Proceedings of The Physiological Society

University of Oxford (2011) Proc Physiol Soc 23, PC124

Poster Communications

The effects of the diving reflex on cerebral blood flow whilst supine with and without mild heat stress: A pilot study.

B. Perry1, Z. J. Schlader1, S. J. Lucas2, T. Mundel1

1. School of Sport and Exercise, Massey University, Palmerston North, New Zealand. 2. Department of Physiology, Otago University, Dunedin, New Zealand.

Heat stress, even when mild, decreases cerebral blood flow (CBF) and thus compromises orthostatic tolerance (OT). Whole-body skin cooling has been shown to attenuate the reduction in CBF during heat stress and improve OT. Forehead-cooling (FC) has also been shown to increase CBF; due, in part, to sympathetic mediated vasoconstriction of the peripheral vasculature (the ‘diving reflex’). However, it is currently unknown whether FC can attenuate the decrease in CBF in response to mild heat stress. In a cross-over design, 8 healthy males completed trials in both 20oC and 40oC. Whilst supine, mean arterial pressure (MAP, Finometer), heart rate (HR), middle cerebral artery blood flow velocity (MCAv, Doppler), ventilation, end-tidal PCO2 (PETCO2), weighted mean skin (TSK) and rectal (TC) temperatures were measured continuously. Cardiac output (Q) was also calculated (Model Flow method). Baseline measures were recorded after 20 minutes of resting supine and 1 min prior to cooling. Cooling was applied via an ice pack (0oC) to the forehead for 2 minutes. Baseline values and percentage change data were averaged over 1 min. FC was effective in decreasing temperature of the head by 8±2 oC. TC was unchanged by heat stress and FC. TSK was greater in 40oC (P<0.01), however FC reduced TSK (P=0.046) from baseline. Heat stress elevated HR (14±6%, P<0.001), Q (26±14%, P=0.008) and decreased MAP (-9±7%, P=0.022). FC tended to elevate MCAv (5 ± 7%, P=0.085) from baseline during heat stress. MCAv was unaffected by FC at 20oC (P>0.05), however, FC tended to decrease HR (P=.154) from baseline values. MAP, Q, PETCO2 and VE were unchanged (all P>0.05) from baseline by FC in both ambient conditions. In summary, we observed a stereotypical cardiovascular response to heat stress and an, on average, increase in MCAv during face cooling. These pilot data show the potential of face cooling as a means to improve orthostatic tolerance during heat stress. Further study is needed to confirm these observations.

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