Apparatus and method for non-invasive measurement of oxygen saturation.

0001The present invention relates to an apparatus and a method for performing non-invasive measurements of oxygen saturation based on the reflection technique.

0002Oxygen saturation is a clinically very relevant parameter to assess the condition of a patient. Particularly in the operating room, the oxygen saturation of the blood gives an indication of the patient's condition, its supply with oxygen and other physiological factors.

0003One possibility to obtain a very precise value of the patient's oxygen saturation is to take a blood sample and analyze it in a blood gas analyzer. Despite the high precision of this method, it is an invasive technique and this means that it cannot performed frequently, i.e. does not allow continuous monitoring. Therefore, significant changes in the oxygen saturation value may be missed. Last not least, it is understood that an invasive technique is not the preferred way to monitor a patient.

0004It is therefore highly desirable to measure oxygen saturation non-invasively. This can be achieved by a technique called oximetry.

0005An oximeter usually comprises two or more light sources of different wave length. The light is irradiated on human flesh, and either the intensity of the light transmitted through the flesh, or the intensity of the reflected light is measured. In more general terms, "light" does not only mean electromagnetic waves in the visible spectrum. For example, the most common oximeters use one wavelength in the visible spectrum and another wavelength in the infrared spectrum.

0006The light transmitted or reflected by human flesh is attenuated according to Lambert-Beer's law, i.e. <maths id="math0001"><math display="inline"><mrow><msup><mrow><mtext>I = I₀e</mtext></mrow><mrow><mtext>-Ecl</mtext></mrow></msup><mtext>   (1)</mtext></mrow></math><img file="EP0613652A2_D0001.tif" /></maths> wherein I represents the intensity of the received light, I₀ the intensity of the incident light, E the molecular extinction coefficient, c the concentration of the absorbing species and l the thickness of the absorbing layer, i.e. the length of the light path through tissue.

0007If we assume that human tissue contains components with constant characteristics over time - such as bones, muscles etc. -, and components with time-varying characteristics - i.e. blood -, equation (1) yields<maths id="math0002"><img file="EP0613652A2_D0002.tif" /></maths> wherein I_DC represents a component constant over time (caused by absorbers of constant absorbance), I_AC represents the time-varying component (caused by blood) and l(t) represents the variation in thickness caused by arterial pulsations. This equation contains two unknown values, c and l(t).

0008As oxygen saturation is defined as the quotient of the concentration of oxyhemoglobin in relation to the concentration of total hemoglobin, i.e.<maths id="math0003"><img file="EP0613652A2_D0003.tif" /></maths> (wherein C_HbO2 is the concentration of oxyhemoglobin and C_Hb is the concentration of reduced hemoglobin), two absorbing species have to be taken into consideration. In this case, equation (2) becomes<maths id="math0004"><img file="EP0613652A2_D0004.tif" /></maths> with E_Hb = the molecular extinction coefficient of reduced hemoglobin and E_HbO2 = the molecular extinction coefficient of oxyhemoglobin. It can be easily seen that, in this case, we have three unknown values, C_Hb, C_HbO2 and l(t).

0009A measurement at two different wavelengths allows to determine oxygen saturation. In fact, measurement at two wavelengths results in two equations for three unknown values, but due to the definition of oxygen saturation one of these values cancels out.

0010As the oxygen saturation value depends also on empirical constants, the two equations obtained from equation (4) (for both wavelengths) are used to calculate a ratio R which, in turn, is used to calculate SaO₂.

0011For more details on the theory of oxygen saturation measurement, reference is made to former publications on this subject, e.g. US 4,167,331 or EP-A-262 778 (the latter patent application contains a quite complete breakdown of the theory).

0012Up to now, most oximeters on the market use the transmission technique, i.e. measure the intensity of the light transmitted through a human organ like the earlobe or the finger. Although the obtained measurement results still are not comparable with the precision of blood sample analysis, this technique even yields better results than the reflection technique. Regarding reflection oximeters, reference is made to EP-A-135 840 which depicts a reflection sensor for the fetus. A further reflection sensor is described in EP-A-329 196. The reflection technique has also been used to measure other physical quantities such as metabolism, see e.g. US 4,223,680.

0013In fact, monitoring a fetus prior to and under birth is an application requiring the reflection technique because it is not possible to place a transmitter on one side of the fetal scalp and a receiver on the other side (besides that, the results would be unusable as the tissue absorbs most of the incident light on a length of approximately 10 cm). The same is true for an adult patient in shock; due to the effect of centralization of the blood, meaningful results can in this case only be obtained from a transducer applied to the trunk (i.e. a reflection transducer), and not from a transducer applied to a limb. Still the results of the reflection technique are more than modest, and no clinically usable reflection sensor has been developed up to now.

0014It is therefore a major objective of the present invention to provide a sensor and an apparatus for measuring the oxygen saturation based on the reflection technique with a precision sufficient for clinical application. It is also an objective of this invention to provide a method therefore.

0015A significant improvement in signal quality may be attained in that the place of application of the sensor is detected and in that the measured values are appropriately corrected. The invention proposes to use sets of predefined signals representative of certain places of application at the human body or representative of certain tissue characteristics and selecting - based on the above mentioned criteria such as amplitude or shape - the set incorporating the most common characteristics with the received signals, and then to correct these signals in dependency of the selected set. The sets of predefined signals may either represent attenuation values of one or more wavelengths, amplitudes of the alternating components or signal shapes. Such detection of the place of application or of a certain tissue characteristic, and appropriate correction, yields considerably better oximeter readings. It has to be pointed out that detection of tissue characteristics and appropriate correction is even useful for a sensor which is - by means of its geometry - adapted for use at a specific human limb; the reason is that there may be differences in tissue characteristics from patient to patient, even if the selected place of application is the same, e.g. caused by the age of the patient or its condition (patients in shock show significant differences, compared to other patients; furthermore, only reflection oximetry is possible in such case as a sensor applied to an arm, a finger or a leg would - due to lack of pulsating blood - not yield meaningful readings).

0016The invention also relates to an apparatus for operating a sensor with more than one light path of a certain wavelength. Such apparatus may comprise conversion means converting the intensity of the received electromagnetic waves into electrical signals, processing means for the calculation of oxygen saturation from said electrical signals, selection input means for selecting the place of application at the human body, and transmitter/receiver selection means responsive to said selection input means and set up to select certain transmitter(s) and/or certain receiver(s) depending on the place of application. When such an apparatus is used in conjunction with one of the aforementioned sensors, the operator may enter the actual place of application, e.g. the chest, the fetal scalp and so on, using the selection input means, and said transmitter/receiver selection means select then the appropriate transmitters for this application. Furthermore, other physiological parameters may be entered such as sex, patient's age or its condition. This allows very precise adaptation to the conditions of measurement by selecting the appropriate transmitters/receivers and therefore of the light paths. To ensure the high precision of such oximeter readings, plausibility checking means may be further provided which compare the received signals with predefined limits or predefined signals representative of the selected place of application and generating an alarm or warning message upon a significant deviation. It can thereby be avoided that errors caused by the medical personnel have any negative impact on oximeter readings.

0017Instead of manual entry of the place of application or other physiologically important values, an advantageous solution proposes to use an apparatus comprising conversion means converting the intensity of the received electromagnetic waves into electrical signals, processing means for the calculation of oxygen saturation from said electrical signals, and application detection means set up to select sequentially certain transmitter(s) and/or certain receiver(s). For each selection of a certain transmitter and a certain receiver, the obtained electrical signals specific to this selection are compared with sets of predefined signals representative of certain places of application at the human body or representative of certain tissue characteristics. The specific electrical signal comprising most common characteristics (amplitude, shape, ...) with a certain of these sets is then selected, and as this specific electrical signal is associated with a certain transmitter and/or receiver, these are selected by selection means for further measurement. Such an automatic adaptation system may be used for all kinds of applications, patients and tissue characteristics; it always selects the best possible transmitter/receiver combination for a particular kind of measurement. The only limitation is the number of stored sets of predefined signals.

0018Preferred embodiments of the invention will now be described - by way of a non-limiting example - with reference to the accompanying drawings in which: <dl id="dl0001"><dt>Fig. 1</dt><dd>depicts a cross-section through a simple reflection sensor,</dd><dt>Fig. 2</dt><dd>is a block diagram of an oximeter,</dd><dt>Figs. 3a and 3b</dt><dd>illustrate an embodiment of a reflection sensor which is designed for adaptance to different applications or tissue characteristics,</dd><dt>Fig. 4</dt><dd>depicts the basic arrangement of a self-adapting reflection sensor,</dd><dt>Fig. 5</dt><dd>shows a cross-section through a fetal sensor, and</dd><dt>Fig. 6</dt><dd>is the view of this sensor in the direction of arrow X of Fig. 5.</dd></dl>

0019Fig. 1 depicts a cross-section through an oximetry sensor of the reflectance type generally outlined as 1. This sensor comprises a carrier body 2 which is applied to a part of the human body generally designated as 3. Human body part 3 may be the chest, the stomach, a leg etc. of a patient. For centralized patients (i.e. patients in shock), the extremities such as arms or legs cannot be used due to a lack of pulsating blood. This is a particular application where reflection oximetry is required; as transillumination through the chest or the stomach is not possible, only a reflection sensor can be applied.

0020Although the carrier body 2 is depicted as a one-component part here, it is understood that the carrier means may also consist of several connected or non-connected components. Carrier body 2 may be fixed to the human body in any convenient manner, e.g. using a surgical glue or a self-adhesive tape (not shown here).

0021Carrier body 2 has three bores 4, 5 and 6. Bores 4 and 6 hold light-emitting diodes (LEDs) 7 and 9, whereas bore 5 houses a photoelectric receiver 8, e.g. a phototransistor or a photodiode. The electrical connection of the wires leading to these LEDs and the photoelectric receiver are not shown in detail here. These connections can be made in usual manner, e.g. they may lead to a connection cable for connection with an appropriate oximeter.

0022LEDs 7 and 9 emit light of different wavelengths into the human flesh. For example, LED 7 may emit light of a wavelength of 650 nms (red), whereas LED 9 may emit light of a wavelength of 940 nms (infrared). Light emitted by these LEDs is transmitted into the human tissue and reflected as indicated by light paths 10, 11 (red) and 12, 13 (infrared). The reflected light is received by photoelectric receiver 8 which is sensitive to both wavelengths.

0023As illustrated in Fig. 1, the depth of penetration into human tissue is different between the two wavelengths, i.e. depends on the wavelength. According to the invention, transmitter LEDs 7 and 9 are <u>not</u> arranged symmetrically with respect to photoelectric receiver 8. Instead, the distances d₁ (distance between LED 7 and receiver 8) and d₂ (distance between LED 9 and receiver 8) are not equal.

0024Distances d₁ and d₂ are selected such that the length of the red light path (l₁ + l₂, i.e. the length of light ways 10 and 11) is equal to the infrared light path (l₃ + l₄, i.e. the length of light ways 12 and 13). As infrared light incorporates a smaller extinction coefficient, i.e. less absorbance and therefore a deeper penetration into human tissue, the infrared LED 9 is placed closer to photoelectric receiver 8 than red LED 7.

0025By means of the selection <maths id="math0005"><math display="inline"><mrow><mtext>l₁ + l₂ = l₃ + l₄</mtext></mrow></math><img file="EP0613652A2_D0005.tif" /></maths> , the ways of the light at different wavelengths through human tissues are made equal. By this measure, it is ensured that the assumptions of the theory are met and leads to reliable oxygen saturation readings. The oximeter (not shown here) to which sensor 1 is connected measures the intensity, i.e. converts the received signal into an analog voltage and/or a digital value and uses the intensities, particularly their AC and DC components, to calculate oxygen saturation as described above.

0026If photoelectric receiver 8 is sensitive to both wavelengths, the sensor must be operated in pulsed mode so that at any point in time only one of the light waves is received. It is also possible to use a combined receiver selective to either of the used wavelengths instead; in this case, the transmitter LEDs may be operated continuously.

0027Distances d₁ and d₂ are selected such that the condition <maths id="math0006"><math display="inline"><mrow><mtext>l₁ + l₂ = l₃ + l₄</mtext></mrow></math><img file="EP0613652A2_D0006.tif" /></maths> is met at the place of application for which the sensor is designed. Instead, it is also possible to use average tissue characteristics - i.e. extinction coefficients - to determine d₁ and d₂; this is particularly useful for a multi-purpose sensor which may be applied to various parts of the human body, e.g. chest, stomach, leg, arm etc.

0028In the shown example - where the surface of the sensor is relatively flat - identical length of the light paths results in different distances d₁ and d₂ between the various transmitter LEDs 7, 9 and the photoelectric receiver 8. It has to be emphasized that such is not mandatory; in case of a sensor which is very complex in geometry, it might also happen that the distances between the transmitter LEDs and the photoelectric receiver are equal. Such a sensor is also covered by the scope of the present invention as long as the light paths at the various wavelengths are substantially equal. Furthermore, there may be additional transmitter LEDs which are used to measure further body parameters; it is understood that, for these additional LEDs, the condition of equal length of light paths must not necessarily be met.

0029Fig. 2 depicts the basic structure of an oximeter, i.e. an apparatus for operating one of the above described sensors. The sensor connected with the oximeter is generally outlined as 43. The signal generated by the photoelectric receiver is fed by a line 44 to a current/voltage conversion unit 45 and then to an analog-to-digital converter 46 which generates a digital representation of the measured intensity. This digital value is then fed to a microprocessor 47 operating under control of a program memory 48. This program memory contains all necessary code for the processor to calculate oxygen saturation. It also contains program code destined to perform a plausibility check on the received and converted signals. Particularly, said program code may compare the received signals with predefined limits or predefined signals and cause the processor to generate an alarm or a warning message to the user if such limits are exceeded or the predefined signals not met. Such is an indication that either the transducer is applied to the wrong place or that it is not attached securely.

0030The program code contained in memory 48 may further contain instructions to the processor 47 to compare the received and converted signals with sets of predefined signals each being representative of a certain place of application at the human body or of certain tissue characteristics. Such comparison may e.g. be performed by cross-correlation. It is also possible to perform such comparison on the basis of attenuation values or amplitudes. The program code then instructs the processor to select those of the sets of predefined signals comprising most common characteristics with the received signal.

0031There may be several reasons for such a check. In case of a sensor which is designed specifically for application to a certain human limb, such check can reveal whether the sensor is applied correctly and in good contact with the human skin. Further, the oximeter can perform self-adaptation to a certain tissue structure (there may be variations in the flesh of e.g. a finger, even from patient to patient). Last not least, in case of a general purpose sensor the place of application may be detected.

0032The program code therefore instructs processor 47 to correct the received signal in dependency of the selected set which yields considerably better oxygen saturation readings.

0033Reference is now made to Fig. 3a. Reflection sensor 49 shown in this figure comprises a carrier body 50 with bores 51, 52, 53 and 54. These bores house transmitter LEDs 55, 56 and 58 as well as a photoelectric receiver 57.

0034Transmitter LEDs 55 and 56 emit light of the same wavelength, whereas transmitter LED 58 transmits light of another wavelength. For example, transmitter LEDs 55 and 56 may emit electromagnetic waves in the red spectrum and transmit LED 58 in the infrared spectrum. Photoelectric receiver 57 is sensitive to both wavelengths.

0035Such a sensor may be adapted to different places of application or different tissue characteristics. In the example of Fig. 3a, the distance between transmitter LED 55 and photoelectric receiver 57 is selected such that the total length of light path 60, 61 is equal to the total length of light path 62, 63 in a certain tissue - e.g. the chest - 59. The other red transmitter LED 56 is positioned such that the total length of light path 64, 65 is equal to the total length of light path 66, 67 in another kind of tissue - e.g. the scalp - 68. This is illustrated in Fig. 3b.

0036It can be readily seen that the depth of penetration, and therefore also the length of the light paths, varies for each wavelength with the kind of tissue. Nevertheless, with the design depicted in Figs. 3a and 3b, it is possible to adapt the sensor to the specific application. Adaptation may be performed for two different purposes: first, a general-purpose sensor may be adapted to a certain place of application, and/or second, a distinction can be made between different tissue structures, even if the place of application is the same (e.g. the chest), but the characteristics of the tissue varies from patient to patient.

0037The selection among transmitter LEDs 55 and 56 may be performed by the oximeter in different ways. First, there may be provided selection input means for selecting the place of application at the human body, i.e. the user may enter the place of application (and/or other characteristics of the measurement, e.g. the age of the patient). The oximeter comprises transmitter selection means responsive to the selection input means and selects the appropriate transmitter. Furthermore, the oximeter may comprise plausibility checking means for comparison of the received signals with predefined limits or predefined signals representative of the selected place of application. If a considerable deviation is detected, an alarm or a warning message may be generated as, in such case, it is likely that the user has entered the wrong place of application.

0038Alternatively, the oximeter may also provide automatic selection of those transmitters which are best suited for a certain application or tissue. Such oximeter selects sequentially all of the available transmitters and/or receivers, i.e. tests all possible light paths. The signals received in any case are compared with sets of predefined signals which are representative of certain places of application at the human body or of certain tissue characteristics. Such comparison may be e.g. performed by cross-correlation. For each transmitter/receiver combination, i.e. for each light path, the degree of correspondence (e.g. the maximum of the cross-correlation function) is stored. When all tests have been performed, the transmitter/receiver combination with the highest degree of correspondence is selected, and further measurements are performed with the associated transmitter/receiver.

0039It is understood that the above principles for sensor construction and oximeter operation may also be "reversed" in the sense that not multiple transmitter LEDs of the same wavelength are provided (like LEDs 55, 56 in Figs. 3a and 3b), but a multiplicity of receivers instead. A further possibility is to provide a multiplicity of transmitter LEDs of one wavelength and a further multiplicity of transmitter LEDs of a second wavelength.

0040Such a sensor is depicted in Fig. 4. Carrier body 69 comprises red LEDs 70a to 70d and 71a to 71d as well as infrared LEDs 72a to 72d and 73a to 73d. The photoelectric receiver is outlined as 74. Such a sensor may be used, in conjunction with an appropriate oximeter, for very precise manual or automatic adaptation to different kinds of measurement by selecting an appropriate red LED/infrared LED combination.

0041A fetal sensor for appliance to the fetal skin and embodying this principle is depicted in Figs. 5 and 6. Sensor case 75 is connected with an oximeter (not shown here) via cable 76. A suction tube 77 is connected with a nozzle 78 which provides connection with a suction ring 79. A suction chamber 80 is in contact with the fetal skin and holds the sensor to it if underpressure is applied to the chamber.

0042Centrally arranged in the sensor are two infrared LEDs 81a and 81b, two red LEDs 82a and 82b as well as a photoelectric receiver 83. Due to the different distances between the LEDs of each wavelength and the photoelectric receiver, adaptation to different tissue characteristics may be performed (as outlined above), either manually or on an automatic basis. The sensor shown in Figs. 5 and 6 may also comprise electrocardiogram contacts (although not shown here).