The Role of the Baroreflex in Diving Bradycardia

Abstract

Large inter-individual differences exist in the degree of bradycardia induced by breath-hold facial immersion. The purpose of this study was to examine baroreceptor sensitivity in subjects who exhibit a strong response and in those who exhibit a minimal response. Thirty-nine healthy volunteers. were screened with three trials of breath-hold facial immersion during mild steady-state cycling. The six subjects displaying the greatest bradycardia were selected as responders and the six showing the least as non-responders. Baroreceptor sensitivity was estimated in each subject by examination of the heart rate and blood pressure responses to a controlled Valsalva manoeuvre and to isometric handgrip exercise. Regression lines for changes in systolic blood pressure over time showed a flatter response in the responders both during isometric handgrip exercise (p<.05) and over the 25 s immediately following release (p<.01). One interpretation of these findings is that the non-responders are less able to maintain a resting level of arterial blood pressure. As well, regression lines for the change in diastolic blood pressure over the period 25 to 55 s post-release of isometric handgrip exercise had different slopes in the two groups (p<.05). A positive mean slope calculated for the responders and a negative mean slope calculated for the non-responders, when plotted with the average intercepts, suggested an undershoot in diastolic blood pressure upon release in the responders. This may represent an attempt to regain resting levels of arterial blood pressure through peripheral vasodilitation. Direct measures by arterial catheter, in a sub-sample of four subjects, suggested that the blood pressure overshoot during the recovery phase of the Valsalva manoeuvre may not have been large enough to demonstrate group differences in baroreceptor sensitivity.