An Electrocardiogram (ECG) is a diagnostic tool used to record the electrical activity of the heart over a period of time. It is a “graphical representation” of the electrical impulses that cause the heart muscles to contract and relax.
The Standard ECG Graph
A typical ECG consists of five waves, designated as P, Q, R, S, and T. Each part of the wave corresponds to a specific electrical event in the cardiac cycle.
The P-Wave
Atrial Depolarization – the electrical excitation that leads to the contraction of both the atria (left and right).
The QRS Complex
Ventricular Depolarization – represents the spread of the impulse through the ventricles, leading to ventricular contraction (systole).
The contraction starts shortly after Q. By counting the number of QRS complexes in a given time period, one can determine the heart rate of an individual.
The T-Wave
Ventricular Repolarization – marks the return of the ventricles from an excited state to a normal state (diastole). The end of the T-wave marks the end of the systole.
Clinical Significance
Deviation from the standard shape of the wave indicates a possible abnormality or disease. For example:
- An enlarged P-wave might indicate an enlargement of the atria.
- An elevated ST segment can be a sign of a myocardial infarction (heart attack).
How it is Recorded (The Setup)
To obtain a standard ECG, a patient is connected to the machine with three electrical leads:
- One on each wrist (left and right).
- One on the left ankle.
For a more detailed evaluation of heart function, multiple leads are attached to the chest region, but the “three-lead” setup is the standard for basic monitoring.
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