Body Posture-Related Differences in Pain Perception and Their Mediation by Cardiovascular Factors.

Pain modulation arising from the cardiovascular system has repeatedly been described. It is mediated by the baroreceptor system, where the activity of baroreceptor afferents inhibits nociception. Body posture changes cause blood volume redistribution that might modulate baroreceptor activity and antinociceptive effects. This study investigated posture-related differences in pain perception and their causation by cardiovascular factors. Pressure pain threshold and tolerance and pain intensity ratings were obtained in 83 healthy individuals (61 women and 22 men) while they were sitting, lying down and standing. Electrocardiography, impedance cardiography and continuous blood pressure were recorded in each posture. Pain threshold was higher while lying down than standing, and pain tolerance was higher while lying down than standing and sitting. Pain intensity ratings for suprathreshold stimulation (4 kg/cm2) were higher while standing than sitting and lying down. Blood pressure, stroke volume, cardiac output, total peripheral resistance and baroreflex sensitivity decreased while standing and increased while lying down. Blood pressure was positively associated with pain threshold in men, but not in women. Total peripheral resistance was positively associated with pain threshold and tolerance and blood pressure was negatively related to pain intensity ratings in the total sample. Lying down is associated with increased venous return, central blood volume and higher baroreceptor load, which may result in stronger antinociceptive effects. The opposite changes along with baroreceptor unloading occur while standing. Assuming a supine position is a functional human response to alleviate acute pain. However, in terms of negative reinforcement, pain reduction while lying down may contribute to passive coping behaviors in chronic pain.