Effect of Increased Parasympathetic and Sympathetic Tone on Internal Atrial Defibrillation Thresholds in Humans

Although changes in autonomic tone affect ventricular defibrillation, little is known about the effect of increased parasympathetic or sympathetic tone on the atrial defbrillation threshold. Methods: To evaluate the effect of reflexly increased parasympathetic and increase α- and β-adrenergic tone on the atrial defibrillation threshold (ADFT), atrial fibrillation was induced in 14 patients. ADFTs, right atrial refractory period (RARP), and monophasic action potential duration (MAPD) were determined before and after autonomic intervention. ADFTs were determined with a step-up protocol using 3/3-ms biphasic shocks delivered through decapolar catheters in the right atrial appendage and coronary sinus. Two groups were studied. Group I (N = 8) had ADFTs determined at baseline, after receiving phenylephrine (PE), and with PE plus atropine (A). Group 2 (N = 6) had ADFTs determined at baseline and after receiving isoproterenol (ISO). Results: Group I: PE significantly increased sinus cycle length (SR-CL) compared to baseline (742 ± 123 to 922 ± 233 ms) without significantly changing RARP, MAPD, or ADFT (2.3 ± 1.3 J vs 2.3 ± 0.8 J). With PE + A, SR-CL significantly decreased (529 ± 100 ms vs 742 ± 123 ms) and MAPD shortened (231 ± 41 ms vs 279 ± 49 ms) without altering RARP or ADFT (1.94 ± 0.9 J vs 2.25 ± 1.25 J). Group 2: ISO decreased SR-CL (486 ± 77 ms vs 755 ± 184 ms) and MAPD (169 ± 37 ms vs 226 + 58 ms) but not RARP or ADFT (2.25 ± 1.21 J vs 2.33 ± 1.75 J). Conclusions: Increasing parasympathetic, α-, or β-adrenergic tone does not affect the ADFT despite causing significant electrophysiological changes in the atria.