CARDIAC-OUTPUT VARIATIONS IN SUPINE RESTING SUBJECTS DURING HEAD-OUT COLD-WATER IMMERSION

Five men, aged 31.2 years (SD 2.3), under semi-nude conditions and res ting in a dorsal reclining position, were exposed to thermoneutral air for 30 min, followed immediately by a cold water (15 degrees C) immer sion for 60 min. Cardiac output was measured using a dualbeam Doppler flow meter. During immersion in cold water, cardiac frequency (f(c)) s howed an initial bradycardia. The lowest values were reached at about 10 min after immersion, 58.3 (SD 2.5) to 48.3 (SD 7.8) beats min(-1) ( P < 0.05). By the 20th min of exposure, f(c) had gradually risen to 70 .0 beats min(-1) (SD 6.6, P < 0.05). This change could be due to the i nhibition of the initial vagal reflex by increased catecholamine conce ntration. Stroke volume (V-s) was significantly increased (P < 0.05) d uring the whole cold immersion period. Cardiac output, increased from 3.57 (SD 0.50) to 6.26 (SD 1.33)l min(-1) (P < 0.05) and its change wi th time was a function of both V-s and f(c). On the other hand, systol ic flow acceleration was unchanged during the period of immersion. The changes in the respiratory variables (ventilation, oxygen uptake, car bon dioxide output and respiratory exchange ratio) during immersion sh owed an initial hyperventilation followed, as immersion proceeded, by a slower metabolic increase due to shivering.