Wolff–Parkinson–White syndrome cardiac action potential arises in the sinoatrial (SA) node, which is located in the right atrium. From there, the electrical stimulus is transmitted via internodal pathways to the atrioventricular (AV) node. After a brief delay at the AV node, the stimulus travels through the bundle of His to the left and right bundle branches and then to the Purkinje fibers and the endocardium at the apex of the heart, then finally to the ventricular myocardium. The AV node serves an important function as a "gatekeeper", limiting the electrical activity that reaches the ventricles. In situations where the atria generate excessively
Sinoatrial block lengthening of conduction time from the SA node to the atria. The p-cells in the centre of the node produce the rhythm at a regular rate, but their conduction across the node to where it meets atrial tissue is where the slowing occurs. A second degree type II, or sinus exit block, is a regular rhythm that may be normal or slow. It is followed by a pause that is a multiple of the P-P interval usually (2-4) . Conduction across the SA node is normal until the time of the pause when it is blocked. A third degree sinoatrial