ELECTROCARDIOGRAPHY(ECG): AN INTRODUCTION

An Electrocardiograph or ECG is a medical device used to perform Electrocardiography which is the detection of the electrical impulses generated by the heart. It produces a graph representing the voltage of the impulses versus time. This information is interpreted by physicians or trained personnel. A lot of useful information can be obtained from the ECG readouts ranging from structural information about the heart to functioning of the cardiac nervous system.

Electrocardiographs typically measure these impulses with the help of 12 leads placed in specific positions. Since the impulses are weak signals (around 100 microvolts to 1 millivolt) we need specialised amplification. We use instrumentation amplifiers to acquire these signals, take the difference between them and amplifying these signals. Because of the low voltage signals, noise isolation is an important factor in the design of these devices. Circuits are included to filter out the 50/60Hz power line noise.

Typical ECGs consist of a wheeled cart on which the device is mounted. A screen and control devices such as keyboards or mouse are connected to it. The leads arise from this unit and is connected to the patient. A printer is often included with the unit.

Earlier devices used all analog circuits to drive a needle to print of graph paper. Modern ECGs use ADCs (Analog to Digital Converters) to obtain a digital signal. Digital signals can then be further manipulated and measured to obtain various other parameters and can apply complicated rhythm analysis algorithms on the signal automatically (although these results must be verified by a physician before taking action) thus enabling such ECGs to offer much more features than analog ones. Another advantage of digital signals is that they can be output easily to digital displays. They can also be recorded easily for later use digitally.

Being a medical device, it has to comply with various safety standards. The machine is usually powered by AC mains. The patient has to be isolated from the AC power source effectively with backup systems in place in case that fails. Failure to implement proper isolation can be fatal to the patient since the heart is very sensitive to AC frequencies. The operator also has to be isolated. ECGs are often connected to people who may require defibrillation. In case defibrillation is required, the ECG is not removed from the body as defibrillation is an emergency procedure and has to be administered as quickly as possible. In these cases the ECG has to be protected from the electrical discharge from the Defibrillator with special protective circuitry. Accidental electrostatic discharge also provides similar problems and requires similar protection.