Clinical testing of thermally stimulated cardiovascular oscillations in man

STUDY OBJECTIVE--The study assessed the physiological validity of an automatic thermal stimulation method to induce synchronised oscillations in the neural cardiovascular control system. DESIGN--Automatic alternating rhythmic warm and cool immersion of different skin areas of 18 males was done at different frequencies and water temperatures. The neurally mediated responses to the periodic thermal stimulation were measured from skin blood flow and heart rate and compared to those of a sham stimulation. Respiration was monitored for control purposes. The reproducibility of the stimulation and responses was examined. SUBJECTS--18 young males volunteered for the study. MEASUREMENTS AND MAIN RESULTS--The water bath method produced reproducible thermal stimulation and responses of skin blood flow and heart rate. Rhythmic thermal stimulation at 0.013-0.096 Hz synchronised the oscillations of the forearm skin blood flow when the thermal stimulus amplitude exceeded 10 degrees C. The increase in the stimulus amplitude or enlargement of the stimulus area did not further increase the oscillatory response of skin blood flow. Sham stimulation or mean temperature of the periodic thermal stimulation in the range 23-33 degrees C did not influence the oscillations of skin blood flow. Local cooling of the stimulated lower legs attenuated the response of skin blood flow. Both thermal stimulation and sham stimulation affected heart rate, but no stable synchronisation of the periodic heart rate variability was found at supine rest. Thermal stimulation of the sitting subjects' forearm instead of legs increased the synchronisation of the periodic heart rate variability. CONCLUSIONS--The response of skin blood flow agreed with the theory of the thermal entrainment. In a supine man, both thermal stimulation and non-specific central nervous influences induced significant and reproducible interactions with periodic heart rate variability and respiration.