Purpose: The purpose of this study was to determine if sinusoidal fluctuations in vagal activity on the ventricular result in an entrainment of the ventricular rhythm in humans and a porcine model of atrial fibrillation.
Methods: : Forced vagal oscillations at two different frequencies were produced by employing deep breathing (0.125 Hz) and neck suction (0.25 Hz) in humans and pigs with AF. In the pig model, glycopyrrolate was administered to block peripheral muscarinic receptors, thus inhibiting vagal transmission to the heart. Vagal activity was evaluated using power spectral analysis of heart rate variability in order to determine the contribution of the vagus to ventricular irregularity in AF.
Results: In 2 of our human subjects, power spectral analysis showed no significant difference between oscillations in heart rhythm during neck suction (0.25Hz) and deep breathing (0.1 Hz) compared to baseline. In addition, the standard deviation of RR intervals was not significantly different with or without neck suction. In one subject, however, there was a significant increase in power during both neck suction and deep breathing. In our pig model, neck suction and deep breathing increased power at 0.25 and 0.125 Hz respectively and vagal blockade abolished power in both frequencies.
Conclusions: In humans, neck suction (vagal activation via baroreflex) did not have a clear effect on RR interval in AF. We are unable to draw a definitive conclusion based on the data in humans because of the small number of subjects. However, data from the pig experiments suggest the vagus does, in part, mediate ventricular irregularity. The difference between the human and porcine models may be attributed to the fact that the humans had chronic AF whereas the pigs had acute AF. Chronic AF may produce changes in baroreflex function, i.e. no change in vagal tone with neck suction. In addition, there may be some level of parasympathetic neuropathy which makes the heart unresponsive to the vagus.