Biyolojik işaretlerin elde edilip işlenmesi

Abstract

Çağdaş yaşamımızın vazgeçilmez bir parçası haline gelen elektronik, biyolojik işaretlerin elde edilip işlenmesinde de pratik çözümler temin etmektedir. İşaret elde etme ve işleme sistemleri, analog olan -fiziksel parametrelerin bulunduğu gerçek dünya ile sayısal hesaplamanın ve kontrolün gerçekleştirildiği yapay dünya arasındaki irtibatı sağlarlar. Sayısal sistemlerdeki mevcut kabiliyet, bu bağlaşım fonksiyonunu önemli kılmıştır. Zira sayısal sistemler karmaşık devreleri çok ucuz, doğru ve nispeten basit bir şekilde gerçekleştirmektedirler ve yaygın olarak kullanılmaktadırlar. Biyolojik işaretlerin özellikleri ismi altında özellikle EKG işaretlerinin oluşumu ve algılama yöntemleri 2. Bölüm 'de, tıp elek troniğinde kullanılan kuvvetlendiriciler başlığı altında kuvvetlen diricilerin genel özellikleri, enstrumantasyon ve izolasyon kuvvet lendiricileri ve CMRR kavramı 3. Bölüm 'de, işaret elde etme teknikleri ismi altında tipik bir bilgi elde etme ve dağıtma sisteminin temel elemanları da 4. Bölüm 'de izah edilmiştir. Yukarıdaki bölümlerde anlatılanları bünyesinde toplayan 'Sistem Tasarımı' isimli 5. Bölümde, hastalara teşhis konulmasında yardımcı olan oniki kanallı EKG işaretlerinin -f filtrelenmesi, örneklenmesi, sayısala çevrilmesi ve bilgisayara gönderilip ekranda görüntülenme si açıklanmıştır. Seçilen kanaldaki EKG işaretinin bilgisayarın monitöründe görüntülenebilmesi için gerekli donanım ve yazılım detaylı bir şekilde izah edilmiştir.

Data acquisition and conversion systems interface between the real world of physical signals, which are analog, and the artifi cial world of digital computation and control. With current empha sis on digital systems, the interfacing function has become an im portant one; digital systems are used widely because complex cir cuits are low cost, accurate, and relatively simple to implement. In addition, there is rapid growth in use of minicomputers and microcomputers to perform difficult digital control and measurement functions. In this study, we are especially dealing with ECG signals. The electrocardiogram (ECG) has considerable diagnostic signifi cance, and applications of ECG monitoring are diverse and in wide use. For example, a diagnostic ECG recording can be made in a doc tor's office in a routine checkup, during which a full 12-lead ECG is take from a resting patient and recorded on chart paper to diag nose cardiovascular diseases. In cardiac intensive care units, a patient's 1-lead ECG may be continuously displayed on a cathode ray tube and monitored for signs of cardiac distress. ECG monitoring capability is incorporated into various other devices, some of which are simple, like the cardiotachometer, which measures heart rate. Some are more complex, like the automatic defibrillator, which must acquire ECG signals to determine both the absence of normal sinus rhythm and the correct instant in the cardiac cycle at which to deliver a high voltage, def ibrillating shock. Modern pacemakers and implantable defibrillators also require ECG acquisi tion capabilities. IV ECG acquisition now finds widespread application in patient monitoring. Some examples of cardiac monitors are portable and battery-powered devices, heart rate monitors used in operating rooms and intensive care units, devices and systems that use tele metry, and other complex devices (such as defibrillators and arrhythmia monitors) that have ECG monitors integral to their de sign. The most common use of ECG monitors occurs in cardiac inten sive care units in hospital. The ECG monitor is used at the pa tient's bedside, usually the ECG from one lead is continuously monitored. In conjunction, other vital signs such as respiration, temperature, and blood pressure may be monitored too. This device may feature a continuous display of the ECG waveform on a cathode ray tube (CRT), as well as a display of heart rate and the status of electrode and lead connections. The nursing staff of the inten sive care unit monitors the patients' ECG and other vital signs for any signs of distress. The primary function of the ECG acquisition system is to amp lify the electrical signal from the heart, and reject environ mental and biological noise and artifacts. A differantial ampli fier is commonly employed for that reason. The electrical signal from the heart is considered a differantial signal and is amp lified. The power-line interference picked up by the body is con sidered a common-mode signal and is rejected. An instrumentation amplifier which has many additional desirable characteristics (such as a high-input impedance). The instrumentation amplifier design is now commonly used in many biomedical applications. The final amplifier stage limits the amplifier response to the de sired frequency range and supplies the amplified signals to the recording signals such as the ECG. An even more important consideration is protection of the sub ject. The guidelines for protection of patients from electrocution are quite severe; only currents less than 10 ^iA may be allowed to pass through the patient. This is a particularly important consider ation for patients who may have a very low resistance path to the heart for electrical currents, such as those being monitored via intracardiac catheters. The principal defense against electrical current leakage is the use of electrical isolation. The electrical isolation circuits introduce a high-impedance and low leakage cir cuit between the front-end amplifiers attached to the patient and the rest of the output circuits and other monitoring instruments. The isolation circuits also protect the front-end electronics from inadvertent high voltages on the patient's body. The most important specifications for the isolation components, optical or t ranu former- based, are (1) ability to withstand defibrillation voltages, (2) very small leakage capacitance across the isolation barrier, and (3) minimum distortion of the signal and no pickup of stray elec trical interference. Consequently, the ECG signal belonging to the channel which is selected by thumbwheel switch is switched with multiplexer. After sampled ECG signal passed through the isolation amplifier which is used as an instrumentation amplifier, it is filtered by low-pass and notch filter. In the digital part of the system which is under control of the microcontroller, analog-to-digital (A/D) converter converts the ECG signal into digital form. Then the microcontroller sends the resultant digital word to a personal computer data bus to monitor the ECG waveform on the secreen by means of program written VI 11 in 'Pascal' programming language and to the digital-to-analog (D/A) converter to monitor the signal on the oscilloscope. ix recording signals such as the ECG. An even more important consideration is protection of the sub ject. The guidelines for protection of patients from electrocution are quite severe; only currents less than 10 ^iA may be allowed to pass through the patient. This is a particularly important consider ation for patients who may have a very low resistance path to the heart for electrical currents, such as those being monitored via intracardiac catheters. The principal defense against electrical current leakage is the use of electrical isolation. The electrical isolation circuits introduce a high-impedance and low leakage cir cuit between the front-end amplifiers attached to the patient and the rest of the output circuits and other monitoring instruments. The isolation circuits also protect the front-end electronics from inadvertent high voltages on the patient's body. The most important specifications for the isolation components, optical or t ranu former- based, are (1) ability to withstand defibrillation voltages, (2) very small leakage capacitance across the isolation barrier, and (3) minimum distortion of the signal and no pickup of stray elec trical interference. Consequently, the ECG signal belonging to the channel which is selected by thumbwheel switch is switched with multiplexer. After sampled ECG signal passed through the isolation amplifier which is used as an instrumentation amplifier, it is filtered by low-pass and notch filter. In the digital part of the system which is under control of the microcontroller, analog-to-digital (A/D) converter converts the ECG signal into digital form. Then the microcontroller sends the resultant digital word to a personal computer data bus to monitor the ECG waveform on the secreen by means of program written VI 11 in 'Pascal' programming language and to the digital-to-analog (D/A) converter to monitor the signal on the oscilloscope.