Right atrial versus left atrial echo zones: a proposed new criterion for determining the atrial site of retrograde preexcitation.

In a patient whose electrocardiogram (ECG) initially (1966) showed a Type A Wolff-Parkinson-White pattern, recurrent supraventricular tachycardia (SVT) developed but never subsequently showed antegrade bypass conduction. Intracardiac pacing studies (1975) revealed that premature high right atrial (induced 250-450 msec after atrial depolarization) or coronary sinus depolarization (250-550 msec) resulted in SVT. Late coronary sinus depolarization resulted in SVT without A-H prolongation. During SVT, P wave morphology changed and the coronary sinus atrial electrogram preceded that from the low right atrium; retrograde ventriculoatrial conduction time was 240 msec. Neither pacing the high right atrium or coronary sinus up to rates of 200 beats/min nor progressive atrial premature depolarizations from the high right atrium or coronary sinus resulted in antegrade bypass conduction. Failure of antegrade bypass conduction does not preclude SVT due to retrograde preexcitation and must be distinguished from atrioventricular (A-V) nodal reentry. Atrial effective refractory period (200 msec) was shorter than the minimal time required for an atrial impulse to return to the atrium (380 msec), suggesting concealed antegrade bypass conduction. Stimulation of the atrium linked to the A-V bypass results in earlier bypass activation and recovery and explains the differing high right atrial vs coronary sinus echo zones.