Relationship between skin blood flow and sweating rate, and age related regional differences

Abstract

To examine the mechanisms and regional differences in the age-related decrement of skin blood flow, 11 young (age 20–25 years) and 10 older (age 64–76 years) men were exposed to a mild heat stress by immersing their feet and lower legs in water at 42°C for 60 min, while they were sitting in near thermoneutral conditions [25°C and 45% relative humidity (rh)]. During the equilibrium period (25°C and 45% rh) before the heat test, no group differences were observed in rectal (T _re) and mean skin (T _sk) temperatures or mean arterial pressure (MAP). During passive heating, T _sk was significantly lower in the older men 20 min after commencing exposure (P < 0.001), although there were similar increases in T _re in both groups. Exposure time and age did not affect MAP. The local sweating rate (m˙ _sw) and the percentage change in skin blood flow by laser Doppler flowmetry (%LDF) relative to baseline values on the chest, back, forearm and thigh were significantly lower in the older men (P < 0.001), especially on the thigh. After starting the heat exposure, three temporal phases were observed in the relationship between %LDF and m˙ _sw at most sites in each subject. In phase A, %LDF increased but with no increase in m˙ _sw. In phase B, m˙ _sw increased but with no secondary increase in %LDF. Finally, in phase C, there were proportional increases in %LDF and m˙ _sw. The increase in %LDF in phase A was significantly lower on the forearm and thigh (P < 0.05) for the older men, but not on the chest and back. In phase C, the slopes of the regression lines between %LDF and m˙ _sw were lower for the older men on the back (P < 0.03), forearm (P = 0.08) and thigh (P < 0.03), but not on the chest. These results would suggest that the age-related decrement in skin blood flow in response to passive heating may be due in part to a smaller release of vasoconstrictor tone and to less active vasodilatation once sweating begins. Regional differences exist in the impaired vasoconstriction and active vasodilatation systems.