An atrioventricular block is a loss of the regular function of the cardiac electroconductive pathways linking the sinoatrial node (SA node) and the ventricles via conduction through the atrioventricular node (AV node). Third-degree AV block indicates a complete loss of communication between the atria and the ventricles. Without appropriate conduction through the AV node, the SA node cannot act to control the heart rate, and cardiac output can diminish secondary to loss of coordination of the atria and the ventricles. The condition can be fatal if not promptly treated.