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Effects of high ambient temperature on parasympathetically mediated cardiovascular reflexes in normal man
W M Banjar1, J Gazzaz, R W Langley
1Division of Psychiatry, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, UK.
Aims:
To examine the effects of high ambient temperature ('heat stressor') on parasympathetically mediated cardiovascular reflexes (power of respiratory sinus dysrhythmia; change in heart rate elicited by change in posture from 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; placebo at 40 degrees C; propranolol 40 mg at 40 degrees C), according to a balanced double-blind design. Heart rate was recorded by ECG, finger tremor (7-12 Hz) with an accelerometer strapped to the middle finger of the nondominant hand, and sublingual temperature by a mercury thermometer. Power of finger tremor and the variations of the R-R intervals of the ECG were obtained 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 obtained at 40 degrees C with those obtained at 22 degrees C were made with Fisher's Least Significant Difference test.
Results:
Heart rate was increased by the heat stressor, and this increase was abolished by propranolol. The heat stressor reduced the power of respiratory sinus dysrhythmia and the 30 : 15 ratio, and increased the power of physiological finger tremor. Propranolol did not affect heat stressor-induced changes in the parasympathetic cardiac reflexes, but reduced the heat stressor-induced enhancement of finger tremor.
Conclusions:
The increase in the power of physiological finger tremor at high ambient temperature is consistent with sympathetic activation, whereas the reduction in the power of respiratory sinus dysrhythmia and 30 : 15 ratio indicates a decrease in parasympathetic activity. These results demonstrate that high ambient temperature may induce vagal withdrawal in the heart.