Rate-dependence of atrial tachycardia effects on atrial refractoriness and atrial fibrillation maintenance

Abstract

Although atrial-tachycardia remodelling is a significant atrial fibrillation (AF) promoting factor, little information is available about how atrial-tachycardia rate determines remodelling effects. This study assessed the effects of atrial tachypacing (ATP) over a range of clinically relevant rates on atrial electrophysiology and AF. Chronically instrumented dogs underwent sequential 7 day ATP at 400, 300, 200, and 160 bpm in random order with 2 day recovery intervals between periods of ATP. ATP at 400, 300, and 200 bpm significantly decreased atrial effective refractory period (ERP) by 41 +/- 2, 37 +/- 3, and 7 +/- 1 ms, respectively, with no significant effects at 160 bpm. Mean duration of induced AF was increased by 400 and 300 bpm ATP (404 +/- 284 and 410 +/- 283 s on day 4, respectively, vs. 12 +/- 4 s at baseline, P < 0.01), but not by 200 or 160 bpm ATP. ATP effects developed slowly with 200 bpm pacing, so we studied 5 week ATP at 200 and 160 bpm in additional dogs. ERP shortened gradually over 3 weeks at 200 bpm (131 +/- 5 ms baseline vs. 112 +/- 4 and 105 +/- 4 ms at 2 and 3 weeks, respectively), but no decrease occurred thereafter (5-week value: 104 +/- 3 ms) and AF duration was not significantly affected. No change in ERP or AF duration occurred at 160 bpm. Because of the limited effects of 200 bpm ATP on AF duration despite significant effects on ERP, we tested 200 bpm ATP effects in the presence of AF substrates. When 200 bpm ATP was induced in the presence of a fibrotic AF substrate induced by 2 weeks of ventricular tachypacing followed by 1 week recovery, no change in AF duration or atrial vulnerability occurred. However, when 200 bpm ATP was followed by 400 bpm ATP, the onset of remodelling and AF duration increases was accelerated. There is a non-linear relationship between atrial rate and the extent of atrial electrical remodelling. Remodelling at rates equivalent to paroxysmal supraventricular tachycardias in man is insufficient to promote AF alone or in the presence of an atrial fibrotic substrate, but can accelerate the remodelling and stabilization of AF when followed by faster atrial tachyarrhythmias.