4.4.1 Sinoatrial block
A sinoatrial block (SA) is a type of heart block which involves an impairment of conduction at the sinoatrial node. Sinoatrial block is delay or absence of the atrial beat due to partial or complete interference with the propagation of impulses from the sinoatrial node to the atria.
First-degree sinoatrial exit block indicates a prolonged conduction time from the SA node to the surrounding atrial tissue. It cannot be recognized on a standard (surface) ECG but requires invasive intracardiac recordings, which can detect this condition indirectly, by measuring the sinus response to atrial premature beats. Second-degree sinoatrial exit block denotes the intermittent insufficiency of conduction of sinus impulses to the surrounding atrial tissue. It is manifested as the intermittent absence of P waves. Third-degree or complete, sinoatrial block is characterized by a lack of atrial activity or by the presence of an ectopic subsidiary atrial pacemaker.