Micro-probe for gas sampling.

0001This invention relates to the sensing of gases dissolved in liquids and is specifically adapted for the in vivo sensing of blood gases. Blood gases have been sensed and analyzed by various prior art methods, one of which being disclosed in U.S. Patents 3,983,864 and 4,016,864. As shown in those patents, a carrier gas is introduced into a special catheter probe, and held in a chamber where the blood gases equilibrate through a gas permeable membrane with the carrier gas and the carrier gas containing the equilibrated blood gases is thereafter withdrawn and analyzed.

0002The catheter is introduced in vivo into the particular blood vessel sought to be analyzed. An equilibration chamber is provided in the probe and allows an equilibration between the carrier gas passing through the probe and the blood gases contained in the blood. Equilibration occurs through a gas permeable membrane that surrounds the equilibration chamber and has its outside surface in direct contact with the blood to be analyzed. The blood gases pass into the equilibration chamber through the gas permeable material until the partial pressures within the chamber achieve blood levels.

0003The carrier gas remains in the equilibration chamber for a specific period of time to insure equilibration is completed, at which time, the carrier gas containing that bolus of carrier gas with the equilibrated blood gases is removed and its content determined by an analyzer such as a gas chromatograph.

0004A vacuum means is used to transport the bolus of equilibrated gases to the analyzer through various valving means.

0005One of the difficulties with such present systems is that a certain finite time is needed for the sample gas to fully equilibrate with the blood gases and thus, the number of samples one can take within any specific period of time is limited. That time is partially dependant upon the ratio of area of the equilibration chamber that directly receives the blood gases through the permeable material to the volume of the equilibration chamber as well as other factors such as the gas permeability of the membrane. To be effective and rapid, the aforementioned ratio should be high, that is, there needs to be a large surface area through which the blood gases pass into the equilibration chamber in relation to the volume of carrier gas in the chamber.

0006The equilibration chamber itself is normally located at the distal end of the catheter and it comprises an active length of the catheter at that distal end. Present catheters have a relatively long active length in order to include a sufficiently large equilibration chamber and therefore a technician utilizing the catheter may be unable to pinpoint the exact location in the blood vessel where the blood gases are being analyzed. It is therefore desirable that the active length of the catheter be minimized.

0007A further disadvantage of prior art catheters is in the carrier gas passing through the catheter through dissimilar pathway areas, that is, an internal mixing takes place when the pathway of the gas though the catheter passes from one cross-sectional area to another that is significantly dissimilar in size. A mixing occurs between the equilibrated bolus and its edges that are surrounded by the carrier gas so that the defined edges of the bolus itself are disrupted.

0008According to one aspect of the present invention, a blood gas catheter probe having a distal end for introduction into a patient's blood stream is characterised by: an inlet for receiving a carrier gas; an outlet for removing carrier gas and blood gas contained therein from said catheter probe; an equilibration chamber at said distal end of said catheter probe forming a continuous gas flow path from said inlet and said outlet; a gas permeable membrane affixed to said catheter probe and surrounding said equilibration chamber, said membrane adapted to contact the patient's blood and allow equilibration of blood gases passing through said membrane into said carrier gas within said equilibration chamber; said equilibration chamber comprising an elongated tortuous path having a high ratio of surface area through which blood gases pass through said permeable membrane to unit volume of carrier gas in said equilibration chamber.

0009According to a further aspect of the present invention, a gas catheter probe having a distal end for introduction into a liquid containing gases, for analyzing gases in said liquid, is characterised by: a capillary inlet for delivering a carrier gas to said catheter probe said inlet terminating by a predetermined length short of the distal end of said catheter probe, a central conduit extending to at or near the distal end of said gas catheter probe and having formed therein, a capillary outlet for removing carrier gas and gas to be analyzed from said gas catheter probe, a wire wrapped spirally about the outside diameter of said central conduit beginning at the termination of said inlet to the end of said central conduit, a gas permeable membrane affixed to said catheter probe and sealed about said wire to form between said outside diameter of said control conduit and said gas permeable membrane, a spiral tortuous equillibration chamber, said gas permeable membrane comprised of a material allowing equilibration of gases passing there through and into said carrier gas within said equillibration chamber,

0010The catheter probe of the present invention includes an annular inlet capillary that receives the carrier gas from the gas source and forms a path for that carrier gas to the distal end of the probe. At the distal end, the carrier gas enters the equilibration chamber that is surrounded by a gas permeable membrane and which is in contact with the liquid containing the dissolved gases to be analyzed. In the preferred embodiment, that gas permeable membrane is introduced directly into the patient's bloodstream.

0011The equilibration chamber comprises the active length of the catheter probe and comprises an elongated, tortuous path through which the carrier gas passes. As will be noted, the elongated, tortuous path is preferably a spiral path formed directly under the inner surface of the permeable membrane so that the equilibration chamber volume is constrained to be a thin layer (or shell) at the outer surface of the probe. As the carrier gas winds its way along the spiral equilibration chamber, its outside surface area is in contact with the gas permeable membrane and is relatively large in respect to the volume of carrier gas passing through the equilibration chamber.

0012Thus, the ratio of active surface area per unit of equilibration volume is high, in the order of 50:1 so that the rapidity of the equilibration process is enhanced.

0013In addition, since the carrier gas passes through an elongated, tortuous path as it passes through the active length of the catheter probe, considerably more equilibration surface area is made available to carry out the equilibration process per unit of linear catheter length. The active length of the catheter probe may be thus reduced, thereby allowing the user to more accurately ascertain the position within the patient's blood stream that is being analyzed. The length of the tortuous spiral path is important since it delineates the length of the blood gas bolus which enters the centrally disposed capillary outlet tube, (the transport capillary).

0014This bolus will be eroded by convection and diffusion effects which cause mixing with the carrier gas during transport through the outlet tube. The mixing effect is evident initially at the leading and trailing edges of the gas bolus; a minimum length of bolus (1) for initial partial pressure information to be preserved within the bolus center can be calculated from the relation: 1 = (4/3 x a² x U x L/D)^1/2 where a = radius of the outlet tube; L = length of the outlet tube; U = carrier gas velocity; D = carrier/bolus coefficient of interdiffusion.

0015A further centrally disposed capillary outlet tube having an open end adjacent the distal end of the catheter probe receives the equilibrated carrier gas and through that capillary outlet tube, the equilibrated carrier gas bolus is withdrawn and directed to an analyzer.

0016As a further feature, the cross-sectional areas of the various paths through which the carrier gas passes as it progresses through the catheter probe are designed to be substantially equal, thus inadvertent mixing of the equilibrated carrier gas with the non-equilibrated carrier gas is minimized. More specifically, the cross-section area of the annular inlet capillary; the cross-section area of the elongated, tortuous path of the equilibration chamber and the cross-section area of the capillary outlet are substantially equal.

0017An embodiment of the invention will now be described, by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which:- <ul id="ul0001" list-style="none"><li>FIG. 1 is a schematic view showing the flow of carrier gas and its control for the system utilizing the present invention;</li><li>FIG. 2 is a cross-sectional view of the catheter probe of the invention showing, in particular, the active length thereof; and</li><li>FIG. 3 is an isometric view, partly cut away, showing the components of the catheter probe constructed in accordance with the present invention.</li></ul>

0018Referring first to FIG. 1, there is shown a schematic view of the overall gas analysis system for which the catheter probe of the present invention is useable.

0019In FIG. 1, catheter probe 10 is shown in schematic form, partially cut away, and having a distal end 12 that is inserted into the patient's blood stream to be analyzed. The catheter probe 10 comprises an annular capillary inlet 16 through which carrier gas is delivered to the catheter probe 10, as will be explained. An outlet capillary 18 having its open end 20 at or adjacent to the distal end 12 of the catheter probe 10 is provided and which withdraws carrier gas from the catheter probe 10.

0020A gas permeable membrane 22 makes up the exterior of the catheter probe 10 and, as will be explained, allows the blood gases to pass there through into the equilibration chamber 24, formed therein. As will be noted, the equilibration chamber 24 receives carrier gas from annular capillary inlet 16 and that carrier gas thereafter passes through the equilibration chamber 24 before entering the open end 20 of outlet capillary 18.

0021Equilibration chamber 24 is formed directly against the inner surface of gas permeable membrane 22 for reasons which will become apparent. A wire 25 is wound around the central conduit 26 in which outlet capillary 18 is formed. That part of the catheter probe 10 in which the equilibration chamber 24 is located is the active length and is indicated in FIG. 1 by the dimension A.

0022A valve 28 is used to control the introduction of gas to and remove gas from the catheter probe 10. A suitable valve for use with the present system is shown and described in U.S. Patent 4,706,700 of Jumeau and therefore is described herein schematically only.

0023Valve 28 comprises an outer housing 30 forming therein a main chamber 32. An inlet 34 is provided in outer housing 30 and receives carrier gas from a suitable source (not shown). The preferred carrier gas is argon, however, other carrier gases, such as helium, may be employed. As noted, the carrier gas is thus introduced into main chamber 32 though inlet 34.

0024The main chamber 32 also communicates directly with annular capillary inlet 16 as further illustrated by reference to arrows B; annular capillary inlet 16 being formed between the outer diameter of central conduit 26 and the inner diameter of tubing 33 and, as noted, forms a path for the carrier gas to the equilibration chamber 24.

0025Valve 28 further includes an inner chamber 36 within main chamber 32 and inner chamber 36 is formed by fixed walls 38 and a moveable wall 40. As shown in the solid line position of FIG. 1, moveable wall 40 is displaced with respect to fixed walls 38 so that inner chamber 36 is open and readily communicates with main chamber 32.

0026As also noted, central conduit 26 is sealed to fixed walls 38 so that outlet capillary 18 communicates with the interior of inner chamber 36. A further capillary conduit 42 also communicates with the interior of inner chamber 36 and forms a path to an outlet conduit 44 leading to the exterior of main chamber 32 and, as will be explained communicates with an analyzer such as a mass spectrometer for analyzing gases.

0027A solenoid 46 is electrically energizable and de-energizable to move moveable wall 40 between its dotted line position seated against fixed walls 38, and its solid line position displaced from fixed walls 38 thus selectively closing or opening inner chamber 36 with respect to main chamber 32. A bleed vent 48 is also formed in outer housing 30.

0028Briefly then, the operation of the system can be described as follows. When the solenoid 46 is suitably energized, the moveable wall 40 is moved to its dotted line position closing inner chamber 36. In this position, carrier gas is continually drawn through the system, including catheter probe 10, by a slight vacuum drawn by the mass spectrometer acting at the outlet conduit 44. That vacuum draws the carrier gas from its source into main chamber 32 of valve 28 and thereafter sequentially through annular capillary inlet 16, equilibration chamber 24, outlet capillary 18, through inner chamber 36 and out through capillary conduit 42 to outlet conduit 44.

0029When it is desired to utilize the catheter probe 10 to take a sample of blood gases, solenoid 46 is activated to move moveable wall 40 to its solid line position opening inner chamber 36 to main chamber 32. Since inner chamber 36 is now fully open to main chamber 32, the vacuum drawn by the mass spectrometer draws carrier gas directly through inlet 34 and through capillary conduit 42 to outlet conduit 44, thereby by-passing any flow to catheter probe 10.

0030The carrier gas thus contained within capillary probe 10 is retained at a standstill and equilibration occurs through blood gases passing from the blood through gas permeable membrane 22 and into carrier gas in the equilibration chamber 24. After a sufficient period of time has elapsed to insure equilibration has been completed, the solenoid 46 returns the moveable wall 40 to its dotted line position, closing inner chamber 36 and again causing flow through the catheter probe 10. This time however, that flow of carrier gas from catheter probe 10 contains a bolus of carrier gas into which the various blood gases have equilibrated from the patient's blood. That bolus continues to the mass spectrometer where it is detected and the blood gases analyzed.

0031Turning now to FIGS. 2 and 3, the details of the construction of the catheter probe 10 of the present invention can be explained.

0032Annular capillary inlet 16 is formed between the inner diameter of tubing 33 and the outer diameter of central conduit 26. Tubing 33 is preferably a commercially available polymer tubing, one example of which is suitable is crystaline polytrifluorochloro-ethylene available under the trademark Kel-F. The requirements of tubing 33 are that it provides some stiffening to the catheter probe 10 and yet is sufficiently flexible so as to be introducible along the interior of a patient's blood vessel. In the preferred embodiment, and to illustrate the size of the various components, tubing 33 has an outer diameter of about .250 millimeters (mm) and an inner diameter of about .160 (mm).

0033Central conduit 26 is preferably vitreous silica coated with several layers of a polyamide for mechanical strength and is commercially available from the Scientific Glass Engineering Co., of Australia. The material is utilized commercially for various fiber optic applications and is preferably of an outer diameter of about .150 mm, and an inner diameter of between .025 to .050 mm. The inner diameter of the central conduit 26 thus forms outlet capillary 18 and in the illustrative embodiment therefore has a cross-sectional area of about 2.0 x 10⁻³ mm². The annular capillary inlet 16 dimensionally therefore has an outside diameter of .160 mm and an inside diameter of about .152 mm and thus can be calculated to also have cross-sectional area of about 2.0 x 10⁻³ mm².

0034The wire 25 is used to form the elongated tortuous path of the equilibration chamber 24. Preferably that wire 25 is wound spirally around the outside diameter of central conduit 26 beginning at the terminal end 52 of tubing 33 and continuing to the open end 20 of central conduit 26 adjacent the distal end 12 of catheter probe 10. Wire 25 is preferably of molybdenum and has a coating that inhibits the condensation of water such as gold. In the dimensions of the preferred embodiment, wire 25 has a diameter of about .05 mm and its pitch is about .10 mm. In construction, the wire 25 is tension wound about and is embedded to some extent in the polyamide coating on central conduit 26. As such, the cross sectional area of the elongated, tortuous path of the equilibration chamber 24 is about 2.5 x 10⁻³ mm².

0035Around the outside of equilibration chamber 24 is secured the gas permeable membrane 22. The gas permeable membrane 22 may readily be made of silicone rubber having a thickness of about 50 microns.

0036As can now be seen, equilibration chamber 24 is, in effect, an elongated, tortuous path that leads from annular capillary inlet 16 to the outlet capillary 18 and along which the carrier gas is passed. Since the equilibration chamber 24 is in direct contact with the inner surface of gas permeable membrane 22, a considerable surface area is provided within the active length, dimension A, of the catheter probe. Thus the ratio of surface area to unit volume of carrier gas sample is relatively high; with the present given dimensions, that ratio is about 50:1. The active length dimension can thus be less than 5 centimeters, preferably less than about 2 centimeters and yet there is sufficient path length along the equilibration chamber 24 to fully effect the desired equilibration.

0037Also, in accordance with the preferred dimensions, the cross-sectional area of the annular capillary inlet 16, the elongated, tortuous path of the equilibration chamber 24, and the outlet capillary are substantially the same, accordingly the cross-sectional areas of the entire path for the carrier gas through the catheter probe 10 are substantially the same, thus mixing of the bolus containing the equilibrated sample and the carrier gas is reduced.