Swallowing-induced cardio-respiratory responses in man.

Swallowing transiently increases heart rate. One of the authors developed pronounced bradycardia while breath holding, particularly after an expiration. The objective, therefore, was to study his cardiac responses during swallowing as pronounced bradycardia developed. When, after a maximum inspiration (supine), the heart rate slowly fell below 50 beats min' well-defined P waves (lead II) disappeared. By swallowing 6 times on command after the P waves disappeared his heart rate increased immediately (68 +/- 1 beats min(-1); n=6). P waves with similar morphology to those pre-swallowing were recorded 0.7 +/- 0.1 s (n=6) after the first swallow. He continued breath holding after swallowing. P waves again disappeared, although at faster heart rates (57 +/- I beats min(-1); n=6). Furthermore, well-defined P waves were observed after the second disappearance at heart rates within the range 30-40 beats min(-1). Small amplitude P waves continued to be recorded from lead I with P wave disappearance in lead II, suggesting a pacemaker shift, although not to the av node. Autonomic nerves can shift the dominant pacemaker within the sa node. The present report indicates that increased vagal tone may be rapidly reversed by swallowing.