Time course of vagal effects studied in clinical electrocardiograms

We consider the transient effects of brief bursts of vagal stimulation on cardiac excitation and conduction as manifested in the clinical electrocardiogram. We hypothesize that such vagal effects may be more frequent than commonly recognized and that they may account for some types of arrhythmias that are commonly ascribed to other mechanisms. We first briefly review the direct chronotropic and dromotropic effects of brief vagal stimulus bursts in animals and man. We next consider the effects of brief vagal bursts on heart period in various arrhythmias in man, particularly ventriculophasic sinus arrhythmia with various degrees of A-V block, and various patterns of A-V dissociation. We illustrate these points with numerous clinical ECGs from our patient population, and with other cases from the literature. We next consider the effects of brief vagal bursts on A-V conduction time, and illustrate this with clinical cases of: (a) prolongation of A-V conduction after premature beats, (b) 'supernormal' A-V conduction, (c) Wenckebach periodicities, and (d) alternation of A-V conduction times. We review the evidence that brief vagal bursts are generated in vivo, and are a naturally occurring reflex phenomenon. We therefore suggest that such brief vagal effects should perhaps be considered more commonly when ascribing mechanisms to some complex and often difficult to interpret arrhythmias.