Medical Pharmacology Topics   

The Electrocardiogram

A normal cardiac cycle starts with a pacemaker cell firing in the sinoatrial (SA) node. The impulse travels through the atria to the atrioventricular (AV) node. The depolarization of the atria is seen in the P-wave of the  electrocardiogram (ECG). The P-wave is relatively small because of the small atrial muscular mass.

The AV node provides a delay to the depolarizing wave, allowing the atria to completely empty into the ventricles before ventricular contraction. Once the action potential emerges from the AV node, the Purkinje system quickly transmit the pulse to the entire ventricular muscle.

The depolarization of ventricles is seen in the QRS complex of the ECG. The distance between the P-wave and QRS complex is the delay imposed by the AV node. A third T-wave indicates ventricular repolarization.