Reciprocating supraventricular tachycardia initiated by paired premature beats.

Abstract

Two cases of paroxysmal supraventricular tachycardia were documented, and its underlying mechanism was discussed. 1) An antegrade A-V conduction pathway was found to be more refractory than a retrograde A-S conduction pathway because an antegrade A-V block was usually accompanied by a retrograde A-S conduction, resetting the sinus cycle. 2) A single supraventricular premature beat did not initiate paroxysmal tachcardia. 3) Paroxysmal tachycardia consisting of an alternate appearance of a retrograde P wave and a QRS complex of normal configuration (reciprocation) always followed a pair of two premature beats. 4) The first premature beat showed an aberrant ventricular conduction, indicating that the impulse spread the same way as that of the preceding sinus impulse. 5) The second premature impulse with an antegrade P wave showed a QRS complex of normal configuration similar to those appearing during paroxysmal tachycardia with a shorter coupling interval, indicating that the second premature impulse spread the pathway which had not been discharged by the preceding premature impulse. 6) Even when the reciprocating supraventricular tachycardia was suppressed by oculovagal reflex, these two premature beats appeared as a pair, indicating that the second premature impulse is causally linked to the first, i.e., an atrial echo via the S-A node. 7) It was inferred that this echo impulse via the S-A node spreads the pathway which has not been discharged by the first premature impulse and turns back to the atria through the pathway which has been discharged by the first premature impulse but has then recovered, thus initiating a sustained reciprocation of an impulse.