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Vol. 84, Issue 4, 1323-1332, April 1998
Noll Physiological Research Center, Pennsylvania State University, University Park, Pennsylvania 16802-6900
During
direct passive heating in young men, a dramatic increase in skin blood
flow is achieved by a rise in cardiac output (
c) and
redistribution of flow from the splanchnic and renal vascular beds. To
examine the effect of age on these responses, seven young (Y; 23 ± 1 yr) and seven older (O; 70 ± 3 yr) men were passively heated with
water-perfused suits to their individual limit of thermal tolerance.
Measurements included heart rate (HR), c (by
acetylene rebreathing), central venous pressure (via peripherally inserted central catheter), blood pressures (by brachial auscultation), skin blood flow (from increases in forearm blood flow by venous occlusion plethysmography), splanchnic blood flow (by indocyanine green clearance), renal blood flow (by
p-aminohippurate
clearance), and esophageal and mean skin temperatures.
c was
significantly lower in the older than in the young men (11.1 ± 0.7 and 7.4 ± 0.2 l/min in Y and O, respectively, at the limit of
thermal tolerance; P < 0.05),
despite similar increases in esophageal and mean skin temperatures and
time to reach the limit of thermal tolerance. A lower stroke volume (99 ± 7 and 68 ± 4 ml/beat in Y and O, respectively, P < 0.05), most likely due to an
attenuated increase in inotropic function during heating, was the
primary factor for the lower c observed in
the older men. Increases in HR were similar in the young and older men;
however, when expressed as a percentage of maximal HR, the older men
relied on a greater proportion of their chronotropic reserve to obtain
the same HR response (62 ± 3 and 75 ± 4% maximal HR in Y and
O, respectively, P < 0.05). Furthermore, the older men redistributed less blood flow from the
combined splanchnic and renal circulations at the limit of thermal
tolerance (960 ± 80 and 720 ± 100 ml/min in Y and O,
respectively, P < 0.05). As a result
of these combined attenuated responses, the older men had a
significantly lower increase in total blood flow directed to the skin.
aging; hyperthermia; cardiac output; skin blood flow; cardiovascular responses
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