All myocardial cells are able to spontaneously generate impulses and initiate the cardiac electrical cycle without the need for external stimulation; this is known as automaticity (Patton [155]). Cardiac conduction normally begins in the sinoatrial (SA) node (see Figure 14.6), located in the wall of the right atrium. Considered the heart's natural pacemaker, the SA node normally initiates impulses at a faster rate than other myocardial cells, generating impulses at a rate of 60–100 beats per minute (Waugh and Grant [207]). The impulse generated by the SA node spreads through the atrial muscle fibres (depolarization) to the atrioventricular (AV) node, causing atrial contraction as it spreads (Aehlert [3]).

The AV node acts as a gateway into the ventricular conduction system, delaying impulses for approximately 0.1–0.2 seconds and creating a short period of electrical standstill before the depolarization spreads through the AV node into the ventricles, allowing the atria to finish contracting before ventricular contraction commences (Waugh and Grant [207], Wesley [209]). From the AV node, the impulse travels rapidly through specialized conduction tissues in the ventricles, firstly through the bundle of His, along the left and right bundle branches, and then more slowly through the mass of ventricular muscle along the Purkinje fibres, resulting in a powerful ventricular contraction (Tortora and Derrickson [199]). See Figure 14.6 for the conduction pathway of the heart.

The normal ECG waveform depicts five deflections or waves known as P, Q, R, S and T waves (Figure 14.13). The P wave (which is small) reflects atrial depolarization, the QRS waves (which are large) reflect the rapid spread of depolarization from the AV node to the Purkinje fibres through the ventricles, and the T wave reflects ventricular repolarization – the return of the ventricular muscle to its resting state (Aehlert [3], SCST [183]). Atrial repolarization is not graphically represented on the ECG as it is hidden in the QRS complex (Wesley [209]).