Each portion of a heartbeat produces a different deflection on the ECG. These deflections are recorded as a series of positive and negative waves. On a normal ECG, there are typically up to five visible waveforms:
- the P wave,
- the Q wave,
- the R wave,
- the S wave,
- and the T wave.
The P Wave
The first deflection of the heartbeat is a small upward wave called the P wave. It indicates atrial depolarisation. The initial portion of the P wave is largely a reflection of right atrial depolarisation and the terminal portion is largely a reflection of left atrial depolarisation.
A fraction of a second after the P wave begins, the atria contract.
The P waves should all look alike and be no larger than 0.3mV (3 mm). Taller morphologies may indicate right atrial enlargement. Wider (or 'm-shaped') morphologies may be caused by left atrial enlargement.
The Q Wave
When visible, the Q wave is any initial downward deflection after the P wave. The normal Q wave represents septal depolarisation.
The Q wave that is seen following a heart attack may be wide and deep. The dead muscle neither conducts nor produces current, so the ECG picks up current flowing away from this muscle, producing a strong negative deflection.
The R Wave
The R wave is the first upward deflection after the P wave (even when Q waves are absent). The R wave is normally the easiest waveform to identify on the ECG and represents early ventricular depolarisation.
The R wave may be enlarged with ventricular hypertrophy, a thin chest wall or with an athletic physique. It may be reduced by a variety of mechanisms including obesity.
The S Wave
The S wave is the first negative deflection after the R wave. It represents the late ventricular depolarisation.
The T Wave
The T wave represents repolarisation of the ventricles. It is normally upright, somewhat rounded, and slightly asymmetric. Its morphology will alter with breath holding and digitalis toxicity.
The T wave may be inverted or flat with myocardial ischaemia, bundle branch block, ventricular hypertrophy, and ventricular ectopic beats. It is tall and peaked with hyperkalaemia (potassium decreases the duration of the refractory period and enhances repolarisation).
The T wave is flat and notched with conditions such as pericarditis, hypothyroid, and cardiomyopathies and flat with hypokalaemia.