Impact of body mass index on risk of acute kidney injury and mortality in elderly patients undergoing hip fracture surgery.

Abstract

This study aimed to examine risk of postoperative AKI and subsequent mortality, by body mass index (BMI) level, in hip fracture surgery patients aged 65 and over. A regional cohort study using medical databases was used. We included all patients who underwent surgery to repair a hip fracture during the years 2005-2011 (n=13,529) at hospitals in Northern Denmark. We calculated cumulative risk of AKI by BMI level during 5days postsurgery and subsequent short-term (6-30days postsurgery) and long-term (31-365days post-surgery) mortality. We calculated crude and adjusted hazard ratios (aHRs) for AKI and death with 95% confidence intervals (CIs), comparing underweight, overweight, and obese patients with normal-weight patients. Risks of AKI within five postoperative days were 11.9, 10.1, 12.5, and 17.9% for normal-weight, underweight, overweight, and obese patients, respectively. Among those who developed AKI, short-term mortality was 14.1% for normal-weight patients compared to 23.1% for underweight (aHR 1.7 (95% CI 1.2-2.4)), 10.7% for overweight (aHR 0.9 (95% CI 0.6-1.1)), and 15.2% for obese (aHR 0.9 (95% CI 0.6-1.4)) patients. Long-term mortality was 24.5% for normal-weight, 43.8% for underweight (aHR 1.6 (95% CI 1.0-2.6)), 20.5% for overweight (aHR 0.8 (95% CI 0.6-1.2)), and 21.4% for obese (aHR 1.1 (95% CI 0.7-1.8) AKI patients. Similar associations between BMI and mortality were observed among patients without postoperative AKI, although the absolute mortality risk estimates by BMI were considerably lower in patients without than in those with AKI. Obese patients were at increased risk of AKI compared with normal-weight patients. Among patients with and without postoperative AKI, overweight and obesity were not associated with mortality. Compared to normal-weight patients, underweight patients had elevated mortality for up to 1year after hip fracture surgery irrespective of the presence of AKI. The absolute mortality risks were higher in all BMI groups with the presence of AKI.