Impact of sympathetic activation on pain threshold in human subjects

This study examined the effect of the sympathetic activation with the head-up tilt test (HUT) on electrocutaneous pain in 56 young normotensive volunteers. Systolic arterial blood pressure (SAP), diastolic arterial blood pressure (DAP), heart rate (HR), sensory and pain thresholds to electrocutaneous stimulation were measured 30 s before (in supine position), during (at 10 s, 90 s, 3 min and 5 min) and 10 min after HUT (in supine position). No relationship was found between resting BP and pain perception. HUT significantly increased DAP and HR, but did not change SAP. During HUT, both sensory and pain electrocutaneous thresholds were significantly increased, indicating reduced nociception. While changes in HR and changes in sensory threshold were correlated, no other correlation could be found between hemodynamic changes (DAP or HR) and changes in sensory and pain threshold. These results suggest that the mechanisms underlying the sympathetic adaptation to HUT and HUT-induced analgesia are different.