Face cooling reveals a relative inability to increase cardiac parasympathetic activation during passive heat stress

Abstract

We tested the hypothesis that passive heat stress attenuates the increase in cardiac parasympathetic stimulation, vascular resistance, and blood pressure evoked by face cooling. During normothermia and when intestinal temperature was elevated by 1.0 ± 0.2°C, eleven healthy young adults underwent 3 min of face cooling. Face cooling was accomplished by placing a 2.5 L bag of ice water (0 ± 0°C) over the cheeks, eyes, and forehead. Primary variables included forehead skin temperature, mean arterial pressure, and systemic, forearm and cutaneous vascular resistances. Indices of heart rate variability in the time domain provided an index of cardiac parasympathetic activity. The magnitude of reductions in forehead skin temperature during face cooling was slightly greater during normothermia (−17.6 ± 1.9°C vs. −16.3 ± 3.0°C, P = 0.03). Increases in heart rate variability evoked by face cooling were attenuated during heat stress. Changes in systemic, forearm, and cutaneous vascular resistances during face cooling were virtually abolished lower during heat stress (P < 0.01). However, when forearm and vascular data were reported as conductance, differences between normothermia and heat stress were not apparent (P ≥ 0.62). Nevertheless, the increase in mean arterial pressure was attenuated during heat stress with face cooling (at 3 min: 2 ± 7 mmHg) compared to normothermia (at 3 min: 19 ± 7 mmHg, P < 0.01). These data indicate that passive heat stress attenuates face cooling evoked increases in cardiac parasympathetic activation, vascular resistance, and blood pressure. However, they also indicate that changes in indices of vascular resistance do not always reflect equivalent changes in conductance.

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