Role of surface area-to-mass ratio and work efficiency in heat intolerance

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Abstract
Postheatstroke (HS) subjects (14) and 9 normal control subjects (C) with similar (.ovrhdot.VO2 max) underwent a 180-min heat-tolerance test (ambient temperature 40.degree. C, relative humidity 40%; 12 steps .cntdot. min-1 on a 30-cm high bench). In group HS 9 subjects (NHS) thermoregulated similarly to group C, while 5 subjects were identified as heat intolerant (HI), characterized by significantly higher heart rate (HR), rectal temperature (Tre), and heat storage (.DELTA.S) than groups NHS and C (at the end of the 2nd h, HR 159 .+-. 9, 124 .+-. 4, and 118 .+-. 3 beats .cntdot. min-1, Tre 38.9 .+-. 0.1, 37.9 .+-. 0.1 and 37.9 .+-. 0.1.degree. C, .DELTA.S 24 .+-. 5, 10 .+-. 2 and 12 .+-. 4 W .cntdot. m-2 in groups HI, NHS and C, respectively). The work efficiency (.eta.) was significantly lower in groups HI and NHS when compared with C (10.2 .+-. 0.7, 10.4 .+-. 0.5, and 11.8 .+-. 0.6%, respectively). Skin surface area-to-body mass ratio (AD/W) in group HI was significantly lower than in group NHS or C (247 .+-. 7, 271 .+-. 5 and 272 .+-. 4 cm2 .cntdot. kg-1, respectively). Multiple linear regression analysis between each one of the dependent variables, HR, Tre and .DELTA.S, mean weighted skin temperature, and sweat rate, against the 2 independent variables, .eta. and AD/W, yielded significant correlations (r = 0.74, 0.73, 0.58, 0.59 and 0.44, respectively). A reduced .eta. and/or a reduced AD/W play a role in heat intolerance, causing increased heat production and a concomitantly less effective heat dissipation.