Apneic Bradycardia : terrestrial and aquatic responses in man under working conditions

Abstract

The focus of this research was the reappraisal in physiological and psychophysical terms of current equivocal theories regarding the onset, course and termination of apneic bradycardia. Sixteen healthy male subjects participated in four separate testing sessions . Maximal oxygen consumption (VOâ‚‚ max) was measured on land and underwater using an identical direct, continuous progressive cycle ergometer test. On each of two other occasions subjects exercised in either environment at 50, 70 and 90% of the appropriate VOâ‚‚ max, during which time heart rate was continuously recorded. An initial apneic bout at each exercise intensity was followed by performance of the same workload without apnea for an equivalent period of time. Ratings of perceived exertion (RPE) were also monitored. While apneic bradycardia occurred at each exercise intensity studied underwater, it was apparent only at 50% VOâ‚‚ max on land. With the exception of between 50 and 90% VOâ‚‚ max on land, the mean apneic heart rates did not differ with varying exercise intensity (p<0.05); nor did the lowest heart rate recorded, although this was lower underwater than on land. Apart from 70% VOâ‚‚ max on land, apneic heart rates were lower than the equivalent values measured during exercise without apnea. Land and underwater heart rates during apneic and non-apneic conditions did not differ until 90% VOâ‚‚ max. The effects of increasing exercise intensity upon the onset of bradycardia were evident in that it occurred earlier at 50% VOâ‚‚ max underwater than at the heavier workloads, and only at 50% VOâ‚‚ max on land. The mean breath-hold duration did not differ between the land and underwater environments, nor was it affected by increasing exercise intensity. The order in which breath-holds was performed did not alter the length of apnea. Land and underwater RPE did not differ and increased with increasing exercise intensity in both environments. During apneic exercise RPE was greater than the equivalent exercise without apnea. Twelve of the original 16 subjects were divided equally into two groups on the basis of vital capacity expressed relative to body surface area. Vital capacity was measured during the first laboratory session. Neither the mean heart rate response to apneic exercise at 50% V0â‚‚ max in both environments, nor the lowest heart rates recorded differed between groups, prompting the conclusion that lung volume did not affect apneic bradycardia. Despite a longer breath-hold duration for Group A (large relative lung volume) than Group B (small relative lung volume), the onset point of bradycardia was the same for either group when expressed relative to total breath-hold duration