Using skin temperature gradients or skin heat flux measurements to determine thresholds of vasoconstriction and vasodilatation

Forearm–fingertip skin temperature differentials (T sk-diff) are used to indicate vasomotor tone, vasoconstriction defined as having occurred when T sk-diff≥4°C (Sessler et al. 1987, 1988a, b). This study was conducted to determine whether T sk-diff or finger pad heat flux (HF) can be used to predict when vasoconstriction and vasodilatation occur. Seven subjects (one female) sat in water at [mean (SD)] 40.7 (0.8)°C until their core temperature (T c) increased by 1°C, ensuring vasodilatation. The water was then cooled [at a rate of 0.6 (0.1)°C.min–1] until T c fell to 0.5°C below pretesting values, causing vasoconstriction. Subjects were then rewarmed in water [41.2 (1.0)°C]. Skin blood flow (SkBF) was measured using laser Doppler flowmetry (LDF) on the left second finger pad [immersed in water at 10.4 (1.4)°C as part of another experiment], and infrared plethysmography on the third finger pad of both hands. T sk-diff and HF were measured on the right upper limb, which remained in air. When vasodilated, the subjects had a stable T sk-diff and HF. During cooling, rapid-onset vasoconstriction occurred coincidental with large gradient changes in HF and T sk-diff (inflection points). In two subjects the original vasoconstriction definition (T sk-diff≥4°C) was not attained, in the other five this was achieved 31–51 min after vasoconstriction. During rewarming, the T sk-diff and HF inflection points less accurately reflected the onset of vasodilatation, although with one exception they were within 5 min of the LDF changes. We conclude that T sk-diff and HF inflection points predict vasoconstriction accurately, and better than T sk-diff≥4°C.