The ECG is an record of the electrical activity of the heart. Its utility stems from the specific changes in the trace caused by various pathologies.
The P waves represent atrial depolarisation. The PR interval is caused by the slow propagation of the depolarisation through the AVN; this allows time for the ventricles to fill. PR is measured from the start of P to the start of R. Once the depolarisation reaches the ventricles, conduction must be fast. The impulse passes though the following hierarchy of structures:
Ventricular depolarisation is recorded as the QRS complex. In practice, the Q, R and S waves are not always present.
The Q wave is defined as any initial downward deflection. The R wave is defined as any deflection upwards. The S wave is defined as any down deflection that is not Q. The T wave is the repolarisation.
Teams of Leads Inferior: II, III, aVF Anterior: I, aVL, V1, V2, V3
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