Second degree block during reciprocal atrioventricular nodal tachycardia.

Of 67 patients with reciprocal atrioventricular (A-V) nodal tachycardia consecutively studied by programmed electrical stimulation of the heart, nine patients showed second degree block toward the ventricle and one patient toward the atrium during tachycardia. In four patients the occurrence of block was critically related to the prematurity of the test stimulus initiating the tachycardia. In three patients block developed following increase in rate of tachycardia. In two patients block could be elicited by introducing premature ventricular stimuli during tachycardia. Our observations indicate that different mechanisms may be responsible for second degree block during reciprocal supraventricular tachycardia. The finding of second degree block during reciprocal supraventricular tachycardia excludes a tachycardia with A-V conduction over the A-V node - His pathway and V-A conduction over an accessory A-V pathway.