Sinoatrial-node entrance block.

Sinus-node function was characterized in 13 patients during cardiac catheterization by the introduction of progressively premature atrial depolarizations. Curves relating the extent of sinus-node depression following these premature depolarizations and their degree of prematurity were constructed. The maximum extent of sinus-node depression following premature depolarizations and A-V-nodal and atrial refractoriness were determined. Maximum sinus-node depression could be demonstrated to be independent of sinus rate. In six patients "early" atrial premature depolarizations demonstrated entrance block into the sinus-node pacemaker. Sinoatrial (SA) node entrance block could be clearly distinguished from atrial echoes and repetitive atrial firing subsequent to stimulation during the atrial relative refractory period. Prolonged refractoriness of tissues conducting impulses between atrium and sinus node was unrelated to parameters of sinus-node automaticity (sinus rate or maximum depression following premature atrial depolarizations) or to the refractoriness of the atrium itself.