Once the sinus node has generated an electrical stimulus, this must be transmitted through the atria, atrioventricular (AV) node and bundle of His to reach the ventricles and bring about cardiac contraction. The time delay while this occurs is mainly taken up by the passage of the electrical impulse through the AV node, which acts as a regulator of conduction. This corresponds to the PR interval on the ECG (Figure 12.1).