The relationship between blood pressure and pain.

The relationship between pain and hypertension is potentially of great pathophysiological and clinical interest, but is poorly understood. The perception of acute pain initially plays an adaptive role, which results in the prevention of tissue damage. The consequence of ascending nociception is the recruitment of segmental spinal reflexes through the physiological neuronal connections. In proportion to the magnitude and duration of the stimulus, these spinal reflexes cause the activation of the sympathetic nervous system, which increases peripheral resistances, heart rate, and stroke volume. The response also involves the neuroendocrine system, and, in particular, the hypothalamic-pituitary-adrenal axis, in addition to further activation of the sympathetic system by adrenal glands. However, in proportion to an elevation in resting blood pressure, there is a contemporary and progressive reduction in sensitivity to acute pain, which could result in a tendency to restore arousal levels in the presence of painful stimuli. The pathophysiological pattern is significantly different in the setting of chronic pain, in which the adaptive relationship between blood pressure and pain sensitivity is substantially reversed. The connection between acute or chronic pain and cardiovascular changes is supported observationally, but some of this indirect evidence is confirmed by experimental models and human studies. The pain regulatory process and functional interaction between cardiovascular and pain regulatory systems are briefly reviewed. Various data obtained are described, together with their potential clinical implications.