Sex differences in cardiac and autonomic response to clinical and experimental pain in LBP patients

Rehabilitation professionals are currently using heart rate (HR) in order to assess the sincerity of effort in certain evaluations. It has been shown that a relation exists between HR and pain but no study has measured cardiac response during both clinical and experimental pain among a patient population using an intra-subject design. Thirty patients with low back pain (LBP) participated in this study including 16 men. Clinical pain was induced by applying a postero-anterior pressure (PA) on a painful lumbar segment for 15 and 30s in order to reproduce the patient's typical LBP at an intensity ranging between 50 and 70/100. Experimental pain was induced with a 15s thermal stimulus at a temperature which reproduced the same pain intensity as the 15s PA. For both reproduced clinical pain durations, we observed a rise in HR ranging between 8.5% and 12.67%. However, unlike men, women's cardiac response failed to show a constant rise in HR during the 30s PA. For all subjects, the rise in HR was much lower during the experimental pain condition (p<0.001), reaching only 5%. On the other hand, galvanic skin responses were significantly higher during the experimental pain condition (p<0.001). During this same condition, women also had a greater rise in galvanic skin responses than men (p=0.04). Finally, a significant correlation was found between both types of pain. These results suggest that pain induced during a clinical evaluation will produce a significant HR augmentation. However, heart rate variability analysis showed greater sympathetic cardiac regulation for men. The sex differences observed in this study call for caution when interpreting HR during pain assessment.