1. INTRODUCTION
1.1 ECG general description
As the heart undergoes depolarization and repolarization, the electrical currents that are generated spread not only within the heart, but also throughout the body. This electrical activity generated by the heart can be measured by an array of electrodes placed on the body surface. The recorded tracing is called an electrocardiogram (ECG, or EKG). A "typical" ECG tracing is shown to the right. The different waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricles. The ECG is recorded at a speed of 25 mm/sec, and the voltages are calibrated so that 1 mV = 10 mm in the vertical direction. Therefore, each small 1-mm square represents 0.04 sec (40 msec) in time and 0.1 mV in voltage. Because the recording speed is standardized, one can calculate the heart rate from the intervals between different waves.
1.2 Limitations of ECG today
· The ECG reveals the heart rate and rhythm only during the time that the ECG is taken. If intermittent cardiac rhythm abnormalities are present, the ECG is likely to miss them. Ambulatory monitoring is needed to record transient arrhythmias.
· The ECG can often be normal or nearly normal in patients with undiagnosed coronary artery disease or other forms of heart disease (false negative results.)
· Many "abnormalities" that appear on the ECG turn out to have no medical significance after a thorough evaluation is done (false positive results).
2. ECG DESIGN
The ECG system is shown in the following figure. The ECG system comprises of following stages :
(1) The first stage is a transduce which converts ECG into electrical voltage. The voltage is generally in the range of 1 mV ~ 5 mV.
(2) The second stage is an instrumentation amplifier, which has a very high CMRR and high gain, with power supply +9V and -9V.
(3Third stage is to isolate the In-Amp and output.
(4) After the opto-coupler is a bandpass.It’s implemented by cascading a low-pass filter and a high pass filter.
Electrode ---------------> Amplifier--------------->Coupler
Output<---------------Filters
A wireless ECG has few more steps, it has RF transmitters and receivers and summing amplifiers. We would talk about its circuitry later.
The final step is displaying the output signal to the oscilloscope.
3. ECG SIGNALS
3.1 The Heart
The basic structure of the heart is shown on Figure 2. Measuring at different region of the heart will retrieve different biopotential. And, so that it will generate different ECG waveforms. The ECG generated by each cardiac cycle is also summarized.
3.2 Cardiac Cycle
There is a continuous cycle, called as cardiac cycle. A cardiac cycle’s summery is provided below
Event | Characteristics | Duration at 75 bpm (0.8 second cycle) |
Atrial diastole Ventricular diastole | AV valves opened. Semilunar valves close. Ventricular filling. | 0.4 seconds |
Atrial systole Ventricular diastole | AV valves open. Semilunar valves closed. Ventricular filling. | 0.1 seconds |
Atrial diastole Ventricular systole | AV valves closed. Semilunar valves open. Blood pumped into aorta and pulmonary artery. | 0.3 seconds |
Table 1
The ECG is converted into electrical voltage by electrodes. A typical surface electrode used for ECG recording is made of Ag/AgCl, as shown in the figure. The disposable electrodes are attached to the patients’ skin and can be easily removed.
The figure show above is the caridac mechanism of a ECG. It shows the pressure difference, ECG wave consisting of P, Q-R-S and T cycles. We measure the ECG by connecting two electrodes on the right and left chest respectively . The body should be connected to ground of the circuits, so that we connect the leg to the ground. Ifthe body is not grounded, no signal would be obtained. This is shown in the following figure.
4. CIRCUITS OF ECG SYSTEM
As discussed earlier, the ECG circuit consists of an amplifier, coupler and bandpass filters. Of course the measured ECG signals are dispayed on an oscilloscope. A wireless ECG has extra circuits of transmitter and receiver.
4.1 Amplifier
This is intended for low level signal amplification where low noise, low thermal and time drifts, high input impedance an accurate close loop gain are required. Besides, high CMRR and high slew rate are desirable for superior performance. As ECG signal is of very low amplitude, instrumentation amplifier is used to amplify it at initial level so that ECG signal will not get loaded. The two signals entering the differential amplifier are subtracted to cancel the common noise present in the signal. Because of instrumentation amplifier common noise gets cancel with advantage of strengthening the signal. A very high CMRR is very essential for Instrumentation Amplifier.The CMRR specified by the AAMI (Association for the Advancement of Medical Instrumentation) is 89 dB minimum for standard ECG and 60 dB minimum for ambulatory recorders.
4.2 Filters
It consists of high pass, low pass and band paas(notch) filters. In order to block DC offset present in the signal, we have to use High Pass Filter to block DC. If DC is not getting blocked at this level it will increase with gain of amplifiers connected after DC blocker, will induce noise .In order to provide gain of 2000 we have to use two amplifiers one is inverting and other is non-inverting.
4.2.1 Notch filters
In order toreduce 50Hz power line frequency notch filter is very essential. It should be highly precise at 50Hz. It is composed of one low pass filter to blocks, and one high pass filter.
The following circuitry is based on the facts and steps discussed above.
The circuit can now be connected to an oscilloscope or matlab display. We can trace a typical ECG on them which would consist of all the P, Q-R-T, S forms of waves & segments.
As dicussed in the introduction, there are some limitations of ECG which can be worked upon, thus making ECGs better than how it is being used today.
