RU2279143C1 - Oxygenic blood pulsing flow optical-mechanical simulator - Google Patents

Abstract

FIELD: medicine.

SUBSTANCE: simulator can be used for tuning, checking and periodical inspection of pulse oxymeters. Simulator has flat light filter with optical radiation transmission factor τλ1 in visible area being smaller τλ2 than in IR-spectrum of radiation (τλ1<τλ2). One or several light filters are mounted additionally in the plane of light filter available. Transmission factors of light filters meet the relation of τλ1>= τλ2. Light filters are optically connected with adjustable diaphragms and they are made in form of sectors and fixed onto transparent disc. Disc is connected with mechanism of rotation that rotates at angular speed of rotation being equal to frequency of heart beat rating. Blood is saturated with oxygen and level of oxygen is brought to borders being close to natural one.

EFFECT: widened range of application.

3 cl, 3 dwg

Description

The proposed device relates to the field of medical instrumentation and can be used to configure, verify and periodically monitor pulse oximeters.

Known imitators of the optical properties of blood used to calibrate laboratory oximeters and devices included in the backbone of the heart-lung machine (AIK). Typically, they use optical glasses that can accurately reproduce the spectral characteristic of the absorption of optical radiation by oxygen-rich blood (Saunders NA, Powles AC, Rebuck AS Ear oximetry: accuracy and practicability in the assessment of arterial oxygenation // Am Rev Respir Dis l976; 114 ; 745-748).

The closest in technical solution to the proposed device is a simulator manufactured by Bentley ("OxySat Meter Kit Operation and Maintenance Instruments. Detailed description" Bentley Laboratories, INC), selected by the authors as a prototype. The Bentley oximeter sensor is mounted on a transparent tube that is included in the AIC circuit and contains red and infrared LEDs and a photodiode located in the same plane and having optical contact with the wall of the transparent tube. The simulator allows connection to an oximeter sensor without a transparent tube. It is made of optical colored glass. Depending on the thickness and brand of glass, one or another value of blood oxygenation is imitated. Usually, a number of glasses with spectral characteristics similar to domestic glasses KS10-KS19 are used (Table 1). The simulator is in optical contact with the LEDs and the photodiode. In order to switch from one oxygenation to another, it is necessary to insert a new optical glass into the simulator. As a result, using such an oximeter, you can check only a number of blood oxygenation values by the number of light filters.

Such a simulator is not suitable for checking pulse oximeters. Since the action of these devices is based on measuring the amplitude of the pulsation of the absorption of optical radiation, therefore, the optical characteristics of the simulator should pulsate in time. Measurement of the variable component of the signal allows you to select arterial blood from other tissues, both stationary and pulsating, according to other laws (venous and capillary blood). Even if the optical properties of the Bentley simulator are modulated, even in this case it is impossible to reproduce the entire range of blood oxygen saturation from 0 to 100%.

Pulse oximetry is based on the selection of pulse waves in a signal obtained by scattering of two-frequency optical radiation by arterial blood.

Pulse oximeters work as follows: first, the heart rate of the heart rate is measured, then the amplitudes of the pulse oscillations are determined, by which the oxygen saturation of the blood is calculated. Optimal in the rms sense is the determination of signal amplitudes by the Fourier method. The zero Fourier harmonics and harmonics are calculated, the frequency of which coincides with the heart rate measured previously. These values are determined by probing tissue with red and infrared radiation. Denote:

R ₀ - zero harmonic of the photocurrent when probing tissue with radiation with a wavelength of 680 nm;

R ₁ - the first harmonic of the photocurrent when probing tissue with radiation with a wavelength of 680 nm;

I ₀ - zero harmonic of the photocurrent when probing tissue with radiation with a wavelength of 940 nm;

I ₁ - the first harmonic of the photocurrent when probing tissue with radiation with a wavelength of 940 nm.

Based on these values, the saturation of arterial blood with oxygen is calculated:

Where J (t) and G (t) is the photocurrent when probing tissue with red and infrared radiation, T is the period of the cardiac rhythm, or, what is the same, the time of one turn of the shutter.

Blood oxygenation is determined by the tabular method. The algorithm of the pulse oximeter is as follows. The object is irradiated with red (λ = 680 nm) and infrared (λ = 940 nm) optical radiation. Signals scattered by pulsating arterial blood (R (t), I (t)) are recorded by a photodetector. The average values of these signals R ₌ , I ₌ and the Fourier amplitudes of the harmonics of the pulse wave R _~ , I _{~ are} determined. Using these values, we calculate the address of the table element x:

Blood oxygenation is determined by the calibration curve f (x), written in table form:

The function f (x) is nonlinear, but in a first approximation it can be represented as:

The pulse oximeter uses sensors that work on the passage and reflection of laser radiation. The latter are often adapted to measure blood oxygenation in the digital artery (finger sensor). Structurally, it resembles a laundry clip, in one half of which two LEDs are installed, generating pulses in the red and infrared ranges. In the other half is an LED. In working condition, the sensor is attached to the finger, and the device registers the modulation of the photocurrent proportional to the perfusion of blood in the digital artery.

A pulse wave simulation for a finger sensor can be performed using an optical modulator. The modulation frequency should be in the range of heart rate. Moreover, it is necessary that the optical transmission modulation be different for red and infrared radiation. At first glance, it seems that it is enough to put a light filter and periodically block the light flux with a shutter. However, calculations show that such a device allows you to simulate only one value of SO ₂ = 85%, regardless of the characteristics of the filter and the size of the shutter. If a hole (window) is made in the plane of the filter, then depending on the ratio of the dimensions of the filter and the shutter, it is possible to simulate a certain range of blood oxygenation. In particular, if the same filter is used as in the prototype (KS19), then the choice of the ratio of the sizes of the filter and the shutter allows you to cover the SO ₂ range from 0% to 85%. The same result can be obtained with any filter with a transmission in the infrared region greater than in the visible region (τ _λ1 <τ _λ2 ). Simulation of large values of SO ₂ is possible only when using filters with a spectral characteristic opposite to the previous one (τ _λ1 > τ _λ2 ). Thus, a full-fledged simulator must contain at least two filters, a window and apertures for regulating the size of filters and a window.

Structurally, it was convenient to use a shutter rotating at a speed of 60-120 rpm. The speed is determined by the range of heart rate (HR) available for measurement with the tested pulse oximeter. The shutter is made in the form of two discs. The first disk of the shutter contains sectors, one of them forms a transparent window, and filters are inserted into the rest. On the second disk of the shutter, adjustable opaque sector diaphragms are installed. The second disk is mounted above the first and attached to it in the center. The mount allows you to fix any angle of rotation of one disk relative to another.

The drawing shows a diagram of the inventive device:

1 - filter τ _λ1 > τ _λ2 ;

2 - light filter τ _λ1 <τ _λ2 ;

3 - sector aperture;

4 - a transparent disk window;

5 - dial with SO ₂ scale;

6 - aperture angle lock;

7 - sensor of the device under test;

8 - optical fibers;

9 - rotation mechanism.

The proposed device operates as follows. To simulate one or another SO ₂ value, before turning on the device, the angle of rotation of one disk of the shutter relative to another is set and fixed. After this, the obturator is untwisted to the required speed. The finger sensor is installed so that the obturator periodically shuts off the flow of optical radiation in the path from the LEDs to the photodiode.

Consider the design of the obturator with a 90 ° sector, a glazed SZS20 glass filter, with a 90 ° sector, a glazed SZS25 glass filter and a window occupying a 180 ° sector.

The transmission coefficients τ _λ1 , τ _{λ2 of} optical filters with a thickness of 2 mm are shown in table 1.

Table 1. Brand of optical Wavelength range (nm) glass 680 ± 10 940 ± 10 KS19 4.2 10 ^-6 0.98 SZS20 3.2 10 ^-4 0.97 SZS25 0.55 0.006

Data taken from the official publication "Optical color glass. Specifications GOST 9411-81" USSR State Committee for Product Quality and Standards Management, Moscow.

The diaphragm contains a darkened sector of 140 °. The rotation of the diaphragm not only changes the area of the used filter to simulate the required value of SO ₂ , but also performs the function of a switch for filters. One of them satisfies the condition τ _λ1 <τ _λ2 , and the other satisfies the opposite τ _λ1 ≥τ _λ2 . This allows you to simulate the entire range of oxygenation from 0 to 100%. The calculation results are shown in figure 2 by a solid line.

New technical results are:

- expanding the range of values of blood oxygenation simulated by the proposed device;

- expanding the scope of the simulator for calibration and verification of pulse oximeters.

New technical results are achieved by the fact that in the plane of the existing filter, one or more filters with optical transmittance coefficients satisfying the relation τ _λ1 ≥τ _{λ2 are} additionally installed; optical filters are optically connected to adjustable diaphragms.

A photograph of the current layout of the proposed device is shown in figure 3. The breadboard contains a collector motor with gear (1). The engine is powered by an adapter that converts the mains voltage (220 V, 50 Hz) into a constant voltage, adjustable in the range of 7-27 V. The angular speed of the gearbox varies from 36-140 rpm.

A shutter (2) is installed on the gearbox axis. It is enclosed in a lightproof enclosure. The obturator consists of a movable diaphragm and a transparent disk onto which the SZS-20 and SZS-25 colored glass sectors are pasted. The diaphragm is made in the form of a blackened sector with a central angle of 140 °. It is installed and rigidly fixed at any angle in the range from 0 ° to 360 °. A scale is marked on the rim of the disc.

Two optical connectors (3) are mounted on the casing of the shutter against each other so that the optical path between them is blocked by the shutter. Connectors are connected via a multicore optical waveguide (4) to a probe (5), the size of which corresponds to the finger sensors of pulse oximeters. The probe has two windows for optical contact with LEDs and a pulse oximeter photodiode. Optical fibers through optical prisms and diffusers are in optical contact with the probe windows.

Tests of the current prototype of the claimed device were conducted in conjunction with two devices: a Philips Medizin System GmbH pulse oximeter, Handheld Digital Pulse Oximeter M45529A (device No. 1) and an OGP-1 oxygemapulsometer manufactured by NPO Astrofizika (the device is certified and received permission from the Russian Ministry of Health for medical use) equipped with a finger sensor (device No. 2).

The voltage of the electric motor was set to 13.6 V. In this case, the angular speed of rotation was 70 rpm, which was checked using a stroboscope.

The diaphragm rotation angles were set in the range from 0 ° to 100 ° in increments of 10 °. For each angle, 3 measurements were performed. Before each measurement, the obturator stopped and again spun up to a speed of 70 rpm. The measurements of SO _{2 were} read from the OGP-1 display.

The measurement results are shown graphically in figure 2 and in table 2.

Table 2. No. p / p Set value Angle of rotation Indications instrument number 1 Indications of the device No. 2 SO ₂ % aperture (degrees) Heart Rate (min ^-1 ) SO ₂ % Heart Rate (min ^-1 ) SO ₂ % one 0 27 137 - 65 10 2 thirty 40 72 40 68 37 3 fifty 48 69 45 70 45 four 60 231 70 59 67 64 5 70 247 72 68 72 75 6 80 265 74 76 73 83 7 90 282 68 87 70 91 8 95 291 69 96 71 96 9 98 297 71 99 69 one hundred 10 one hundred 302 75 99 72 one hundred

On the abscissa axis, the angles of rotation of the diaphragm relative to the plate with light filters are plotted, along the ordinate axis, the measurement results of the oxygenation of the blood in percent with the OGP-1 device are plotted. The average value for three measurements is marked (*) in figure 2.

As a result of the tests, it was found out that the standard error of the pulse rate measurement for both devices is not more than 2 rpm, which corresponds to the technical requirements. The device No. 1 at a given value of SO ₂ = 0% measured the frequency of the second harmonic. This is explained by the fact that at the edge of the studied oxygenation range, the amplitudes of the 1st and 2nd harmonics are comparable. In addition to this point, the 1st harmonic was confidently detected in the entire range, which indicates a high-quality imitation of the pulse signal.

The measurement error of SO ₂ in the range of 80-100% is not more than 2%. Smaller SO ₂ values are measured with an error of 4%. It complies with the technical requirements.

The most important range of measured oxygenation is 85-100%. In this range, the difference between the instrument readings and the simulation results is less than 2%. In the range of 40-85%, the difference is less than 5%, and for SO ₂ <40% the difference is less than 10%. The last two ranges are of little significance for practical medicine and errors of 5-10% are acceptable.

Currently, the problems of calibration and verification of pulse oximeters have not been resolved. When creating new devices, volunteers are used, who, through the introduction of special drugs, artificially reduce the oxygen saturation of arterial blood. At the same time, it is possible to check the device in the range of 85-98%. Other values are not checked. Instruments are tuned by selecting the transient response of the manufactured instrument according to the reference ones tested by volunteers. Instead of calibrating instruments, many firms tighten the spectral characteristics of LEDs and photodiodes. An input control is carried out with a significant rejection of these elements, which makes it possible to guarantee acceptable accuracy without adjusting the computational algorithm. However, this increases the cost of the device by an order of magnitude. In addition, with this approach, it is impossible to take into account the aging of the LEDs. It is very difficult to repair the device, associated with the replacement of light-emitting elements. Considering that the global market for pulse oximeters currently exceeds 500 million US dollars, the proposed device allows you to get significant savings when checking and calibrating them.

Claims (3)

1. An optical-mechanical simulator of a pulsating flow of oxygenated blood, containing a flat filter with an optical radiation transmittance in the visible region τ _λ1 less than that in the infrared τλ ₂ (τ _λ1 <τ _λ2 ), characterized in that one additional filter is installed in the plane of the filter or several filters, the transmittance of which in the visible region is equal to or greater than in the infrared (τ _λ1 ≥ τ _λ2 ), while the filters are optically connected with adjustable diaphragms, made in the form of sectors and strengthened s on a transparent disk, which is connected to the rotation mechanism with an angular velocity equal to the heart rate. 2. The optical-mechanical simulator according to claim 1, characterized in that the diaphragms are made in the form of sectors connected to the center of the disk and have the property of fixing the rotation angle. 3. The optical-mechanical simulator according to claim 1, characterized in that the disk is optically connected to the sensor of the device under test.

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