Nadir heart rate as a novel risk marker of postoperative complications - A propensity matched analysis

Aim: Although vital signs have a well-described 24-h cyclic variation (circadian rhythm), this variation is often not considered in clinical vital sign monitoring despite being influenced by physiologic stress from complications. This study aimed to evaluate the lowest heart rate at night (night-nadir) (NN-HR) as a novel risk marker for serious adverse events (SAE) in postoperative patients, rather than the average nocturnal heart rate. Methods: This was a propensity-matched nested case-control study of continuously monitored vital sign data (heart rate (HR), respiration rate, oxygen saturation and blood pressure) for up to 5 days after major non-cardiac surgery. The primary outcome was any SAE that occurred up to 24 h after a circadian value calculation. We compared NN-HR between patients who developed SAE and those who did not during similar postoperative periods. Results: Out of 588 patients, 104 (17.8 %) experienced an SAE during the monitored period and were matched with a median of 201 controls. The NN-HR was significantly higher in patients with impending SAE (median 74.0 bpm [IQR 68.2–81.8] vs. 68.3 bpm [IQR 61.5–76.0], p < 0.001). An NN-HR threshold of >67 bpm identified patients at increased risk (risk ratio 2.43, 99 % CI 1.24–5.00) for SAE, however with only moderate predictive performance (F1 score 0.58). Conclusion: Absence of HR below 67 bpm at night, was significantly associated with increased SAE risk, highlighting the potential value of NN-HR as a monitoring target.The potential clinical utility of monitoring NN-HR to prevent SAE warrants further prospective investigation.