Reciprocal rhythm in patients with normal electrocardiogram: Evidence for dual conduction pathways.

The present study refers to six patients in whom an A-V reciprocal rhythm could be documented; in four cases it took the form of sustained tachycardia. None of the patients showed any ECG feature of ventricular pre-excitation (PR interval of more than 0.12 sec. and normal QRS configuration). The extrastimulus method showed, at first, that the A-V conduction time of the premature beat varied only slightly with the decrease of the coupling interval. From a critical A1A2 interval there was a sudden lengthening of A2H2 preceding the occurrence of re-entrant beats. The curve of H1H2 responses reflected these changes, showing two distinct parts. The second part following the slowing of the impulse included the initiation zone of atrial echoes and of reciprocating tachycardia. These results suggest the existence of two A-V pathways, one fast and the other slow. The point at which the break between the two parts of the curve occurred might be related to the effective refractory period of the fast pathway. In the same way, when atrial pacing reached a critical rate, it induced an abrupt increase of AH in five cases. In the sixth patient, A-V conduction time remained unchanged up to 170 per minute. Ventriculoatrial conduction was always observed, the delay of which did not lengthen with the rate. In one case tachycardia could be induced by a premature ventricular beat without lengthening of the V-A time. It is concluded that in spite of a normal PR interval, the presence of dual A-V pathways may be implied in the genesis of reciprocal rhythm.