Heart rate variability is associated with postoperative pain intensity and pain trajectory after orthopedic surgery.

BACKGROUND: Accurate assessment of postoperative pain is essential for rehabilitation and for identifying patients at risk of chronic postsurgical pain. However, pain assessment often relies on self-report, which may be difficult in older adults and patients with impaired communication. Heart rate variability (HRV) has been proposed as a noninvasive objective indicator of pain, but its association with postoperative pain recovery remains unclear. OBJECTIVE: To investigate whether postoperative HRV is associated with pain intensity and the trajectory of pain reduction during the acute postoperative period. METHODS: Fifty-eight patients with postoperative pain after orthopedic surgery participated. Electrocardiographic recordings were obtained for 10 minutes at rest. The root mean square of successive differences (RMSSD) and the standard deviation of NN intervals (SDNN) were calculated and averaged across 3 consecutive days during early and later postoperative periods. Pain was assessed daily using the Numerical Rating Scale (NRS). Pain trajectory slopes over 7 and 14 days were obtained by linear regression. Associations were examined using nonparametric statistics. RESULTS: Pain intensity was significantly lower, and both RMSSD and SDNN were significantly higher, in the later than in the early postoperative period (all p < .001). Mean early postoperative NRS was negatively correlated with RMSSD (rho =  -0.31, p = .009) but not with SDNN. The standard deviation of NN intervals was negatively correlated with the slope of the pain trajectory over 7 days (rho =  -0.28, p = .019) and 14 days (rho =  -0.24, p = .040), indicating that higher SDNN was associated with faster pain reduction. CONCLUSION: Postoperative HRV was associated with both pain intensity and the trajectory of early pain recovery. These findings suggest that HRV may reflect autonomic state during postoperative pain recovery, and should be considered a promising complementary marker rather than a validated stand-alone measure of postoperative pain.