Ackland, GarethAljurais, Saleh2026-03-292025https://hdl.handle.net/20.500.14154/78529Abstract Background: Postoperative complications remain a significant cause of morbidity and mortality worldwide. Traditional risk assessment methods are useful, however often fail to capture real-time physiological reserve. Heart rate variability (HRV), a non-invasive measure of autonomic function, has become known as a potential biomarker for surgical vulnerability. This dissertation reviews the evidence for preoperative HRV, focusing on the root mean square of successive differences (rMSSD) and high-frequency (HF) power, as a predictor of postoperative complications. Methods: A systematic review was conducted following PRISMA 2020 guidelines. PubMed was searched for studies published between 2015 and 2025 reporting preoperative HRV in surgical patients with and without postoperative complications. Data extraction included study design, surgical type, HRV metrics, and complication outcomes. Risk of bias was assessed using the Quality in Prognostic Studies (QUIPS) tool. Narrative synthesis was employed due to heterogeneity in methods and outcomes. Results: Sixteen prospective studies involving patients across cardiac, orthopaedic, abdominal, vascular, and thoracic surgeries were included. HF power was reported in 15 studies and rMSSD in 12. Across the literature, reduced vagally mediated HRV mainly lower HF power, was frequently observed in patients who experienced postoperative complications. However, methodological heterogeneity, variable outcome definitions, and limited multivariable adjustment constrained the strength of conclusions. Conclusion: Preoperative HRV, in particular HF power, has potential as a low-cost, non invasive marker for postoperative risk. Nevertheless, it remains an investigational tool. Well designed, standardised, prospective studies are needed to determine its independent prognostic value and clinical applicability45enHRVPerioperativeThesis