Abstract:

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 °C; propranolol 40 mg at 22 °C; placebo at 40 °C; propranolol 40 mg at 40 °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 °C with those obtained at 22 °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.

Keywords: cardiovascular reflexes; heart rate; heat stressor; physiological finger tremor; propranolol

Document Type: Research article

DOI: 10.1046/j.1365-2125.2000.00284.x

Affiliations: 1: Division of Psychiatry, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, UK