The influence of low blood pressure and baroreceptor activity on pain responses.

The influence of baroreceptor modulation on pain perception has been extensively studied in normal and hypertensive subjects, but not in hypotensive subjects. The present experiment was performed to verify the following hypotheses: 1. Hypotensive subjects exhibit an increased pain response following baroreceptor activation compared to normals; 2. unlike the hypotensives, normal subjects, with normal baroreceptor reflexes, would learn to choose more often the baroreceptor activation condition compared to the opposite condition, during painful stimulation; 3. sensory and pain thresholds are correlated with blood pressure. The subjects (39 men), divided according to low and normal blood-pressure groups, participated in an experiment in which electrically-induced pain ratings and pain-evoked potentials were measured. Baroreceptor manipulation was performed by means of the PRES procedure. Whereas hypotensive subjects did not perceive any pain difference between the two baroreceptor conditions, activation and inhibition, normotensive men were able to perceive the baroreceptor activation condition as less painful. Similarly to subjective ratings, pain-evoked potentials of the low blood-pressure group showed a reversed trend compared to normals: larger somatosensory evoked potentials (N150-P260) to the baroreceptor activation condition and the opposite to the baroreceptor deactivation condition. Furthermore, results showed a negative correlation between diastolic blood pressure and sensory perception threshold, and a positive correlation between systolic pressure and pain threshold. Contrary to expectations, the rate of choice of baroreceptor conditions during painful stimulation did not show, in either group, any preference for baroreceptor activation.