Electrocardiogram (ECG) – recording electrical signals from the heart

Electrocardiogram (ECG) – recording electrical signals from the heart

The heart generates its own electrical signals which are recorded by an ECG machine (electrocardiograph). The tracing or record is known as an electrocardiogram. The electrical signals control the orderly contraction of the heart in a regular manner to ensure smooth flow of blood into various parts of the body. The modern electrocardiograph was invented more than a century back. An electrocardiograph records the electrical activity of the heart from the surface of the body. It is one of the simplest and widely available investigations to assess the heart. ECG is an important part of cardiac evaluation, especially in the emergency room when someone presents with chest pain.

In spite of the availability of many modern high tech investigations, the role of a simple ECG in the diagnosis of acute myocardial infarction (heart attack) still holds good. Other than heart attack, ECG is useful in getting knowledge about the nature of various parts of the heart and its function. If certain chambers of the heart are enlarged, it is manifested with a specific pattern in the ECG. Disorders of cardiac rhythm have their on electrocardiographic patterns, depending on the site of the abnormality. Abnormalities in the pathways of conduction of electrical signals and blockage of the signals in these pathways all have their own signatures on the ECG. Even abnormalities in the electrolytes in the blood like low potassium levels, high potassium levels, low calcium levels etc manifest as abnormalities in the electrocardiogram as well, because of their effects on the genesis and conduction of electrical signals of the heart.

An ECG machine has several electrodes (leads) to collect the electrical signals from the body and an amplifier to magnify the signals. The signals are then fed to a strip chart recorder which records the electrical signals on a moving graph paper known as ECG paper. Simple ECG machines record only one channel at a time and are known as single channel ECG machines.

Sophisticated machines can simultaneously record 3, 6 or even 12 channels. Conventional single channel ECG paper has a width of 5 cm and a recording area width of 4 cm. The paper moves at a speed of 25 mm/sec. The paper has a heat sensitive coating which is marked by a heated stylus for the recording. The recording stylus is attached to a moving coil galvanometer and movement of the stylus corresponds to the instantaneous voltage detected by the galvanometer. Modern day electrocardiographs have digital acquisition of ECG data and prints them even on usual printing paper. Many of the modern ECG machines also have diagnostic algorithms incorporated in them so that they give printed interpretations as well. Though the printed information is often useful regarding the various electrocardiographic intervals, you have to be careful with the diagnostic reports which may not tally well with the clinical situation. The diagnostic ability of different commercially available softwares also vary widely.