DE3629447A1 - Method and device for oxymetry - Google Patents

Abstract

The oxygen saturation of the blood of a patient is determined by guiding pulses of red and infrared radiation from radiation sources through a body part of the patient. A detector determines the intensity of the radiation which has passed through the body part, and delivers an output signal which indicates this intensity and is filtered and split into two signals, specifically into a red channel and an infrared channel, by means of sample-and-hold circuits which are synchronised with the pulses for the red and infrared light sources. The outputs of the sample-and-hold circuits pass through low-pass filters which deliver output signals which specify the intensity of the red light and infrared light radiation picked up by the detector. In addition, the red and infrared signals are led through high pass filters in order to produce signals which contain only the time-variable component in the emitted intensities. These signals are fed to a digital control circuit which determines the non-temporally varying values of the red and infrared signals, determines the peak-to-peak values of the temporally varying components of the signals and forms a ratio of the peak-to-peak amplitudes to the non-temporally varying values of the red and infrared signals. This ratio can then be used for the purpose of determining the oxygen saturation as a function of the ratio.

Description

The invention relates generally to the field of devices for measuring the oxygen content of blood and specifically for external use Devices that measure blood oxygen levels the basis of light transmission Measure body part.

It has long been known that the intensity of Light coming through a relatively thin body part, like for example a finger or an earlobe is allowed by the oxygen saturation of the Blood is affected. The intensity of the by the Body of transmitted light changes with that Oxygen content of the blood, as it is enriched with oxygen Hemoglobin (oxyhemoglobin) a different color exhibits as hemoglobin to which no oxygen is bound is. Because the light absorption of the two aforementioned Substances hemoglobin and oxyhemoglobin at one individual wavelength deviates from each other, the relative concentration of each component in the Blood determined from the relative absorption of light  that goes through the blood in unison with the law of "Beers", see e.g. B. "Real Time Oximetry" by Yelderman and Corenman in "Computing in Anesthesia and Intensive Care ", 1983, Martinus Nighoff Publishers.

The law of "Beers" relates to light absorption of a substance on its thickness and its optical properties as an exponentially decreasing function the molecular extinction coefficient, the concentration and thickness of the substance. Generally speaking assumed that blood consists of cells that are only two Contain ingredients, namely oxyhemoglobin and Hemoglobin, and that oxygen saturation as that Ratio of concentration of oxyhemoglobin to Total concentration of oxyhemoglobin and hemoglobin is defined. In fact, body tissues contain through which light is passed, not just mixtures from hemoglobin and oxyhemoglobin, but also venous blood, bones and solid tissues as well the red blood cell tissue. It also contains blood often hemoglobin attached to molecules other than Oxygen is bound, for example carboxyhemoglobin and cyanomethemoglobin.

There are already various oximetry devices been proposed trying to reduce the hemoglobin and arterial blood oxyhemoglobin content of separate other factors by translucency changes at two different wavelengths be measured when the blood pulsates. Such Devices generally have to Exponential equations of the "Beers" law  each wavelength solve the oxygen saturation to determine, using logarithmic amplifiers, computer supported logarithmic transformations or complex signal processing need to the need for logarithmic transformations to avoid. Such known devices need complex circuits, in particular essentially analog signal processing devices, and generally require substantial Time to receive and process data with which the saturation is determined. Also request the known devices have a significant blood volume set, the during an impulse to exact Get results whatever the uses such devices in patients who have a have poor circulation, for example under Shocked patients. Accordingly, there is a desire for a quick, simple, reliable and inexpensive, compact, portable yet accurate device for measuring oxygen saturation in a clinical environment.

In accordance with the present invention are saturation values using a selected linear approximation to the law of "Beers" obtained so that you get the solution for saturation as a linear function of the measured transmitted Light intensity instead of an exponential one or non-linear function. By suitable choice of approximation can the invention in a microprocessor control device, that both in terms of hardware and  Software versus currently available oximeters is significantly simplified, responds faster and is still highly accurate.

In the device according to the invention there are two light sources different wavelengths, e.g. B. Red and Infrared, alternately pulsed. The light from the Light sources are through a body part, for example a finger, and is guided by a detector recorded and in a time-varying tension converted, which can be filtered to signal overlays to eliminate by ambient light. The Detector output signal is divided into two separate signals divided according to the changing time Intensity of radiation from the detector at each which has received two wavelengths. The Separation of the detector signal can be done in this way achieve that two separate channels are provided, from each of which has a sample and hold circuit which the output signal at the time of the associated pulse scanned from the source, and still a low pass filter contains to high frequencies from the output the sample and hold circuit and any high frequency Eliminate environmental disturbances. The exit of the Low pass filter is a time varying signal with an average that represents the average level of the Radiation intensity at the selected wavelength corresponds to that which has passed through the body part. The Output of the low-pass filter is still a High pass filter passed that the mean component suppressed and the temporally changing component amplified so that the time-varying component of the signal that remains the fluctuations  corresponds to the transmitted light intensity, that occur during the blood pulse. The four signals from the two channels are sent to a control device fed, for example a microprocessor system, saturation as a function of relationships the values that do not change over time for each channel and the peak-to-peak values of the to determine the time-varying proportion of each channel. By determining saturation in this way there will be no errors in the measurement result of differences in the absolute values of that of different ones Individuals of different skin color and Radiation transmitted by the thickness of a finger could result.

To continue fluctuations in measurement results as a result of changes in light transmission between Minimizing individuals becomes that of radiation sources supplied energy regulated in such a way that the average intensity, that from the detector of everyone Source is received, within a desired range lies. The intensity setting will executed directly in a linear fashion, which is very run quickly when the investigation begins becomes.

The probe is preferably two-emissive Provide sources for each wavelength that are symmetrical are arranged around a center point, around a transmission path to generate by the patient who for Light of both wavelengths is essentially the same is. The sources can be convex with an increased Be covered lens, which on the one hand serve light  focus from the sources, and on the other hand serves to press on the body tissue to this way to favor that light intensity reaches the detector and the intrusion of ambient light is reduced. The higher concentration of Light from the sources and strain on the tissues allows reliable signals from the detector too preserved, even at very low blood flow levels. Because the detector signal is reliable and a relatively high signal / interference ratio at low Has blood flow, the time-varying component of the detector signal are amplified so far until it falls into a desired area. Because the time varying components of the signals in both Wavelengths will be amplified to the same extent the saturation value, which is a function of the ratio of these time-varying components is not affected by changes in gain.

As a result of the great simplifications in hardware, which are required to determine the required variables and the simplified software for that Microprocessor system, the device can be relatively inexpensive are reliable and compact Construction and also allows battery operation, so that it can be made portable.

Other objects, features and advantages of the invention go from the following detailed description below Reference to the drawings. It shows:

Figure 1 is a block diagram of the essential functional units of the oximeter according to the present invention.

Fig. 2 is a circuit diagram of the analog circuit components of the invention;

Fig. 3 is a circuit diagram of the digital control and processing system;

FIGS. 4 to 9 are flow charts of the operation steps in the program of the digital control system;

FIG. 10 is a graphical representation of the functional relationship between the ratio R and the oxygen saturation S;

Figure 11 is a plan view of the light source in the probe. and

FIG. 12 shows a cross section through the probe along the line 12-12 of FIG. 11.

The attenuation of electromagnetic radiation, e.g. B. red and infrared light, when passing through a substance depends on the thickness of the substance and their optical properties according to the law from Beers and can be expressed as:

where I is the intensity of the transmitted light, I _{0 is} the intensity of the incident light, E is the molecular extinction coefficient of the substance, C is the concentration of the substance and d is the thickness of the substance.

A part of the body, for example a finger, in which Blood flowing in pulses can be seen as a model which consists of two separate parts that the light falls one by one. The first part represents the absorption of light due to time-constant substances, such as Bones, skin, venous blood, etc. The second part represents the time-varying substance, namely the pulsating arterial blood in the finger. The total absorption by the two separate parts that is Product of the exponential absorption expressions for the two parts, or

where A ( t ) is the time-varying quantity, which is a function of the extinction coefficient of the blood (constant), the concentration of the blood (constant) and the thickness of the blood (time-varying) and B is a constant which is the non-time-varying Absorption represents.

If the non-temporally varying component of the intensity

then the time-varying intensity I ( t ) can be defined as follows:

or

Since blood pulsates through the finger or through the other part of the body through which the light is directed, the function A ( t ) has a fairly constant peak-to-peak value. If one defines Ap as a half of the peak-to-peak value of A ( t ), then the peak-to-peak value I _{ac of} the time-varying components I ( t ) is the difference in the value of I ( t ) at Ap and - Ap , which leads to the equation:

The ratio of I _ac to I _dc is therefore as follows:

Let the exponential functions of equation (7) develop as a series of factors, and if so the first or linear term of this series expansion is used as an approximation, then the ratio:

This linear approximation to the exponential leads to a relatively small deviation from the real exponential for ratios that are likely to be encountered. For example, for the ratio I _ac / I _dc = 0.25, the percentage error is approximately 1%. This means that the time-varying component of the light intensity can be up to 25% of the total transmitted light before the approximation error reaches 1%. In current practice, the ratio is usually between 5% and 20%, resulting in an error that is much less than 1%. In the manner to be described below, the error can be reduced to 0.2% or less above the normal measurement range because the final saturation value is calculated by dividing the approximate ratios for two light wavelengths, so that an error trend tends to be in proportion to reverse the error trend in the other ratio. In addition, since the ratios of the time-varying and constant intensities are used, any constant factor that could influence the light transmission, such as bone or muscle thickness or skin color, is automatically deleted when the ratios are formed and evaluated.

The size 2 Ap is the peak-to-peak value of the time-variable absorption A ( t ). Defining D _P as the total peak-to-peak change in blood thickness gives 2 Ap = ECD _p , where E is the quenching coefficient and C is the concentration.

Oxygen saturation S in the blood is defined as the ratio of the concentration C _O of oxygenated hemoglobin (oxyhemoglobin) to the total of the concentration of oxyhemoglobin and the concentration C _H of hemoglobin, ie S = 100 C _O / ( C _O + C _H ) . To determine the oxygen saturation, the absorption properties are measured at two different light wavelengths, usually in the red and in the infrared range. The oxyhemoglobin and hemoglobin can be assumed to be uniformly mixed in the thickness D _{p of} the blood, which leads to the following equations for the red and infrared ratios.

And the ratio R of the infrared ratio to the red ratio is as follows: where in the indices I stand for the infrared sizes, R for the red sizes, O for oxyhemoglobin and H for hemoglobin.

Using the expression for oxygen saturation S in the above equation gives:

This expression can be resolved after saturation as follows:

The preceding expression gives a very good approximation of oxygen saturation, where all time-varying components result from changes in hemoglobin and oxyhemoglobin. However, other factors can contribute to the pulsating signals, for example hemoglobin combined with compounds other than oxygen, for example carboxyhemoglobin or cyanomethemoglobin, but also pulsations in the capillaries which cause a small error since the blood in the capillaries has partially decreased. To account for these error terms, the ratio of infrared to red sizes is redefined by introducing a substance labeled X to account for possible errors, as follows:

Furthermore, the percentage of oxygen saturation is redefined as S = 100 C _O / ( C _O + C _H + C _X ), where C _{X is} the concentration of substance X. If the equation for oxygen saturation S is solved for C _H and inserted into the equation for the ratio, then the following expression for the ratio R can be derived:

The preceding equation shows that the quantity S / C _O only depends on the sum of the concentration of the three substances. Assuming that a change in oxygen saturation alone shifts the blood concentration between C _O and C _H and the concentration C _X of substance X is constant, then the sum of the three concentrations is constant. The last terms of the above equation can then be assumed to be constants K _I and K _R , so that the equation can be rewritten as follows:

If you solve the above equation after saturation S , you get:

The values K _R and K _I can be quickly determined empirically by comparing the saturation values resulting from the above equations with saturation values determined using direct measurements of the oxygen content of blood as a standard. For example, approximate values K _R = 5,400 and K _I = 1,200 have shown that they give essentially accurate results with 1% or less deviation from the standard measurements. More generally, the saturation can be determined in accordance with the following function: where K ₁ , K ₂ , K ₃ and K _{4 are} constants which can be determined empirically by adapting equation (18) to a saturation curve which has been determined by directly measuring the oxygen content of blood. Alternatively, the value of S as a function of R can be determined empirically for a number of data points, the functional relationships being stored in a look-up table.

A block circuit of an oximeter that determines saturation in accordance with the previous steps is generally indicated at 20 in FIG. 1. The device 20 contains a probe 21 which is specially designed for clamping onto a finger and for this purpose has a fixed support body 22 and a movable clamping part 23 which is mounted on the support body by means of a joint 24 . The clamping part 23 is spring-loaded to press against the support body 22 and includes a first radiation source 25 , e.g. B. a light emitting diode that emits visible red light and a second source 26 , z. B. a light emitting diode that emits infrared light. The light that passes through the finger is detected by a detector 28 , e.g. B. a photodiode, which provides an electrical output signal on line 29 which is a function of the intensity of red or infrared light received by the detector. The detector can be mounted in a plastic foam cushion (not shown) so that the finger and the cushion are compressed around the detector to block out ambient light. An ambient light filter 30 is preferably installed in the device and serves to suppress the transmission of any signal resulting from ambient light which has been picked up by the detector 28 . This environmental filter can also serve to convert the current output signal of the photodiode detector 28 into a voltage.

The output of the ambient filter 30 has a first channel 31 for the red signal component and a second channel 32 for the infrared signal component. The signal from the ambient filter 30 in the red channel is fed to a first sample and hold circuit 34 , which samples the output of the ambient filter during the time in which the red pulse is emitted by the source 25 and holds this value until the next red pulse is delivered. A low pass filter 35 receives the output of the sample and hold circuit and filters out the high frequency components that have been introduced into the signal by the sample and hold circuit as well as any other pre-frequency interference. The output of the low-pass filter 35 is fed to a system controller and processor 38 on a line 36 . The voltage on line 36 corresponds to a reconstructed time-varying voltage that indicates the intensity of the red light that passes through the finger during a time when the blood in the finger is pulsing. This voltage signal V _rdc contains both an average _{signal amplitude} and a superimposed, time-changing component. The output of the low-pass filter 35 is also fed to a high-pass filter 39 , which filters out the unchangeable or direct current component from the red voltage signal and outputs an output signal on a line 40 to the control system 38 , which is a voltage V _rac , which is the time-varying component of the red intensity signal, which has been recorded by the detector 28 .

Analogously, the signal on line 32 in the infrared channel is sampled by a sample and hold circuit 42 at those times when the infrared source 26 is turned on, and the sample and hold circuit 42 holds the sampled value until the next pulse. The output of the sample and hold circuit 42 is fed to a low-pass filter 43 , which filters out the high-frequency components which have been introduced into the signal by the sample and hold circuit 42 , as well as all other noise. The output voltage V _irdc from the low-pass filter 43 contains both temporally variable and temporally unchangeable portions of the output of the detector 28 for infrared radiation and is fed directly on a line 44 to the control circuit 38 and further to a high-pass filter 45 , which thus filters the same component out of the signal only the time-varying portion of the voltage V _{irac remains} on the line 46 to be supplied to the control circuit 38 .

The control _circuit calculates the non-time varying values of the voltages V _rdc and V _idc and uses these values as the DC _{intensity analog} values I _dci and I _dcr . It has been shown that the peak values of V _rdc and V _idc can be conveniently used as non-time-varying intensity _{analog values} I _dci and I _dcr , because the saturation is calculated using a ratio of these quantities. Of course, the current averages of V _rdc and V _{idc could be used} as non-time varying intensity _analog values. The peak-to-peak values of the time-varying voltages V _rac and V _iac are used by the control _circuit 38 to determine the peak-to-peak intensity _{analog values} I _aci and I _acr . The control circuit then determines saturation as a function of the ratio

and outputs the saturation value for display on the front panel 48 to a user. The control circuit 38 also controls the operation of the system, including the timing for the pulsed operation of the sources 25 and 26 , which are operated, for example, by LED current amplifiers 49 for red and 50 for infrared, and controls the red sample and hold circuit 34 and the infrared accordingly. Sample and hold circuit 42 to suitably match the operation thereof to the pulse operation of LEDs 25 and 26 .

An electrical circuit diagram of the analog signal processing components of the oximeter 20 is shown in FIG. 2. The output signal from the photodiode 28 , a current which emits the intensity of the incident light radiation, is fed on line 29 to the input of an operational amplifier 51 within the ambient filter section 30 , which converts the current signal into a voltage signal at its output. The voltage output signal from the operational amplifier 51 is fed via a capacitor 52 to the input of an operational amplifier 53 , which amplifies the signal and makes it available to the red channel line 31 and the infrared channel line 32 . An operational amplifier 55 with a diode 56 in a feedback loop to its negative input is connected to the amplifier 53 and clamps the baseline input voltage to the amplifier 53 at substantially zero volts, thereby interacting with the capacitor 52 to remove any signal component due to ambient light block the detector.

The pulse signal from amplifier 53 is fed to red channel line 31 , which is connected to a controllable amplifier 58 within sample and hold circuit 34 . The amplifier 58 receives a sample control pulse from the system control circuit 38 on a line 59 through a resistor 60 and a transistor 61 . When a pulse on signal appears on line 59 , amplifier 58 is activated to pick up the voltage signal level on line 31 and provide a voltage level on its output line 62 that is proportional to the input voltage. This voltage is held on line 59 until the next control pulse signal. In the same way, the output signal on the infrared channel line 32 is fed to a controllable amplifier 64 , which receives a control pulse signal on a line 65 via a resistor 66 and a transistor 67 . The output of amplifier 64 on line 68 is a voltage proportional to the input at the time of the control pulse and this voltage is held until the next pulse.

The output on line 62 is fed via resistors 70 and 71 to the non-inverting input of an operational amplifier 72 , which has a feedback capacitor 73 and a grounded input capacitor 74 and is part of the low-pass filter 35 and is a two-pole filter, which is preferably a corner frequency of about 15 Hz to filter out any radio frequency interference and smooth the output signal from amplifier 58 . The filtered signal from amplifier 72 is provided on output line 36 to system control circuit 38 and also on line 76 to high pass filter section 39 .

Similarly, the output on line 68 is passed from the sample hold circuit 42 in the infrared channel via resistors 78 and 79 to the non-inverting input of an amplifier 80 which has a feedback capacitor 81 and a grounded capacitor 82 around the filter section 43 , again a two-pole Form filter, which preferably has a corner frequency of about 15 Hz. The output of amplifier 80 is fed to system control circuit 38 on an output line 44 and is further fed to filter section 45 on an output line 83 .

The filtered output signal on line 76 passes through a series capacitor 85 to the non-inverting input of an operational amplifier 86 in high-pass filter section 39 . This input of amplifier 86 is also grounded via a resistor 87 . Feedback and ground resistors 88 and 89 are connected to the inverting input of amplifier 86 . This arrangement creates a single-pole high-pass filter, which preferably has a corner frequency of 2 Hz, in order to block the non-temporally varying portion of the signal on line 76 . The output of amplifier 86 is also supplied via a resistor 90 to the inverting input of an operational amplifier 91 , the output of which is clamped to a minimum of zero volts by a diode 92 and is fed back to the inverting input via a resistor 94 and a capacitor 95 connected in parallel . The non-inverting input of operational amplifier 91 receives a reference voltage via line 96 and from line 97 via voltage dividing resistors 98 and 99 . The reference voltage on line 97 is provided by system control circuit 38 to adjust the average level of the time-varying voltage at the output of amplifier 91 which is supplied on line 40 to system control circuit 38 to ensure that the output signal voltage is within a desired range for conversion to a digital signal for the reasons described below.

In the same way, the output on line 83 is fed to high-pass filter part 45 , which consists of a series capacitor 101 , which is connected to the non-inverting input of an operational amplifier 102, which is also connected to ground via a resistor 103 . A feedback resistor 104 and a grounded resistor 105 are connected to the inverting input of amplifier 102 . The output of amplifier 102 is connected via a series resistor 106 to the inverting input of an operational amplifier 107 , the output of which is conducted via a diode 108 and is fed back to the input via a resistor 109 and a parallel capacitor 110 . Amplifier 107 similarly receives a reference voltage at its non-inverting input on line 111 from the connection between resistors 98 and 99 so that the range of output voltage provided on line 46 from amplifier 107 is regulated by system control circuit 38 can.

A block diagram of the functional components of the digital system control circuit is shown in FIG. 3. The control circuit includes a microprocessor 114 , which is connected in a conventional manner to an oscillating crystal 115 , which provides a reference clock signal. The control circuit includes address bus output lines 116 , data bus lines 117, and read and write lines 118 and 119 . A clock output from the microprocessor on line 120 is provided to an interrupt counter 121 which counts down the number of pulses from the clock pulse generator and supplies a pulse to the interrupt input 122 of the microprocessor 114 after a certain number of counted pulses. The control system also includes a read only memory (ROM) 124 connected to the address and data bus lines, a random access memory (RAM) 125 connected to the address and data bus lines, a chip select decoder 126 which inputs from the address bus and supplies the chip selection outputs on lines 127 to the various components in a conventional manner. The input is via a keyboard and setting switch 129 on the front panel of the device for a data lock and a buffer 130 , which is connected to the data bus 117 . The output of the system is provided by an output data latch and buffer circuit 132 to a display unit 133 , e.g. B. a liquid crystal display unit on the front panel of the device.

The system receives data on lines 36, 40, 44, and 46 from the analog processing components for an analog-to-digital converter 135 , which converts the analog signals to digital signals that are provided on the data bus 117 . A reference voltage is provided to the A / D converter 135 on a line 136 by a D / A converter 138 which receives its input from the data bus 117 . D / A converter 138 also generates an AC reference signal on line 97 to adjust the average output levels from amplifiers 91 and 107 . The D / A converter 138 generates a separate output reference voltage based on the input on the data lines from the microprocessor on a line 139 via voltage dividing resistors 140 and 141 for the positive inputs of a driver amplifier 143 within the driver part 49 for the red light-emitting diode channel and a driver amplifier 144 within the Driver part 50 for the infrared light-emitting diode channel. The voltage from D / A converter 138 on line 139 provides a reference level for drivers 133 and 134 to allow adjustment of the output intensity of the LEDs under the control of the microprocessor. Variations in the optical absorption from finger to finger can be compensated for by changing the operating current for the light emitting diodes such that the DC output voltage level on lines 36 and 44 at full capacity (e.g. 4/5 of the full capacity) of the A / D converter 135 lies. In this way, the light-emitting diode intensities can be set in such a way that they deliver the greatest signal / interference ratio without saturation of the input amplifier 51 . Because the ratio of the amplitudes of the time-varying and non-time-varying signals is used in the calculations instead of using absolute values, no relative change in voltage influences the calculation of the saturation value. The drivers 143 and 144 are switched on and off by a gate signal on lines 146 and 147, respectively, via interface transistors 148 and 149 . The driver currents from the amplifiers 143 and 144 are supplied to the red light-emitting diode 25 and the infrared light-emitting diode 26 on output lines 150 and 151, respectively.

The control signals for drivers 49 and 50 and the synchronized sample hold control signals on lines 59 and 65 are provided by a decoder 153 which receives inputs from the microprocessor on address bus lines 116 and data bus lines 117 .

The analog to digital converter 135 is preferably ratiometric so that the digital output supplied to the microprocessor 114 represents the ratio of the input voltage on a selected one of the lines 36, 40, 44 or 46 with respect to the reference voltage on the line 136 . The microprocessor, via D / A converter 138, changes this reference voltage if necessary to keep the output of the A / D converter close to its full range, so as to achieve the greatest possible accuracy of the ratio calculation in which the ratio of the Peak-to-peak values of the signal on lines 40 and 46 are determined. The relationships are simply determined by integer division; however, such integer division introduces larger errors as the AC component values decrease. The A / D converter is therefore conveniently sized separately for each of the four inputs to ensure that the ratios are as large as possible in order to achieve the greatest possible accuracy in the division; the ratios are later corrected by the size (step size) to which the A / D converter was measured.

The operations performed by the microprocessor and associated circuitry within control circuit 38 have two main operating routines. The first is the clock interrupt routine, which samples the A / D converter, analyzes the pulse waveforms from the converter to average values on lines 36 and 44 , the peak-to-peak values of the time-varying signals on lines 40 and 46, and the period of the time-varying signals, and sends the data thus determined to the main program. The main program routine runs continuously in the background, waiting for the clock interrupt analysis routine to determine the end of the blood pulse. When the full pulse has been determined, the interrupt routine sends a broadcast to the main program loop, which then operates to calculate the pulse rate and oxygen saturation values. The results of the calculation are fed to a data rejection and mean value routine which averages the pulse frequency and the oxygen saturation over several pulses and then displays the mean values thus calculated for the user on the display unit 133 .

The functional operations of the control system are shown in greater detail in the flow diagrams of Figs. 4-9. 4 shows the main program loop flow diagram. When the device is turned on at 160, the system runs initial tests, including a self-test to check the display and read the alarm limits that have been set by the user. The program then checks the switches at 162 to determine whether a "test" switch has been set by the user or whether an "audio reset" has been turned on. If the test switch is on, the system continues to perform the system test (block 163 ). If the audio reset switch is set, the audio alarm is disabled for three minutes if enabled, or the audio alarm is enabled if initially disabled (block 164 ). After checking the switches, the program checks from the A / D data collection routine (block 166 ) and then checks at block 167 to determine if a pulse has been detected; if a pulse has been detected, the program goes to subroutine 5 (block 168 ) to process the new pulse. If no pulse rate has been determined, or if processing of a new pulse rate is complete, the program returns to block 162 to again test the switches.

The pulse data processing subroutine 5 is shown as a flowchart in FIG. 5. Upon initiation of the program (block 170 ), data values are captured by the data collection routine (block 171 ) and then the collected DC voltage values compared to predetermined set values (block 162 ) to determine if the values are within a range. If not, control signals are provided to the D / A converter 138 to appropriately increase or decrease the output level on line 139 to change the LED brightness (block 173 ). Then the program goes back to the main loop. If the DC or non-time varying values are in the range, the program then checks to see if the AC or time variable values are in the range (block 175 ). If not, the program checks whether the A / D converter reference voltage is at a minimum (block 176 ) and if not, the A / D reference voltage is decreased (block 177 ). When the A / D reference voltage is at a minimum, an output signal is provided to display unit 133 to produce a "lo perf" display (block 180 ) which indicates to the user that finger perfusion is too low, to make a reliable measurement. If the AC values have been found to be in range (block 175 ), the pulse rate and oxygen saturation values are calculated and displayed in a subroutine 6 (block 182 ) before the subroutine returns to the main loop program at 183.

The oxygen saturation and pulse rate subroutine is shown as a flow chart in FIG . After entering the subroutine (block 135 ), the updated AC _values ( I _aci , I _acr ) and the DC _values ( I _dci , I _dcr ) are used to calculate a new ratio value R (block 186 ) and then the pulse rate and the oxygen saturation value S calculated (block 187 ) as a function of R. The functional relationship between the saturation S and the ratio R can be determined in various ways. As shown in Fig. 10, the relationship between S and R can be expressed as a series of data points determined from empirical data, at which data determined by the device is compared to saturation data determined using standard direct saturation measurements have been. The numerical data relating to S and R are then stored in ROM 124 (e.g. EPROM) in a look up table with interpolation between data points if desired. Alternatively, the system can be programmed so that the equation is solved, where E _{HR is} the quenching coefficient for hemoglobin in red light, E _{HI is} the quenching coefficient for hemoglobin in infrared light, E _{OR is} the quenching coefficient for oxyhemoglobin in red light, E _{OI is} the quenching coefficient for oxyhemoglobin in infrared light and K _R and K _I constants, so are selected so that the calculated saturation is corrected to match standard data. This equation can be expressed as where K ₁ , K ₂ , K ₃ and K _{4 are} constants. Instead of calculating these constants from the cancellation coefficients, it is also possible to determine the constants using a best match of the above equation with empirical data relating to the saturation S and the ratio R. The above equations can be solved for each new saturation value. However, it is advantageous to reduce the computation time to solve the equation for a set of R values over the expected range at the start, to store the computed S values in a lookup table in RAM, and to store the S values from the lookup table during the patient's observation.

The range of the calculated saturation is then checked (block 188 ). If the oxygen saturation so determined is not between 50% and 100%, the data as out of range is ignored (block 189 ) and the program returns to the main loop (block 190 ). If the saturation is between 50% and 100%, the program then checks whether the heart rate is between 0 and 199 beats per minute (block 192 ). If not, the data is again ignored (block 199 ) and the program returns. If the heart rate is correctly in the range between 0 and 199 beats per minute, the program then checks whether the number of consecutive acceptable pulses falls into three ranges, one subsequent pulse, 2 to 25 subsequent pulses or 16 or more subsequent pulses (blocks 193 and 194 ). If the number of subsequent pulses accepted is 1, the heart rate and oxygen saturation averages are initialized (block 195 ) and the program returns (block 196 ). If the number of subsequent accepted pulses is 2 to 15, the new heart rate and oxygen saturation data are averaged with the previously accepted values (block 198 ) and the new heart rate and oxygen saturation averages are displayed (block 199 ) before the program returns. If the number of subsequent accepted pulses is 16 or more, only the data that has been accepted for the last pulses is averaged (block 197 ) and the new mean heart rate and oxygen saturation values are displayed (block 199 ) before the program returns .

The interrupt routine is shown in Fig. 7 for the periodic scan interrupt. If an interrupt pulse is given by interrupt counter 121 (block 201 ), the program checks whether the last sample comes from the red channel (block 202 ). If so, the infrared light emitting diode is pulsed (block 203 ) and the infrared sample and hold circuit is then triggered (block 204 ). If the last sample was not red, the red light emitting diode is pulsed (block 205 ) and the red sample and hold circuit is triggered (block 206 ). The program then checks to see if a 2.5 ms delay has passed, and if so, the program proceeds to subroutine 8 (block 208 ) to receive new A / D converter data before it returns. If the 2.5 ms delay time has not elapsed, the program immediately returns (block 209 ) without taking new A / D converter data.

The A / D data collection flowchart is shown in FIG . At block 210 , the program is entered and initiated by storing data from the last A / D conversion (block 211 ). The program then continues to set the reference level for the next conversion (block 212 ) and then the next conversion is started (block 213 ). The program then determines whether the latest data was for the red or infrared AC channel or for the red or infrared DC channel (block 214 ). If the last data was for the red or infrared AC channel, the program determines the minimum and maximum values of the input data (block 215 ) and stores the difference between the maximum and minimum values found in this way as time-varying analog values I _aci or I _acr , and then awaits the end of a pulse (block 216 ). If a pulse has been determined, the pulse period is determined (block 217 ) based on conventional criteria, such as the time between successive maximum or minimum values, and a "pulse detected" message is sent to the main loop (block 218 ), before the program returns (block 219 ). If the end of the pulse is not found at block 216 , a "no pulse" message (block 220 ) is sent to the main loop and the program returns. If the last channel on which data has been converted has been determined in block 214 as the red or infrared DC channel, the peak value of the data is determined (block 222 ) and is determined as the non-time varying analog value I _dci or I _{dcr is} saved and the program then returns.

The oximeter 20 is preferably capable of performing a self-check on power up to ensure that all components are working properly. If the self-check switch has been closed by the operator, as checked during initialization (block 162 ), the system enters a self-check subroutine of Figure 9 (block 225 ). First, the alarms are blocked and the display is suppressed (block 226 ). Control circuit 38 then provides signals to light emitting diode drivers 49 and 50 (block 227 ) to vary the intensity of the pulsed outputs from light emitting diodes 25 and 26 as a cyclic function of time (e.g. sawtooth) to accommodate changes in intensity due to To simulate pulse beats. A piece of semi-transparent filter material is preferably inserted into the probe 21 between the light emitting diodes and the detector 28 during this test. Saturation and heart rate values are determined in accordance with normal routines, and the program waits for a selected period of time to allow data averaging to go to sleep (block 228 ). The calculated saturation and heart rate values are then compared to predetermined values for the device (blocks 229 and 230 ). It has been found that, under the preceding test conditions and with the device functioning properly, the calculated saturation value always corresponds to a certain numerical value, regardless of the attenuation characteristic of the filter used, provided that sufficient light reaches the detector 28 in order to be able to carry out the measurements. Therefore, if the saturation value equals the predetermined value, a "test OK" signal is displayed and normal operation is enabled (block 231 ). If the oxygen saturation value does not reach the predetermined value, a "Check System" sign is displayed and normal operation is blocked (block 232 ) and the program stops (block 233 ) for the operator to check the system.

Common commercially available digital circuit components can be used in the present system. For example, suitable commercially available components for the digital circuit according to FIG. 3 can be used, namely NSC800 for the microprocessor 114 , 27C64 for the ROM 124 , 6116 for the RAM 125 , 4599 for the decoder 153 , 74HC138 for the chip selection decoder 126 , 4040 for the interrupt clock counter 121 , 74HC541 for the data buffer 130 , 80CO844 for the A / D converter 135 , AC7528 for the D / A converter 138 , CA3094 for the LED current drivers 143 and 144 . For the analog components of FIG. 3 can be LTI013 80, used for the amplifiers 50, 53, 55, 72, 86, 91, 102 and 107 and CA3280 for the amplifiers 58 and 64.

A preferred mounting of the light sources 25 and 26 within the probe 21 is shown in FIGS. 11 and 12. As shown in FIG. 11, two red light-emitting light-emitting diodes 25 and two infrared light-emitting light-emitting diodes 26 are arranged symmetrically around a center point on the probe jaw 23, light-emitting diodes of the same type being arranged diametrically opposite one another at the corners of a square. Each pair of light emitting diodes 25 and 26 are preferably each connected in series to be supplied with current from drivers 49 and 50 and in this way produce twice the light intensity for a specific current level compared to a single light emitting diode. Because the four light-emitting diodes are arranged symmetrically around a center point, which preferably faces directly the detector 28 when the probe is closed, the light from the two light-emitting diodes 26 travels through the body tissue in essentially the same way as the light from the light-emitting diodes 25 . A lens 240 , which is transparent and capable of refracting both infrared light and red light, is placed over the light sources 25 and 26 to focus the light from these light sources onto the detector 28 . The lens 240 , which can be made from a suitable transparent epoxy resin in which the light emitting diodes 25 and 26 are preferably embedded, is convex and extends outward over the adjacent surface 241 of the clamping section 23 . The above convex lens not only serves to focus the light from the light sources, but also presses into the finger tissue and stresses it, so that the pulse beats in the finger become clearer. As a result of the focusing of the light in the tissue and the stress on the tissue, it is possible to obtain reliably determined light intensities that run through the tissue, even if the blood circulation is only very low.

Claims (28)

1. Device for determining the oxygen saturation of blood in a patient, characterized by :
a) a source arrangement ( 25, 26 ) for generating electromagnetic radiation with two different wavelengths;
b) a detector ( 28 ) for detecting the intensity of the radiation from the source assembly ( 25, 26 ) after it has passed through the patient and for generating an output signal indicative thereof;
c) a separator ( 30 ) for separating the output signal of the detector ( 28 ) into a first signal which corresponds to the intensity of the radiation received by the detector ( 28 ) at the first of the two wavelengths, and into a second signal which corresponds to the intensity corresponds to the radiation received by the detector ( 28 ) at the second of the wavelengths;
d) a device ( 38 ) for determining the mean values of the first and second signals and for determining the peak-to-peak amplitudes of the time-variable components of the first and second signals for calculating a ratio R from the product of the mean value of the second signal with the Peak-to-peak amplitude of the first signal on the one hand and the product of the mean value of the first signal and the peak-to-peak value of the second signal on the other hand and for determining the oxygen saturation in accordance with the equation: where E _{HR is} the quenching coefficient of hemoglobin at the first wavelength, E _{HI is} the quenching coefficient of hemoglobin at the second wavelength, E _{or is} the quenching coefficient of oxyhemoglobin at the first wavelength, E _{oi is} the quenching coefficient of oxyhemoglobin at the second wavelength and K _r and K ₁ are selected constants. 2. The apparatus of claim 1, wherein the source arrangement electromagnetic radiation at the wavelength of red light and at the wavelength of infrared light gives and where the extinction coefficient for the red light wavelength and for the infrared light wavelength are. The apparatus of claim 1, wherein the source arrangement includes at least two light emitting diodes ( 25, 26 ), one emitting red light and the other emitting infrared light, the detector including a photodiode ( 28 ) that is sensitive to both red light and infrared light responds and emits an output signal for the intensity of the same. 4. The apparatus of claim 1, wherein the source arrangement includes two sources ( 25, 26 ) that emits the electromagnetic radiation at two different wavelengths and further including means ( 38 ) for alternately pulsing the two sources at a selected clock frequency, and containing means ( 30 ) connected to the output of the detector ( 28 ) for filtering the detector output signal to eliminate the effects of ambient light not from the two sources ( 25, 26 ). The apparatus of claim 1, wherein the source arrangement includes two separate sources ( 25, 26 ) that generate the two different wavelengths of electromagnetic radiation, and means ( 38 ) is provided to alternate one of the two sources ( 25, 26 ) - and off, and wherein the separator includes first and second sample and hold circuits ( 34, 42 ) to sample the output signal of the detector ( 28 ) and to provide a constant amplitude output signal for a predetermined period of time which is proportional to the sampled signal at the time of sampling, each sample and hold circuit ( 34, 42 ) samples the output of the detector ( 28 ) at a time synchronized with the pulses of an associated one of the sources ( 25, 26 ) to reconstruct the time-varying signal from the detector ( 28 ) that corresponds to the radiation intensity received by the detector ( 28 ) only from the associated one of the sources ( 25, 26 ), and w further comprising a low pass filter arrangement ( 35, 43 ) connected to the outputs of the sample and hold circuits ( 34, 42 ) to filter out the high frequency components in signal introduced by the sample and hold circuits ( 34, 42 ). 6. The apparatus of claim 1, wherein the source arrangement comprises two pairs of light emitting diodes ( 25, 26 ), one pair of red light and the other pair emitting infrared light, the diode pairs ( 25, 26 ) being arranged symmetrically about a center point in a probe body , and wherein the detector has a photodiode ( 28 ) which is arranged substantially opposite the center point of the light emitting diodes ( 25, 26 ) in the probe body such that a patient's finger between the light emitting diodes ( 25, 26 ) and the detector ( 28 ) can be brought. 7. Apparatus according to claim 6, including a convex lens ( 241 ), which is arranged in the probe body above the light emitting diodes ( 25, 26 ) to focus the light emitted by them, and which extends outward over the surrounding area of the probe body to squeeze the tissue of a patient's finger to help determine pulse rates. The apparatus of claim 1, wherein the means for determining the non-time varying values and the peak-to-peak amplitudes of the first and second signals includes high pass filters ( 39, 45 ) to filter the first and second signals to to remove the time-invariant part from the signal and to supply output signals which only correspond to the time-varying parts of the first and second signals. 9. The device according to claim 1, comprising a device ( 38 ) for determining the pulse rate of the patient from the time-variable values of the first and second signals. 10. The apparatus of claim 8, wherein the means for determining the non-time varying values and the peak-to-peak amplitudes of the first and second signals further includes: a microprocessor ( 114 ), an analog-to-digital converter ( 135 ) which is provided for receiving the first and second signals and the temporally varying portions of the first and second signals from the output of the high-pass filters ( 39, 45 ) and which supplies the digital equivalent thereof to the microprocessor ( 114 ) under the control of the microprocessor ( 114 ) , a digital / analog converter ( 138 ) for receiving data from the microprocessor ( 114 ) and for outputting corresponding analog output signals, and two drivers ( 49, 50 ) for the sources ( 25, 26 ) of electromagnetic radiation, and wherein the microprocessor ( 114 ) outputs data via the digital / analog converter ( 138 ) in order to operate the drivers ( 49, 50 ) at a level such that the average intensity is received by the detector ( 28 ) NEN radiation has a desired level. 11. Device for determining the oxygen saturation of blood in a patient, characterized by:
a) a probe ( 21 ) which is set up to record a part of the patient's body and contains a source arrangement ( 25, 26 ) for generating alternating pulses of red and infrared light which is directed onto the patient, and a detector ( 28 ) for recording the intensity of the red and infrared light emitted by the source arrangement ( 25, 26 ) after it has passed through the body part and for generating an output signal relating thereto;
b) separation means ( 34, 42 ) for separating the output signal of the detector ( 28 ) into a first signal which corresponds to the intensity of the red light received by the detector ( 28 ) and a second signal which corresponds to the intensity of the from the detector ( 28 ) corresponds to received infrared light, and
c) a high-pass filter arrangement ( 39, 45 ) which is set up to receive the first and second signals and generates output signals which correspond only to the temporally varying portions of the first and second signals;
d) a device ( 38 ) for receiving the first and second signals of the outputs from the high-pass filter arrangement ( 39, 45 ) corresponding only to the temporally varying portions of the first and second signals in order to adjust the non-temporally varying values ( I _dci and I _dcr ) determine the first and second signals and the peak-to-peak amplitudes I _aci and I _{acr of} the time-variable components of the first and second signals; and
e) means ( 38 ) for determining the oxygen saturation of the blood in the patient as a function of the ratio R of the non-time varying values of the first and second signals and the peak-to-peak amplitudes of the first and second signals, wherein 12. The apparatus of claim 11, wherein the source arrangement (25, 26) has two light-emitting diodes couples (25, 26), of which a pair of outputs (25) of light of red wavelength and the other pair (26) light to infrared wavelength, the diode pairs ( 25, 26 ) on the probe body are arranged symmetrically around a center point and the detector contains a photodiode ( 28 ) which is arranged essentially opposite the center point of the light-emitting diodes ( 25, 26 ) and on both red light and infrared light responds to generate an output signal that corresponds to the received light intensity. 13. Apparatus according to claim 11, comprising a device ( 30 ) to which the output signal of the detector ( 28 ) is fed in order to filter it and to suppress from it signal components which originate from ambient light which does not come from the source arrangement ( 25, 26 ) has been submitted. 14. The apparatus of claim 11, wherein the separator comprises a first and a second sample hold circuit ( 34, 42 ) for sampling the output signal of the detector ( 28 ) and which emit a signal of constant amplitude for a predetermined period of time, which is proportional to the sampling signal at the time of sampling wherein each sample hold circuit ( 34, 42 ) samples the output of the detector ( 28 ) at a time synchronized with the pulses of the associated sources ( 25, 26 ) to reconstruct the time varying signal from the detector ( 28 ) which corresponds to the intensity of the radiation received by the detector ( 28 ) only from the associated one of the sources ( 25, 26 ), and further comprising a low pass filter arrangement ( 35, 43 ) which is connected to the output of each sample and hold circuit ( 34 , 42 ) is connected to filter out high frequency components in the signal which have been introduced by the sample and hold circuits ( 34, 42 ). 15. The apparatus of claim 11, wherein the means for determining the non-time varying values and the peak-to-peak amplitudes of the first and second signals further includes: a microprocessor ( 114 ), an A / D converter ( 135 ) which receives the first and second signals and the temporally varying portions of the first and second signals from the output of the high-pass filters ( 39, 45 ) and delivers the digital equivalent thereof to the microprocessor ( 114 ) under the control of the microprocessor ( 114 ), a D / A converter ( 138 ) which receives output data from the microprocessor ( 114 ) and generates corresponding analog output signals, and two drivers ( 49, 50 ) for the sources ( 25, 26 ) for red and infrared light, the microprocessor ( 114 ) Provides data via the D / A converter ( 138 ) to operate the drivers ( 49, 50 ) at a level that brings the average intensity of the radiation received by the detector ( 28 ) to a desired level. 16. The apparatus of claim 11, wherein the probe ( 21 ) includes a convex lens ( 241 ) attached to the probe body above the source assembly ( 25, 26 ) to focus the light emitted therefrom and after extends outside over the surrounding surfaces of the probes ( 21 ) in order to press on the tissue of a patient's finger in order to favor the determination of pulse beats. 17. The apparatus of claim 11, wherein the means for determining oxygen saturation as a function of the ratio R includes a memory ( 124 ) for storing predetermined values of saturation for a series of values of R over the expected range of saturation values that are accessible; when the ratio R is calculated to give the saturation value corresponding to this ratio. 18. Device for determining the oxygen saturation of blood in a patient, characterized by:
a) a probe ( 21 ) with two light-emitting sources ( 25, 26 ), one of which emits light with a red wavelength and the other light with an infrared wavelength, which are arranged adjacent to one another in a probe body, and a photodetector ( 28 ), which is arranged in the probe body at a location such that it receives light from the sources ( 25, 26 ) after it has passed through a body part of a patient which is between the light-emitting sources ( 25, 26 ) and the photodetector ( 28 ) has been introduced, the photodetector ( 28 ) emitting an output signal which is dependent on the intensity of the red or infrared radiation received by it;
b) an amplifier ( 30 ) for converting the output signal of the photodetector ( 28 ) into a voltage signal;
c) a red channel circuit ( 31 ) containing a sample and hold circuit ( 34 ) which receives the output signal of the amplifier ( 30 ) and samples it at those times which are synchronized with pulses from the red light emitting source ( 25 ), and Outputs a signal which is constant with an amplitude which is proportional to the amplitude of the output signal of the amplifier ( 30 ) at the time of sampling, a low-pass filter ( 35 ) which is connected to the output of the red signal sample and hold circuit ( 34 ) to the high frequency components to filter out which has entered the signal through the sample and hold circuit ( 34 ) and which outputs an output signal, a high-pass filter ( 39 ) which receives the output signal of the low-pass filter ( 35 ) to produce an output signal which contains only a time-varying component ,
and a red channel circuit ( 32 ) containing a sample and hold circuit ( 42 ) for receiving the output signal of the amplifier ( 30 ) and for sampling the same at times synchronized with the pulses of the infrared light emitting source ( 26 ) and which has an output signal outputs a constant with an amplitude proportional to the amplitude of the output signal of the amplifier ( 30 ) at the time of sampling, a low-pass filter ( 43 ) connected to the output of the infrared sample and hold circuit ( 42 ) to filter out the high frequency components which are inserted into the signal through the sample and hold circuit ( 42 ) and which provide an output signal, and a high-pass filter ( 45 ) which receives the aforementioned output signal from the low-pass filter ( 43 ) to give an output signal which is only a time-varying component having;
d) a digital control device comprising:
(1) an A / D converter ( 135 ) which receives the outputs of the red and infrared low-pass filters ( 35, 43 ) and the red and infrared high-pass filters ( 39, 45 ),
(2) a microprocessor ( 114 ) connected to the A / D converter ( 135 ) to control its operation and receive the digital signals therefrom,
(3) a D / A converter ( 138 ) to which the microprocessor ( 114 ) is connected to supply data to control the operation thereof,
(4) adjustable drivers ( 49, 50 ) connected to the red light source ( 25 ) and the infrared light source ( 26 ) that receive the output signals from the D / A converter ( 138 ), and wherein the microprocessor ( 114 ) receives the non-time-varying values I _dcr and I _{dci of} the red and infrared signals received by it and the peak-to-peak amplitudes I _acr and I _{aci of} the time-variable components of the red and infrared signals received by it, determined and a ratio of calculated, determining the oxygen saturation S as a function of R , and wherein the microprocessor ( 114 ) controls the D / A converter ( 138 ) to output the source drivers ( 49, 50 ) by the intensity of the sources ( 25, 26 ) adjust emitted light such that the mean values of the red light and infrared light signals are essentially before a selected desired level. 19. The apparatus of claim 18, wherein the digital controller includes a memory ( 124 ) that stores predetermined values of saturation S for a series of values of R over the expected range of saturation values, and the memory ( 124 ) operatively associated with the Microprocessor ( 114 ) is connected to allow it to access when a ratio R is calculated to determine the saturation value corresponding to that ratio. 20. Apparatus according to claim 18, including means for alternately pulsing the two sources ( 25, 26 ) at a preselected clock frequency, and including means ( 30 ) connected to the output of the photodetector ( 28 ) to control the output of the Filtering photodetectors ( 28 ) in order to free the output signal from influences of ambient light which has not been emitted by sources ( 25, 26 ). 21. The apparatus of claim 18, including a device to determine the patient's pulse rate from the time-varying amplitudes of the first and second signals. 22. The apparatus of claim 18, wherein the A / D converter ( 138 ) provides a digital output related to a ratio of its input to a reference voltage level, and wherein the microprocessor ( 114 ) provides a reference voltage to the A / D- Transducer ( 138 ) supplies such that the digitized output of the converter ( 138 ) corresponding to the outputs of the red and infrared high-pass filter ( 39, 40 ) is within a desired range. 23. A method for determining the oxygen saturation of blood in a patient, characterized by:
a) alternately directing red light and infrared light through a body part of the patient;
b) determining the intensity of the red light and infrared light that has passed through the body part and generating a signal that is representative of the intensity of the quantities of light that have passed through the body part.
c) separating the determined signal into a first signal that corresponds to the radiation that the detector has received in red light and a second signal that corresponds to the intensity of the radiation that the detector has received in infrared light;
d) calculating the non-time-varying values I _dcr and I _{dci of} the first and second signals and the peak-to-peak amplitudes I _acr and I _{aci of} the time-varying components of the first and second signals;
e) Calculating a ratio f) determining the oxygen saturation S from the calculated value of R in accordance with a predetermined functional relationship between S and R. 24. The method of claim 23, wherein the infrared light and the red light alternating through the Patient is passed through in pulses and the signal determined being separated into two signals  becomes different for each of the pulse types Wavelengths by scanning the determined Signal at the time of the red light pulse, Holding the determined amplitude of the output signal until the time of the next red light pulse and accordingly separate sampling of the determined signal at the time of the infrared pulse and holding the determined amplitude of the Output signals until the time of the next Infrared pulse. 25. The method according to claim 24, characterized in that the Intensity of infrared and red radiation, the directed towards the patient varies in such a way is that the value that does not vary over time in the determined red light and infrared light signal in is substantially at a predetermined level. 26. A probe for use in determining oxygen saturation in a patient's blood, characterized by
a) a solid housing body;
b) a movable clamp member attached to a hinge on the housing body;
c) two pairs of light emitting sources attached to the clamp member, one pair emitting red light and the other pair emitting infrared light, the light sources being arranged symmetrically about a center point;
d) a photodetector disposed on the fixed housing body at a location substantially opposite the center when the probe is closed around a patient's finger; and
e) a convex lens located on the clamp member above the sources to focus the light thereof. 27. The probe of claim 26, wherein the lens outwards from the surrounding areas of the terminal part the probe extends, causing it to A patient's finger tissue squeezes to determine from improving heartbeats. 28. The probe of claim 27, wherein the sources LEDs are and the diodes in one solidified epoxy body are embedded, which for Red light and infrared light is transparent and which differs from the terminal part with a convex Surface extends to form the lens.