Hemodynamic and sympathetic nerve responses to painful stimuli in normotensive and borderline hypertensive subjects

Observations in animals and humans show that pain sensitivity might be lower (and pain tolerance higher) in hypertensive as compared to normotensive subjects. One hypothesis, derived from experimental studies, assumes that enhanced activation of baroreceptors leads to an enhanced central inhibition. A central hypothesis assumes changes in the central (endogenous) control of the nociceptive system. To investigate these two hypotheses we quantitatively assessed the minute-by-minute changes in mean arterial pressure (MAP), central venous pressure (CVP) heart rate (HR), muscle sympathetic nerve activity (MSNA), and individual pain ratings during noxious mechanostimulation in 10 normotensive (NT) and 13 borderline hypertensive (BH) subjects. Linear regression analysis indicated a close negative correlation for the overall data between resting levels of MAP and pain ratings (r = -0.57, P < 0.0001). The BH group exhibited a lower pain sensitivity compared to the NT group (P < 0.001). The extent of baroreceptor activation during the application of pain was not different between the two groups (P = NS) as assessed by almost identical increases in MAP (+8 +/- 1 vs. +9 +/- 1 mmHg NT vs. BH group), CVP (+0.7 +/- 0.1 vs. +0.5 +/- 0.1 mmHg), HR (+2 +/- 1 vs. +2 +/- 1 beats/min), and MSNA (+5 +/- 1 +4 +/- 1 bursts/min). The NT subjects exhibited significant correlations between the pain ratings and the increases in MAP (r = +0.52; P < 0.05) and MSNA (r = +0.49; P < 0.05) whereas the BH subjects did not show such a relationship. Thus, the increased pain tolerance in human hypertension cannot be explained by hemodynamically mediated differences in the activation of baroreceptors or by an altered baroreflex sensitivity during the application of pain. We conclude, that the reduced pain sensitivity in hypertensive humans is more likely related to central changes.