Effects of high ambient temperature on parasympathetically mediated cardiovascular reflexes in normal man

Aims To examine the effects of high ambient temperature ('heat stressor') o n parasympathetically mediated cardiovascular reflexes (power of respirator y sinus dysrhythmia; change in heart rate elicited by change in posture fro m lying to standing ['30 : 15 ratio']). Methods Twelve healthy male volunteers participated in four weekly sessions , each of which was associated with one treatment condition (placebo at an ambient temperature of 22 degrees C; propranolol 40 mg at 22 degrees C; pla cebo at 40 degrees C; propranolol 40 mg at 40 degrees C), according to a ba lanced double-blind design. Heart rate was recorded by ECG, finger tremor ( 7-12 Hz) with an accelerometer strapped to the middle finger of the nondomi nant hand, and sublingual temperature by a mercury thermometer. Power of fi nger tremor and the variations of the R-R intervals of the ECG were obtaine d from Fourier transformations of the data. Data were analysed by analysis of variance, with repeated measures using a significance criterion of P < 0 .05; individual comparisons of active treatment with placebo and of data ob tained at 40 degrees C with those obtained at 22 degrees C were made with F isher's Least Significant Difference test. Results Heart rate was increased by the heat stressor, and this increase wa s abolished by propranolol. The heat stressor reduced the power of respirat ory sinus dysrhythmia and the 30 : 15 ratio, and increased the power of phy siological finger tremor. Propranolol did not affect heat stressor-induced changes in the parasympathetic cardiac reflexes, but reduced the heat stres sor-induced enhancement of finger tremor. Conclusions The increase in the power of physiological finger tremor at hig h ambient temperature is consistent with sympathetic activation, whereas th e reduction in the power of respiratory sinus dysrhythmia and 30 : 15 ratio indicates a decrease in parasympathetic activity. These results demonstrat e that high ambient temperature may induce vagal withdrawal in the heart.