Loud first heart sound with long P-R intervals

A case of partial atrioventricular block with Wenckebach periods and a loud first heart sound (S1) associated with the longest P-R interval of the cardiac cycles is described. Although the surface electrocardiogram provided no clues to this paradoxical behavior, the intraatrial electrogram revealed that the long P-R interval with the loud S1 was accompanied by atrial echoes (reciprocal atrial beats). The loud S1 with a very long P-R interval was always caused by an atrial contraction synchronous with the QRS complex.