İki kanallı EKG monitörü

Abstract

Bu çalışmada, EKG işaretinin oluşması, işaretteki gürültülerin azaltılması yöntemleri incelenmiştir. Devre gerçekleştirilmesinde > hastadan elde edilen İşaretlerin uygun bir elektronik devre ile işlenerek elde edilen kanallardan ikisinin seçilmesi ve bu kanalların bir ADC vasıtasıyla bilgisayara aktarılması sağlanmıştır. Bilgisayarda, özel bir yazılım vasıtasıyla EKG işaretlerinin bilgisayar ekranında gerçek zamanda gösterilmesi, dosyaya kaydı, dosyadan geri çağırma t tekrar görüntüleme, işaretin istenen anda dondurulması ve istenilen bölümlerine ulaşılması sağlanmıştır. Gerçekleştirilen cihazın, tıbbi yönetmeliklere uyum göstermesine çalışılmıştır.

The electrical activitiy of human hearth can be detected on the body surface and, though quite small (about 1 mV), can be recorded as an electrocardiogram (ECG) Electrocardiography began late in the nineteenth century when, with the aid of cappillary electrometers, Burdon - Sanderson and Page obtained cardiac potentials from animals and Waller obtained these from humans. Higher fidelity measurements became possible when Eindhooven developed a string galvanometer to trace rather minute biopotentials. A few of Eindhoven's experimental techniques still survive,among them positioning of three electrode leads on the limbs or thorax in a closed equilateral triangle called Eindhoven's triangle. Geddes and Baker trace the fascinating history of biopotential recording and the instrumentation developed in the early days.The string galvanometer could detect electrical activity from the hearth but could not provide the amplification necessary for its clear visulation. Amplification became possible with the development of vacuum tube-based electronic components. Eventually, as in most other areas of electronics, transistors and integrated circuits (ICs) almost completely replaced the ancient string galvonometer and vacuum tube devices. Now, the circuits for ECG machines are designed sometimes from transistors, but mostly from special - purpose ICs. The invention of braun tube a precursor of the modern oscilloscope, made it possible to visualize biopotentials with higher fidelity and in real time. Although the oscilloscope is still used in patient monitors, modern vi ECG machines use digital sampling techniques to produce "nonfade" or scrolling displays. The first ECG recorders employed waxed paper, and later a carbon -black coated surface, to acquire visible and recorded copies of ECG signals. These older recorders have now been replaced by chart-paper recorders by ink, thermal, or laser - based techniques to produce permanant copies of excellent quality. Computers have had perhaps the greatest impact on ECG technology, making automatics data acquisition, analysis, and diagnostic routine. The electrocardiogram has considerable diagnostic significance, and applications of ECG monitoring are diverse and in wide use. For example, a diagnostic ECG recording can be made in a doctor's office in a routine checkup, during which a full 12-lead ECG is taking from resting patient and recorded on chart paper to diagnose cardiovascular disease. At this realized unit a patient's 2-lead ECG are continuously displayed on a personal computer - monitor. 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 instance in the cardiac cycle at which to deliver a high voltage, defibrillating shock. Modern pacemakers and implantable defibrillators also require ECG acquisition capabilities. The primary function of the realized ECG system is to amplify the electrical signal from the heart, and reject enviromental and biological noise and artifacts. The differential amplifiers are commonly employed for this reason. The electrical signal from the heart is considered a differential signal and is amplified. The power line interference picked-up by the body is considered a common mode signal and is rejected. The instrumentation amplifiers used at realized unit are advanced types of differential amplifiers which have many additional desirable characteristics (such as a high - input impedance ). The instrumentation amplifier design is now commonly used vli in many biomedical applications. There is also a stage which limits the amplifier response to the desired frequency range and supplies the amplified signals to the non isolated section to the personal computer for display and recording. This ECG monitor system is suitable for use on patients hazardous enviroments such as cardiac intensive care. Consequently, the amplifier designed to limit the rise current flowing into the patient. This is achieved by careful design of front - end of the amplifier and isolation of the circuit from the leakage paths to the ground. When a differential amplifier records biopotentials, the voltage of patient with respect to the amplifier's common is called common mode voltage Vc. Since Vc can be transformed by amplifier into an interfering differential signal, it is desirable to minimize Vc by attaching a electrode to the right leg of the patient. This electrode provides low impedance path between the patient and the amplifier common so that Vc is small. Connecting the electrode directly to the common is undesirable for two reasons. 1) If the circuit is not isolated, dangerous currents could flow though the right leg electrode. 2) A poor electrode contact may present up to 100 K.ohm of resistance between the patient and common. Most common and effective use of right leg electrode is to connect it to a driven - right - leg circuit. This circuit overcomes both of problems listed above. It reduces the effective electrode resistance by several orders of magnitude, and it allows only safe amount of current to flow through the right leg electrode. Although the circuit is used extensively in modern biopotential amplifiers, little has been written on optimal design technique, presented design approach that results in minimization of Vc. Hovever, a non optimal driven - leg - circuit may seem to work as well as one that minimizes Vc because : 1) the non optimal circuit reduces the interference to a level that is below the sensitivity of recorder, or 2) other sources create more interference than Vc. vi i i Manually selected two of twelve channel ECG signals are monitorized at real time at the meanwhile signals are recorded a file, remonitorized of recorded signals, accessibility to desired region of signals by specific software There are two monitoring mode, these are ;normal mode and narrow mode Monitoring durations are 5 sec at normal mode 9.5 sec at narrow mode. Specifications of used components in the realized circuitry are mentioned at the end of the report. There are many factors that must be into consideration in the design and application of the electrocardiograph. Not only must the medical engineer be aware of these factors, but also the individual who operates the electreocardiograph must be aware of them. In the following paragraphs, I shall describe a few of the more common problems encountered, and indicate some of their causes. Frequency distortion : The electrocardiograph does not always meet the frequency - response standarts. When this happens, frequency distortion is seen in the ECG. At the realized system frequency response is -3dB between 0,04 Hz and 125 Hz.. If signal is high frequency distorted then signal is in rounding off the sharp corners of the waweforms, as well as in diminishing the amplitude of the QRS complex. If signal is low frequency distorted then distortion is at the base line. Saturation or cutoff distortion : High offset voltages at the electrodes or improperly adjusted amplifiers in the electrocardiography can produce saturation or cutoff distortion that can greately modified the appearance of the ECG. In this case, the combination of input - signal amplitude and offset voltage drives a portion of the amplifier in to saturation. The peaks of QRS complex are thereby cut off, since the output of the amplifier cannot exceed the saturation voltage. At the realized system is quite satisfy these conditions. ix Technical specifications of system are as folows : Electrical isolation : 5000 V dc Leakage current from the power line : 2 uA Isolation capaticance : 7 pF Input impedance :70M.ohm (approximately) Amplifier gain :50 - 60 dB (adjustable) Maximum input signal can be amplified : 15 mVpp Common mode rejetion ratio :78 dB Frequency response :0.04 Hz - 125 Hz (-3 dB) As seen above specifications, values quite satisfy the ECG safety regulations.