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

What is the central question of this study? Does passive heat stress attenuate the increase in cardiac parasympathetic stimulation, vascular resistance and blood pressure evoked by face cooling? What is the main finding and its importance? Passive heat stress attenuates the capacity to increase cardiac parasympathetic activation and impairs the ability to increase vascular resistance during sympathoexcitation, which ultimately results in a relative inability to increase blood pressure. These findings cast doubt on the efficacy of face cooling at augmenting blood pressure during orthostasis while heat stressed.

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, 10 healthy young adults underwent 3 min of face cooling. Face cooling was accomplished by placing a 2.5 litre 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 reduction in forehead skin temperature during face cooling was slightly greater during normothermia (-17.6 ± 1.9 versus -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 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 with 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.