Significance of retrograde conduction in A-v dissociation.

In certain circumstances the discharge rate of the A-V node may exceed that of the S-A node. The A-V node then becomes the dominant pacemaker of the heart. If, in addition, there is retrograde unidirectional A-V block, the atria remain under control of the S-A node, and A-V dissociation results. When a sinus impulse reaches the A-V node during the non-refractory phase, it is conducted to, and captures control of, the ventricles for one beat-known as a capture beat; occasionally 2, or rarely 3, consecutive capture beats occur. An analogous situation may arise when the ectopic pacemaker is situated in the ventricles. This communication reports further observations on A-V nodal conduction in A-V dissociation, with special reference to conduction in consecutive capture beats.