Dual-channel optical pulse oximeter on the base of laser sensors to solve actual problems in medical practice
Pulse oximetry method as the diagnostic measurement of blood oxygen saturation and blood oxygen level is nowadays rather perspective in use and researches in medicine.
As much as therapy problem (registration of the result "effect-the body’s reaction", i. e. response) becomes so important, blood level registration during therapy is one of the most important parameters ("responses") of therapeutic effect [2, 3]. Signal isolation of photobiological effect is the main problem using therapeutic, physiotherapeutic, diagnostic laser and other equipment. The challenge of response "effect-the body’s reaction" realization is the creation of metrologic methods due to this tasks, medical and technical development of technologies and their application.
Existing urgent task of the body’s response adequate parameters on therapy effect including electro-light-laser effect is the part of the main problem of multiparametric biodosimetry. 1) determination of functional dependence between different exposure parameters and correlated body’s response parameters; 2) application of this functional dependence for exposure parameter control.
That’s why pulse oximetry method of blood oxygen level after certain therapy effect is the most important feedback (the body’s response) and it contains information for further activities and biodosimetric control.
As is known the main purpose of pulse oximeter is timely detection and prevention of patient’s hypoxia (low tissue oxygenation) and anoxemia (low blood oxygenation). One of the pulse oximeter’s advantages is a possibility to provide noninvasive, continuous, safe and effective patient’s blood research. Normal oxygen content in healthy man’s blood is 90–100%, so change even 1 percent affects all body’s physical state. That’s why there is a necessity in exact determination of this parameter in clinical conditions, which has been got by calculation on the base of photoplethysmogram. (FPG) [4, 5].
At present new biocontrolled dosimetric methods of therapy effect parameters, including electro-light-laser therapy, are in demand . Electro-light-laser therapeutic procedure performing is closely related to the exposure quantity on the biological body and individual adequacy control of the effective quantity taken by the body (Arndt-Shulz law). Application of pulse oximeter as a body state recorder helps doctor not only to fix qualitative changes during therapy period but to automate therapy process. Photoplethysmogram here shows blood supply dynamics of the body. It must be mentioned, that a lot of existing pulse oximeters cannot always satisfy medician’s demands in precision characteristics, there is no feedback from physiotherapeutic device when certain values are achieved as so as recorded data dynamics. So we have another urgent medical problem –automation of the treatment process, in other words, there is a need in automated workplace of the doctor (AWP), which is in demand .
As of this writing there have already been articles about dual-channel optical pulse oximeter’s application (on the base of LED sensors (λ1 ≈ 940 ± 20 nm; λ2 ≈ 640 ± 20 nm)). Application of these sensors in pulse oximeters leads to "physiological disturbance" during FPG- signal recording because of the difference in light absorption by the main parts of blood structure- oxy- and deoxyhemoglobin. Consequentlyо, there is impossible to get required accuracy-less 5%, frequency and adequacy of FPG- signal (important biosignal parameters). Such pulse oximeters are useless for the medical goals, for example, in biodosimetry, that’s why the third kind of wavelength has to be used (λ = 805 ± 0,75 nm) [1, 8, 9]. Emission wavelength 804,25…805,75 nm is "isobestic point" for oxy- and deoxyhemoglobin, i. e. there is the wavelength, where spectral characteristics of these components match. So, we can avoid "physiological disturbance" on photoplethysmogram, use it for therapeutic parameter optimization and for automation of the treatment process with the help of photopletismographic dosimeter. A lot of researches showed that in modern models of pulse oximeters the reason of pulseoximeter’s "alarm" activation mainly lies in artifacts, not in real danger. The simplest models of pulse oximeters without special disturbance safe systems often show artifact information. But even in expensive models from known companies you can get response of device on artifact stronger than on the real danger.
Application of pulse oximetry method in medicine has not solved problems related to low perfusion of peripheral blood veins, so some clinical or technical circumstances can hinder the process of getting the data and to interpret them. All above concerning medical "desires’ must be fulfilled in new developed pulse oximeters.
TECHNICAL GOALS SETTING
Well, let emphasize and generalize some medical practical problems we see while using pulse oximetry, and they are not solved yet in a lot of modern models of pulseoximeters:
• some clinical or technical circumstances can hinder the process of getting the pulse oximeter’s data and to interpret them;
• low perfusion of peripheral blood veins
• sensibility to artifacts;
• impossibility to integrate pulse oximeters into automated doctor’s workplaces.
The author of present article has analysed the ways of solutions of the problems above and as a result new pulse oximeter PSO-2CL (dual-channel optical pulse oximeter) was developed.
Main parameters comparison between modern devices and the author’s one (Table).Having analyzed data from Table1 we can resume, that developed pulse oximeter PSO-2CL is not only the same in main parameters of such devices, but is the better in some of them. It has been reached by application of some new technical methods, given below.
METHODOLOGICAL BASE OF OPERATION PRINCIPLE OF PSO-2CL
The developed pulse oximeter principle of operation is based on optical pletismography method. An optical pletismography method is based on the fact that some biological subject facilities are associated with its optical characteristics (coefficient of absorption, scattering, refraction, etc.) and changes of body’s facilities leads to changes in its optical characteristics. In PSO-2CL the radiation of wavelength range from 640 to 1100 nm, is used, so called "therapy window" . This radiation goes through body’s tissue and is modulated by tissue optical characteristics changes because of body’s physiologic processes. In PSO-2CL large veins pulse blood supply is registered. PSO-2CL registers the intensity modulation of optical radiation, caused by coefficient of transmittance. This modulation is directly associated with pulse blood supply, so we can estimate the blood circulation intensity due to the change curve of recorded radiation intensity threw the body’s tissue.
Optical pletismography method is divided into two groups depended on plethysmogram registration conditions: registration of light signal and back scattering from turbid environ using Lambert law. While registration of light signal coefficient of attenuation makes the greatest contribution into coefficient of transmittance of the body. While registration of and back light scattering – the contributions of coefficients of absorption, scattering and refraction are equal. Examples of pulse oximeter constructions with different types of optical pairs (light source+ photodetector) are shown on the Pict.1 and Pict.2.
Each group has its positive and negative sides. One of the most important parameters is a signal intensity, getting from the body. получаемого сигнала. Light quantity going through thin tissues is more than back scattering quantity, besides light going through tissue is located in certain place. Consequently, light intensity is higher in transmittance sensors.
The main disadvantage of transmittance sensors is connected with its limited application only for peripheral parts of the body – fingers, toes, earlobes, nose. Back light scattering pulse oximeters can be located on the any place of the body.
There are following methods to eliminate the errors in modern pulse oximeters:
• shielding of sensor and device cable (to avoid detector brightening and effect of closely located electromagnetic equipment);
• parameter indication averaging;
• sensor sensibility increase;
• increase of photodetector "signal/noise" ratio, etc.
However the most serious problem is the artifact as the body’s moving, especially of its part with the sensor. It will lead to venous addition and as a result to photoplethysmogram disturbance. This artifact causes serious errors, and some methods and signs were designed to avoid it:
• pulse signal of environing tissues and following changes of the base signals;
• the difference between artifact signal and "normal" signals in wave channel;
• calibration curve special construction for the moving body when breathing exact quantities of oxygen.
But in addition to all methods above we can observe another one-accelerometer. Application of such kind of devices is widely used in different systems. Accelerometer measures acceleration of moving body. Using up to 3 components we can consider acceleration for each of three coordinate axis.
JUSTIFICATION AND STRUCTURE OF DEVELOPED DUAL-CHANNEL OPTICAL PULSE OXIMETER
Some new technical decisions were used in proposed dual-channel optical pulse oximeter PSO-2CL (structure diagram is shown on Picture 3), with the help of wich some tasks can be solved. First to decrease artifact effect significantly so to get stable measurements and useful information. Therefore proposed dual-channel optical pulse oximeter can provide high-precision recording of blood vessels state, blood supply, vein pathology on certain places of the body, that is very important at the beginning of decease, it can fix these data and transfer them to computer system of AWP.
The choice of sensor light source. LED (light emission diodes) are the most popular at developers because they are cheaper than lasers. That’s why they are widely used in pulse oximeters with low-precision data. Such measurement instruments are not allowed for exact researches and patient state monitoring under treatment. LED"s main disadvantage is connected with a disability to give narrow light stream with minimal deviation (±5ч10 nm). Lasers succeed in it. Besides they can provide the stronger light stream, that leads to the stronger signal on detector. Moreover, laser can work in different states- as in constant, so in pulsed, in order to avoid laser fatal effect to biological material.
Developed PSO-2CL is composed of several blockes, including laser sensor or pulse oximeter sensor, connected with device with cable (Pic.4):
• pulse oximeter sensor (SPO);
• block processing of medical information (DPMI);
• display (D);
• power supply (PS).
NEW TECHNICAL SOLUTIONS IN PSO-2CL.
PSO-2CL registers photoplethysmogram with the help of laser sensor. The first converter of optical radiation intensity into electrical current is photodiode in SPO. In industrial photoplethysmographs as well as in pulse oximeters the detector does not have photodiode signal amplification system, that’s why registered signal quality depends on cable, connecting sensor and device (cable length, number of screens, kind of dielectric, determinating triboelectric effect level) as well as pulse current value for light diode acting. To avoid the problem in developed PSO-2CL the amplifier is located in SPO. This approach allows to decrease some requires to the cable, connecting SPO and device, and to minimize laser diode pulse current effect to photoplethysmogram signal.
Photoplethysmogram signal as it was already mentioned is strongly influenced by moving patient artifact. Different producers of photoplethysmographs and pulse oximeters use various methods of photoplethysmogram signal treatment and verification. These methods serve for patient moving artifact detection by signal analyses. It must be mentioned that artifact detection is rather complicated task for producers as movement artifact has to be distinguished from arrhythmia. Accelerometer used in PSO-2CL in SPO allows to simplify photoplethysmogram signal treatment and verification and in the same time to increase signal validity and to distinguish movement artifact from arrhythmia. Accelerometer in SPO records acceleration value because of the patient moving. Directions of this moving can be different, that’s why triaxis accelerometer is taken for SPO.
Photoplethysmogram signal is registered with the help of laser SPO located on certain places of the body. SPO consists of two semiconductive laser diodes and one photodiode which records optical radiation from patient tissue – transmitted, reflected and diffused. Signal goes from photodiode to DPMI where photoplethysmogram becomes digital. From DPMI output digital signal goes to display D, where user can get information in convenient image.
Power supply PS is needed for DPMI and D supply (constant currency). Incoming voltage for PS is alternating current with voltage 220V and 50Hz frequency.
There are two laser diodes (LD), one photodiode (PhD), photosignal amplifier (APS) based on operational amplifiers, one accelerometer (A) in SPO composition.
Two LD and one PhD form two optical pairs. LD transmit optical impulse in certain moment, and impulses from these two LD are shifted in time. Accelerometer (А) is used for moving artifact detection.
Block processing of medical information (DPMI) of pulse oximeter sensor (SPO) (Pic. 6) consists of photosignal amplifier (APS), two current sources precision (CSP), analog-to-digital converter (ADC), synchronizing device (SD), data processing device (DPD), random-access memory (RAM), read-only memory (ROM), display (D).
Analyses of modern methods of the human body state control graphic registration showed that application of dependence between therapeutic parameters and photoplethysmogram and further conclusions based on blood oxygen level value is one of the most informative methods for medical tasks. Pulse oximetry is based on photoplethysmographical method, which let pulse oximetry to develop. Pulse oximetry is a noninvasive method of measurement, easy in use, available, rather high-precision, providing information about blood oxygen saturation, heart frequency and pulse.
In spite of some difficulties, new ways were found in order to solve some medical problems. These ways are based on signal detection as a response to photobiological effect, further activities with physiotreatment biodosimetric control (including transfer to AWP). After analyses of different types of pulse oximeters the optimal kind of the pulse oximeters was chosen as well as surface signal detector, working with back diffused light using three laser radiation wavelength (working- λ=640 nm, λ = 940 nm; main – λ = 805 nm).
While working on PSO-2CL existing actual problems of medical practice were taken into account, we have told about them in this article.
Metrological methodic based on existing tasks allowed to provide optical pulse oximeter with characteristics better than in some of modern analogues.
The author’s device PSO-2CL has characteristics better than in some of modern analogues and has new technical solutions: in pulse oximeter sensor SPO laser diode is used as a light source. Thanks to laser diode pulse blood supply of local part of vein can be exactly registered. Thanks to new technical solutions in PSO-2CL-triaxis accelerometer in detector- we can avoid from movement artifact as the main reason of disturbance. The photodiode light brightening SPO level is decreased.
In PSO-2CL there is a plethysmogram curve treatment with the help of contour analyses for the aim of registration of peripheral vessels signal especially for vessels with low perfusion. We can get such parameters as stiffness index, reflection index, augmentation index, vessel age index, pulse frequency.
Developed optical pulse oximeter PSO-2CL can be included into the doctor’s automated workplace by data transfer to computer system for biodosimetric control purposes.
Author is very thankful to the Technical Director of "Yarovit-Yar" Company A. P. Kuzmich and d. t.s. prof. A. I. Larushin for the help in testing of dual-channel optical pulse oximeter PSO-2CL.