Atrial depolarisation is transmitted to the ventricular myocardium by the AV node and intraventricular conducting system. The time between the onset of atrial depolarisation and the release of depolarisation into the ventricular myocardium from the terminal branches of the conducting system is represented by the PR interval on the ECG. Dysfunction of the AV node or diffuse damage to components of the ventricular conducting system can result in a delay or even failure of transmission of atrial depolarisation into the ventricular muscle mass. This situation is referred to as atrioventricular or AV block. Three degrees of AV block are recognised. First degree AV block is defined by transmission of all P waves to the ventricular myocardium but with prolongation of the PR interval beyond the upper limit of normal on the ECG. Second degree AV block is defined by failure of conduction of some P waves into the ventricles. In third degree or ‘complete’ AV block, no P waves are transmitted to the ventricular myocardium.