Demonstration of dual atrioventricular nodal pathways utilizing a ventricular extrastimulus in patients with atrioventricular nodel re entrant paroxysmal supraventricular tachycardia

In patients with atrioventricular (A V) nodal re entrant paroxysmal supraventricular tachycardia (PSVT), atrial extrastimulus technique frequently reveals discontinuous A₁-A₂, H₁-H₂ curves suggestive of dual A V nodal pathways. To further test the hypothesis that these curves in fact reflect dual A V nodal pathways, a ventricular extrastimulus (V₈) was coupled either to A₂ at a fixed A₁-A₂ interval which reliably produced an A V nodal re entrant atrial echo (E) with a constant A₂-E interval in two patients, or to QRS complex (V) during sustained PSVT with a constant E E interval in one patient. Three response zones were defined: at longer A₂-V₈ or V-V₈ coupling intervals, V₈ manifested no effect on the timing of E (Zone 1). At closer A₂-V₈ or V-V₈ coupling interval, V₈ conducted to the atrium, shortening the apparent A₂-E or E E interval (Zone 2). At shortest A₂-V₈ or V-V₈ coupling interval V₈ was blocked retrogradely, and no E was induced (Zone 3). The ability of V₈ to preempt control of the atria (Zone 2 response) strongly suggests the presence of dual A V nodal pathways in these PSVT patients. If only a single pathway were present, V₈ would of necessity collide with the antegrade impulse and could not reach the atria. The Zone 3 response occurs because of retrograde refractoriness of the fast pathway. Failure of the echo during Zone 3 probably reflects concealed conduction to the fast pathway, or possibly interference in the slow pathway.