Habituation and recovery of vascular responses in calf and forearm and of the level of pain sensation during the cold pressor test in man

Summary

Forearm and calf blood flow were measured using a mercury-in-silastic gauge during immersion of one foot in water at 4° C. The subjects were asked to assess the level of pain on a scale 0-10. The effects of one session of repeated immersions on the vascular response in the calf, forearm and on subjective pain sensation were studied. In 6 of the 14 subjects vasodilatation was elicited in both forearm and calf during the first immersion. During the first session the vasodilatation diminished, while subjective pain sensation remained at the level of 7 on the scale. In 4 subjects vasoconstriction in both calf and forearm occurred during the first immersion. During repeated immersions forearm vasoconstriction subsided, while in the calf it remained at the same level. The level of the subjective pain sensation diminished from 7.5 to 5 in 1 subject. Repeated immersions in six to ten daily sessions led to reduction of vasodilatation in calf and forearm while there was a small change in the pain sensation, i.e. reduction from the level of 6.9 to 5.7 (P < 0.01). In subjects in whom vasoconstriction in forearm and calf was observed no clear trend in vascular responses was observed during the repeated daily sessions while the pain sensation in 1 subject was reduced from 6 to 4.8. In 3 subjects the change of vascular responses was different in the forearm and calf. After complete habituation of the vasodilatation in the forearm and calf at the end of the ten daily sessions, re-tests were carried out every 7 days to see whether the vasodilatation would reappear. The vasodilatation recovered, but the time required for partial recovery in forearm or calf varied from individual to individual. The shortest time for partial recovery was 7 days, the longest more than 70 days. In all subjects but 1, the subjective pain sensation returned to its initial value or even exceeded it. In 1 subject the pain remained suppressed during the last re-test. In some subjects the changes in the vascular response in the calf and forearm did not develop in parallel. The subjective pain sensation was not reflected in vascular changes. It is concluded that the vascular response in the calf and forearm and the subjective pain sensation elicited by a noxious stimulus have independent central neural control.