Heat pain sensitivity of human skin after mild heat injury and its lack of dependence on the local blood flow

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Abstract
Heat pain thresholds have been determined on the forearm of a group of inexperienced subjects using a radiant heat stimulator. During heating, the skin temperature was estimated using a small thermocouple pressing on the skin. Linear rates of rise of skin temperature were produced by automatic feedback control of the heating lamp. Pain threshold was reached at an average temperature of 50.7.degree. C when the rate of rise of temperature was 1.0.degree. C/s. With slower rates of heating the pain threshold was reached at a lower temperature. Heat pain thresholds fell by an average of 5.5.degree. C following suprathreshold heating to 57.5.degree. C. They did not fall significantly following heating to 55.degree. C. Heat pain thresholds in hyperalgesic skin (and in normal skin) were unaffected when the blood flow was arrested for up to 17 min by inflating a sphygmomanometer cuff around the upper arm. The lack of effect of stopping the blood flood is compared with other studies showing similar results for heat injury of nociceptors in rabbit skin and contrary findings for hyperalgesia following other types of tissue damage with slower onset in man.