measured values, to determine a value difference. The value difference is then compared to a reference value to indicate pressure characteristics of the breathing gases and/or control the operation of nebulizer <b>1</b>.

P-0104[0104]FIG. 10 shows a further embodiment of a control apparatus <b>500</b> suitable for use in the nebulizer apparatus and method of the claimed invention. Voltage controlled oscillator <b>502</b> provides an adjustable frequency voltage through signal amplifier <b>504</b> to piezoelectric element <b>56</b> in nebulizer <b>1</b>. The admittance of piezoelectric element <b>56</b> is measured, by one of the techniques/means described above in circuitry <b>506</b> and provided through analog/digital converter <b>508</b> to microprocessor <b>510</b>. To determine the resonance frequency of the composite transducer containing piezoelectric element <b>56</b>, microprocessor <b>510</b> operates voltage controlled oscillator <b>502</b> through digital/analog converter <b>512</b> to scan a predetermined frequency range in which the resonance frequency point should be found. The admittance of piezoelectric element <b>56</b> is measured by element <b>506</b> during the scanning to determine the peak value of the admittance that indicates the resonance frequency. The resonance frequency is stored in a memory in microprocessor <b>510</b>. After the resonance frequency is found, microprocessor <b>510</b> operates voltage controlled oscillator <b>502</b> to provide alternating voltage energization of the resonance frequency, or another desired operating frequency.

P-0105[0105] After the operating frequency for piezoelectric element <b>56</b> has been established, microprocessor <b>510</b> operates valve <b>18</b> to provide liquid to mesh plate <b>52</b>. As described above, an impedance measurement is carried out using mesh plate <b>52</b> and electrode <b>38</b>. The impedance measurement signal is provided to comparator <b>514</b>. The amount of liquid provided by valve <b>18</b> may be controlled by a reference signal to comparator <b>514</b>. The output signal from comparator <b>514</b> is provided to microprocessor <b>510</b> to operate valve <b>18</b>.

P-0106[0106] During operation of nebulizer <b>1</b>, the impedance measurement detects when the column of liquid supplied to mesh plate <b>52</b> is reduced or disappears due to the atomization of liquid through the mesh plate. The signal from comparator <b>514</b> causes microprocessor <b>510</b> to open valve <b>18</b> to resupply liquid until the impedance measurement detects that a sufficient quantity is again present in nebulizer <b>1</b> and the above described control loop continues to function in the foregoing manner during operation of nebulizer <b>1</b>.

P-0107[0107] During the atomization of the liquid, the liquid on rear surface mesh plate <b>52</b> is forced through holes <b>53</b> in mesh plate <b>52</b> to front surface <b>54</b>. The motion of mesh plate <b>52</b> must exceed the cohesive forces of the liquid being atomized in order to discharge atomized liquid from front surface <b>54</b>. To ensure the amplitude of vibration of mesh plate <b>54</b> is sufficient to cause atomization of the liquid, the amplitude of vibration may be controlled through signal amplifier <b>504</b> to alter the amplitude of the alternating voltage applied to piezoelectric element <b>56</b>. Beneath a certain piezoelectric element voltage excitation amplitude, and corresponding mesh plate <b>52</b> vibration magnitude, the atomization of liquid will cease. This means that by reducing the amplitude of the alternating voltage excitation to piezoelectric element <b>56</b> to a level below that at which atomization occurs, the composite transducer incorporating piezoelectric element <b>56</b> may be used for pressure measurement in the manner described above.

P-0108[0108] One way in which the foregoing may be carried out is shown in FIG. 11 for a case in which a subject is being mechanically ventilated by ventilator <b>3</b>.

P-0109[0109] The voltage excitation level for piezoelectric element <b>56</b> provided by amplifier <b>504</b> is shown at the left side of FIG. 11<i>a </i>as at a lower amplitude level <b>520</b>. Level <b>520</b> is sufficiently low that atomization of the liquid does not occur in nebulizer <b>1</b>. Measurement of the pressure of the breathing gases in breathing circuit <b>2</b> is carried out by the composite pressure transducer in the manner described above using microprocessor <b>510</b>. The microprocessor may provide a readout of gas pressure magnitude. As, the subject is being ventilated by mechanical ventilator <b>3</b>, the pressure of the breathing gases will rise as the inspiration phase of the breathing cycle is commenced, as shown by graph <b>522</b>. When the pressure of the breathing gases exceeds a predetermined triggering level <b>524</b> established in microprocessor <b>510</b>, microprocessor <b>510</b> operates signal amplifier <b>504</b> to increase the amplitude of the voltage excitation to piezoelectric element <b>56</b> to a higher amplitude level <b>526</b>. Excitation amplitude level <b>526</b> is sufficient to cause atomization of the liquid to occur in nebulizer <b>1</b>.

P-0110[0110] After a predetermined time, and no later than the start of expiration, the amplitude of the excitation to piezoelectric element <b>56</b> is reduced to level <b>520</b>, at which time the measurement of the pressure in breathing circuit <b>2</b> resumes.

P-0111[0111] Operation of piezoelectric element <b>56</b> in the manner described above is advantageous in that it provides atomized liquid to the breathing gases in the initial portions of the inspiration phase of the respiratory cycle, thereby to ensure that an atomized liquid, such as a drug, will enter the lungs of the subject during inspiration. It is also advantageous in that power consumption by transducer is reduced inasmuch as the higher amplitude energization <b>526</b> is used only during atomization.

P-0112[0112] To reduce power consumption even more, the amplitude of the energization for piezoelectric element <b>56</b> may be reduced to zero for a predetermined time immediately following the conclusion of the atomization, as shown in FIG. 11<i>b</i>. This is shown by signal level <b>528</b> in FIG. 11<i>b</i>. The energization of piezoelectric element <b>52</b> is restored to the low amplitude level <b>520</b> as the end of the expiration phase of the subject's respiratory cycle concludes to ensure that proper detection of the leading edge of the inspiratory phase is carried out.

P-0113[0113] The techniques shown in FIG. 11 have the advantages of reducing power consumption over approaches, such as that shown in FIG. 9, in which the piezoelectric element is continuously operated at a high voltage excitation level.

P-0114[0114] A further technique to measure gas pressure magnitude and flow direction of breathing gas flow is shown in FIG. 12. A circuit for measuring gas pressure magnitude and flow direction based on electrical phase differences between the voltage and current in the composite circuit transducer is shown in FIG. 12<i>a</i>. The manner in which the circuit operates is shown in FIG. 12<i>b</i>. In FIG. 12<i>a </i>components corresponding to those in FIG. 10 are identified by similar reference numerals, it being understood that the circuitry shown in FIG. 12<i>a </i>may use appropriate digital signal processing components of the type shown in FIG. 10.

P-0115[0115] The circuit of FIG. 12<i>a </i>includes resistor <b>520</b> in series with piezoelectric element <b>56</b>. Resistor <b>520</b> corresponds to resistor <b>228</b> shown in FIG. 7<i>c</i>. The voltage across resistor <b>520</b> is an indication of the current through piezoelectric element <b>56</b>. The output of signal amplifier <b>504</b> containing the amplified output of voltage controlled oscillator <b>502</b> is an indication of the voltage applied to piezoelectric element <b>56</b>. The current signal from resistor <b>520</b> and the voltage signal from amplifier <b>504</b> are applied to phase displacement detection system <b>522</b> in conductors <b>524</b> and <b>526</b>, respectively. Phase detection system <b>522</b> determines the phase difference between the two signals, as by detecting zero crossings or some other appropriate technique, and provides a phase difference output signal in conductor <b>528</b>.

P-0116[0116] At the resonance frequency of the composite transducer, when there is no mechanical loading on the transducer due to a zero breathing gas pressure or “normal” condition inside patient limb <b>10</b>, the phase difference between the current and voltage is zero or close to zero.

P-0117[0117] Gas pressures in patient limb <b>10</b> are shown by graph <b>540</b> in FIG. 12<i>b</i>. The zero breathing gas pressure condition or other baseline condition is shown as <b>541</b>. For a spontaneously breathing subject, when the subject breathes in at the commencement of inspiration, an underpressure occurs in patient limb <b>10</b>. This shifts the resonance frequency of the composite transducer, for example, to a lower frequency than the frequency of the output signal of voltage controlled oscillator <b>502</b> which is typically at the resonance frequency of the transducer in the zero gas pressure state. These conditions result in a phase difference <b>542</b> between the current, as reflected in the signal in conductor <b>524</b> measured across resistor <b>520</b> and the voltage output of amplifier <b>504</b>. The phase difference may be one in which the phase of the current <b>544</b> is behind the phase of the voltage <b>546</b>. If the voltage signal <b>546</b> is used as a reference, the phase difference may be deemed a “negative” phase difference, i.e. the current lags the voltage.

P-0118[0118] In the circuit in FIG. 12<i>a</i>, this “negative” phase difference is detected by phase detection system <b>522</b>. Phase detection system <b>522</b> then controls voltage controlled oscillator <b>502</b> by, for example, decreasing the oscillator control voltage <b>547</b> shown in FIG. 12<i>b </i>in conductor <b>528</b> to alter the frequency of the voltage controlled oscillator to minimize the phase difference. As the oscillator control voltage is decreased, the oscillator output frequency also decreases and the phase difference between the current and the voltage decreases. As the magnitude of negative pressure inside the patient limb stops decreasing at <b>548</b>, the need to decrease the oscillator control voltage also lessens. Finally, when the negative breathing circuit pressure inside patient limb <b>10</b> has reached its minimum value <b>550</b>, the phase difference again becomes zero, due to the fact that the energization frequency from voltage controlled oscillator <b>520</b> has been set to the resonance frequency of the composite transducer at the minimum gas pressure condition. At this point, the oscillator control voltage <b>547</b> from phase detection system <b>522</b> in conductor <b>528</b> is minimum. The change in oscillator control voltage <b>547</b> provided by phase detection system <b>522</b> is an indication of the breathing gas pressure in patient limb <b>10</b> which may be provided as gas pressure readout. The direction of change of the voltage is an indication of the direction flow of the breathing gases in patient limb <b>10</b>.

P-0119[0119] As the breathing gas pressure inside the patient limb starts to revert back to its original pressure, as the subject breathes out, the phase difference between the current and the voltage again increases but in the opposite direction, i.e. a “positive” phase difference <b>552</b>. The new resonance frequency point which was established at breathing gas pressure <b>550</b> shifts back to the original resonance frequency as the breathing gas pressure returns to the zero pressure or baseline condition at <b>554</b>. The “positive” phase difference is detected by phase detection system <b>522</b>, which then increases the oscillator control voltage <b>547</b> for voltage controlled oscillator <b>502</b> in conductor <b>528</b> toward its original value to again minimize the phase difference between the current and voltage in the composite transducer. As the oscillator control voltage is increased, the oscillator output frequency also increases and the phase difference in the transducer is decreased. As the breathing gas pressure inside patient limb <b>10</b> reaches its original value <b>541</b>, the phase difference becomes minimized, or zeroed, as shown at <b>556</b>, as the oscillator control voltage and oscillator output frequency reach the same values that established the original zero phase difference.

P-0120[0120] In the period during which the subject has a pause <b>554</b> between inspiration and expiration, the breathing gas pressure inside patient limb <b>10</b> remains constant. The phase difference between the current and voltage remains zero and the oscillator control voltage <b>547</b> is constant at the nominal potential that establishes the frequency of the energization of piezoelectric element <b>56</b> at its original resonance frequency.

P-0121[0121] When the subject breathes out, during the time interval <b>556</b> shown in FIG. 12<i>b</i>, the operation of the control circuitry resembles that during inspiration but is now opposite to that described above. This control voltage <b>547</b> to voltage controlled oscillator <b>502</b> is now increased over the nominal voltage to increase the energization frequency of piezoelectric element <b>56</b> to bring about the phase difference minimization at peak positive breathing gas pressure at point <b>558</b>. As before, the amount and direction of change of oscillator control voltage <b>547</b> is an indication of the pressure and flow direction of the breathing gases in patient limb <b>10</b>.

P-0122[0122] The oscillator control voltage <b>547</b> is controlled by phase detection system <b>552</b> as the phase differences occur between the voltage and current in the composite transducer and thus immediately as the breathing gas pressure changes inside patient limb <b>10</b>. FIG. 12<i>b </i>shows the change in oscillator control voltage <b>547</b>, i.e. the signal in conductor <b>528</b> in correspondence with changes in breathing gas pressures in patient limb <b>10</b>. An upward arrow indicates that the oscillator control voltage is increased, a horizontal arrow indicates that the voltage is constant, and a downward arrow indicates that the voltage is decreased.

P-0123[0123] In the circuit shown in FIG. 12<i>a</i>, the output of phase detection system <b>522</b>, i.e. the oscillator control voltage <b>547</b>, which is a voltage proportional to the breathing gas pressure in patient limb <b>10</b> also goes to comparator <b>570</b>. Comparator <b>570</b> compares the voltage to a set trigger level voltage in conductor <b>572</b> which determines the pressure level for the start of nebulization during inspiration, as described above, in connection with FIG. 11. As noted above, comparisons can also be made in which the fall/rise time and/or the duration of fall/rise of the breathing gas pressure signal are compared to one or several previously measured values and the change or the difference between the values is compared to some predetermined value. In that case, the comparison of the breathing gas pressure signal is based on a differential signal and, for example, drift or slow disturbances have limiting effect on the triggering action provided by comparator <b>570</b>.

P-0124[0124] The triggering signal from comparator <b>570</b> goes to amplifier <b>504</b> and is used to establish the amount of amplification occurring in the energization of piezoelectric element <b>56</b> during breathing gas pressure measurement and during nebulization. During breathing gas pressure measurement, the signal level in the amplifier output is adjusted below the signal level used for nebulization, as shown in FIGS. 11<i>a </i>and <b>11</b><i>b</i>. Signal levels may, for example, be adjusted to a 1-100 millivolt range during pressure measurement and to a 1-10 volt range during nebulization.

P-0125[0125] The supply of liquid from reservoir <b>17</b> in nebulizer <b>1</b> has its own control loop in the manner described in connection with FIGS. 10 and 11. The impedance sensing means <b>38</b>, <b>52</b> detects the amount of liquid on mesh plate <b>52</b>. The signal from the impedance sensing means goes to comparator <b>514</b> which performs a comparison to a set liquid level reference in conductor <b>574</b>. If the amount of liquid is less than the set level, valve <b>18</b> is open to supply more liquid from reservoir <b>17</b> until the desired amount is reached, after which the comparator output closes the valve. The liquid may be supplied to mesh plate <b>52</b> only during nebulization so that the load effect on the composite transducer is avoided or minimized during pressure measurement. However, if the liquid supply does have a load effect on piezoelectric element <b>56</b> and thus its resonance frequency, the signal from phase detection system <b>522</b> can be filtered, biased, or removed whenever the liquid is supplied to mesh plate <b>52</b>.

P-0126[0126]FIG. 13<i>a </i>shows another embodiment of a structure that can be used to obtain atomization of a liquid in nebulizer <b>1</b>. In the schematic cross-sectional view of FIG. 13<i>a</i>, a bi-morph structure <b>290</b> comprises a ring-shaped piezoelectric element <b>292</b> attached with adhesive <b>294</b> to a disk-shaped metal plate <b>296</b>. Plate <b>296</b> is made of a conductive, generally rigid material, such as brass, having a center hole for the attachment of a mesh plate <b>298</b> similar to the mesh plate <b>52</b>. The bi-morph structure <b>290</b> is vibrated in a bending mode illustrated by the dashed line <b>300</b> in FIG. 13<i>a </i>by an alternating electrical energization applied to the terminals, such as terminals <b>35</b> and <b>36</b> which contact the structure A liquid is simultaneously supplied to mesh plate <b>298</b> and is discharged through the holes in the mesh plate. The bi-morph structure <b>290</b> may be used for gas pressure and flow direction measurements in the same manner as plate <b>50</b> and element <b>56</b> when incorporated in nebulizer <b>1</b>.

P-0127[0127]FIG. 13<i>b </i>shows a schematic cross-sectional view of another type of a transducer structure <b>302</b> that can be employed in nebulizer <b>1</b>. Structure <b>302</b> comprises a ring-shaped piezoelectric element <b>304</b> attached with adhesive around the periphery of a disk-shaped plate <b>306</b> made of a conductive, generally rigid material, such as brass, having a center hole for the attachment of a mesh plate <b>308</b>. When an alternating electrical energization is applied to terminals contacting the piezoelectric element <b>304</b>, the element <b>304</b> vibrates in a radial mode as shown by the dotted lines in FIG. 13<i>b</i>, with the vibrating force directed to plate <b>306</b>. The plate <b>306</b> then starts to vibrate in a bending mode, as indicated by the dashed line <b>310</b> in FIG. 13<i>b</i>. Liquid supplied onto one side of mesh plate <b>308</b> is then discharged through the holes in the mesh plate. The transducer structure <b>302</b> can also be used for gas pressure and flow direction measurements when incorporated, for example, in nebulizer <b>1</b> shown in FIG. 2.

P-0128[0128]FIG. 14 shows an alternative embodiment of a piezoelectric structure for discharging a liquid into a receiving gas. In the structure of FIG. 14, a piezoelectric element in the form of a piezoelectric rod <b>600</b> is mounted at one end of elongated housing <b>602</b>. An outlet <b>604</b> is provided at the other end of housing <b>602</b>. Liquid inlet <b>606</b> opens into housing <b>602</b> intermediate the ends of the housing. Piston <b>608</b> is fastened to piezoelectric element <b>600</b>.

P-0129[0129] In operation, when piezoelectric element <b>600</b> is supplied with an alternating voltage, piston <b>608</b> is reciprocated in housing <b>602</b>. In a withdrawn state of piston <b>608</b>, inlet <b>606</b> is opened to provide a column of liquid in the lower portion of housing <b>602</b>. In an extended state of piston <b>608</b>, liquid is driven from outlet <b>604</b> of housing <b>602</b> into a receiving gas. The pressure of the receiving gas will be transmitted through the liquid column and piston <b>608</b> to piezoelectric element <b>600</b> so that resonance frequency and admittance value changes in the piezoelectric element can reflect the pressure and flow direction of the receiving gas. If it is desired to more finely atomize the liquid, appropriate means, such as a mesh plate, may be provided for the liquid discharged from housing <b>602</b>.

P-0130[0130]FIG. 15<i>a </i>shows circuitry suitable for use in a passive embodiment of the present invention in which piezoelectric element <b>56</b> is not actively energized as by source <b>700</b>. Rather, the voltage appearing at the terminals of transducer <b>68</b>, as element <b>56</b> is subjected to mechanical loading by member <b>50</b>, is measured and used as an indication of the pressure of the receiving gas. For this purpose, piezoelectric element <b>56</b> is connected to the input of amplifier <b>800</b>. The output of the piezoelectric element may be filtered by filter <b>802</b>, if desired, to remove noise and to provide a bandpass for signals having desired frequency characteristics. The output of amplifier <b>800</b> is provided to gas pressure indicator <b>804</b>. Changes in the output of amplifier <b>800</b> may be used to detect the commencement of the inspiratory phase of the patient's respiratory cycle to cause the nebulizer to provide atomized fluid to the breathing gases.

P-0131[0131] To detect gas pressure changes to carry out a triggering action and/or to provide an indication of gas pressure, the output of amplifier <b>800</b> may be compared to a predetermined reference value from reference value generator <b>806</b> in comparator <b>808</b> shown in FIG. 15<i>b</i>. The output of comparator <b>808</b> provides a differential gas pressure measurement at indicator <b>804</b> and/or operates nebulizer <b>1</b>. Or, the measured value may be compared to some other reference value, such as a previously measured value or an average of previously measured values produced by value generator <b>810</b> shown in FIG. 15<i>c</i>. The difference between the two values determined by comparator <b>812</b> is then compared to a predetermined reference quantity from reference value generator <b>814</b> in comparator <b>816</b> for us in controlling the operation of the nebulizer or as an indication of gas pressure.

P-0132[0132] It will be appreciated that the frequencies present in the embodiment of the invention shown in FIG. 15 will be much lower than those found in the active embodiments of the invention, described above. That is, the signal frequencies will tend to be those of the subject's respiratory rate. The same is true of the output signal levels and the amounts by which the signal levels change in the course of operating of the liquid discharging apparatus. Output signal levels, and changes therein can be enhanced by mechanically “tuning” the transducer to a resonance frequency corresponding to that of the mechanical loading to which it is subjected. This may be accomplished by appropriate selection of the physical characteristics of element <b>56</b> and member <b>50</b> and/or the composition of these elements.

P-0133[0133] The embodiment of the invention shown in FIG. 15 provides an economical version of the present invention but having somewhat less accuracy in gas pressure measurement.

P-0134[0134] Various other types of elements that function in the same manner as the foregoing could also be substituted for those described in detail, above. Thus, it is recognized that other equivalents, alternatives, and modifications aside from those expressly stated, are possible and within the scope of the appended claims.