Sinus node re-entry and sinus node tachycardia.

Five patients are reported with SN echoes which could be produced by the technique of APD. The RA was paced at the basic rate and the SEI was measured repeatedly. SN echoes were diagnosed on the basis of: (1) A1A3 interval shorter than the SEI; (2) upright P-waves in Leads II and III; (3) activation of high RA preceding the activation of low RA; (4) lack relation to critical delay in the A-V node or HPS; (5) definite echo zone. In one of the cases, attacks of reciprocating tachycardia through the SN occurred spontaneously and also could be initiated by an SN echo. These were terminated by a single APD or by atrial pacing.