Face Immersion Bradycardia: Comparison of Swimmers and Nonswimmers

A profound bradycardia may be exhibited by waterfowl and aquatic mammals when they dive underwater in search of food. A similar response occurs in humans diving underwater or simply wetting the face while breathholding. This bradycardia is mediated by the parasympathetic nervous system (vagus). We investigated whether the extent of this apneic face immersion bradycardia, or bradycardia during dry apnea, is greater in actively training competitive swimmers than in nonswimmers. Eight competitive swimmers and eight age/sex matched nonswimmers each performed apneic face immersion and dry apneic maneuvers while prone. Resting cardiac cycle (interval) duration was not significantly different between these groups, but the swimmers had a significantly longer interval duration (lower heart rate), P<0.05, and a greater percentage decrease of interval duration during both apneic face immersion and dry apnea. Swimmers—but not nonswimmers—demonstrated cardiac arrhythmias considered to be vagally mediated during face immersion. These results indicated that competitive swimmers in active training may have a greater range of parasympathetic nervous system (vagal) control of the heart as well as the well-accepted greater resting level of vagal tone. OHIO J. SCI. 81(2): 88, 1981 Certain waterfowl and aquatic mammals exhibit a profound bradycardia upon diving underwater, involving heart rates as low as 6 beats per minute (bpm) and persisting despite the increased physical activity as the animal searches for food. The bradycardia is accompanied by intense peripheral vasoconstriction preserving the body's oxygen stores for the vital organs. This response pattern, called the "diving reflex," has also been observed in humans (Irving 1963, Eisner et al 1966, Olsen et al 1962). Irving (1963) observed that diving bradycardia was more pronounced and appeared more regularly in persons accustomed to swimming than in nonswimmers. Craig (1963), however, reported that diving bradycardia was "as prominent in poor swimmers as in those subjects who were familiar with the water." Craig's diving data were determined when the subjects, dressed in wet Manuscript received 30 November 1979 and in revised form 1 December 1980 (#79-56). suits, were submerged to the bottom of a pool 2.3 m deep, and control measurements were made when the subjects were prone on the surface of the water breathing through a snorkle tube. Thus, the variable was submersion and not face wetting. This is an important distinction because the reflex has been demonstrated in response to merely wetting the nasal area of the face, or "face immersion," especially if the face immersion is accompanied by apnea (Eisner et al 1966, Whayne and Killip 1967, Hong et al 1970, Str0mme et al 1970, Bove et al 1968, Hutinger 1971, Frey and Kenney 1977). Whayne and Killip (1967) compared face-immersion bradycardia in sedentary subjects with that in oarsmen who were actively training. The resting heart rate of the oarsmen was lower, but the percentage change with apneic face immersion or breathhold was no greater than the percentage change in the sedentary subjects. Str0mme et al (1970) Ohio J. Sci. FACE IMMERSION BRADYCARDIA 89 also observed significant bradycardia with apneie face immersion in healthy subjects 15 to til years of age, but found no relationship between level of physical fitness, as measured from the Harvard step test, and degree of bradycardia. Other researchers have observed a, significantly greater degree of bradyeardia with apneie face immersion in individuals who are physically trained. Hong el al (1970) reported that experienced divers demonstrated greater faeeimmersion bradycardia than did nondivers. Bove el al (19(iS) observed an augmentation of the bradycardic response to apneie face immersion after a nonswimming training program, and Hutinger (1971) reported that the bradycardic response to face immersion was exhibited by competitive swimmers, 12 to 22 years of age. The purpose of our study was to determine whether the degree of bradycardia elicited by face immersion is greater in competitive swimmers who are regularly exposed to the water than in nonswimmer control subjects. METHODS Two groups of students matched for age and sex were studied. Group I, the swimmers, consisted of 6 males and 2 females, 15-20 years of age, all of whom had been swimming competitively 4 or more years, and who at the time of the study regularly spent large periods of time in the water. The swimming program for these subjects involved two 45 min workouts each day. A 400 m warm-up swim was followed by a period of interval training during which the subjects swam 100 m sprints with a recovery period between sprints. Training distance totaled approximately 2000 m each day. This program had been in effect approximately 'A months when the experiments were performed. The subjects spent many additional hours a day in the water, and most participated in additional year-round swimming programs. Group II, the nonswimmcrs, also consisted of (> males and 2 females between 15 and 22 years of age. Their physical activity ranged from "no regular activity" to occasional jogging or social basketball. The face immersion/1 >reathhold procedures used with both groups were described previously (Frey and Kcnney 1977). Brcathholding with or without immersion was performed with subjects prone on a special table which incorporated a water-filled sink at one end. Subjects were instructed to hold their breath after a slightly larger-than-normal inspiration and to maintain the breathhold as long as they could with comfort, i.e., to each subject's individual endpoint. For face immersion maneuvers, subjects lowered their faces into the water to a depth sufficient to wet the nasal and mouth areas while keeping the ears dry. A sling across the sink supported the forehead. Water temperature was maintained at 27 °C. An electrocardiogram was recorded for measurement of cardiac cycle duration on a beat-by-beat basis; it was examined also for evidence of cardiac arrhythmias. The single longest beat interval and the longest 5 consecutive-beat period for which the mean interval length was determined were identified for each participant during each experiment. Group I and Group II means for these indicators of apneie face immersion or dry apneie bradycardia were compared by t test, with P<0.05 used as the criterion for statistical significance.