The effect of type of anaesthetic on delirium after surgery for acute hip fracture: An instrumental variable analysis to assess causation

Delirium is the most common in-hospital complication affecting older adults with acute hip fractures. Current evidence demonstrates inconsistent associations between anaesthetic type for acute hip fracture surgery and postoperative delirium. Using the Australian and New Zealand Hip Fracture Registry database, we conducted a retrospective cohort study of patients aged 50 years and over who underwent acute hip fracture surgery between 2015 and 2020. The incidence of delirium in patients who received general anaesthesia alone or combined with a regional technique, versus those who received spinal or regional anaesthesia was assessed. Multivariable multilevel logistic regression was used to test associations between anaesthetic type and delirium controlling for known confounders. Finally, given hospital variation in preference for anaesthetic type, an instrumental variable analysis was performed to include the effect of both known and unknown confounding. Of 35,252 patients, 25,682 (72.9%) patients received general anaesthesia, and 9570 (27.2%) patients received spinal or regional anaesthesia for their hip fracture surgery. A higher proportion of patients who received general anaesthesia developed delirium than those who received spinal or regional anaesthesia (40.6% vs. 35.7%, odds ratio (OR) 1.23, 95% confidence intervals (CI) 1.18 to 1.30, P < 0.0001). After adjusting for known confounders, general anaesthesia patients were at slightly increased odds of developing delirium (OR 1.14, 95% CI 1.04 to 1.25, P = 0.0052). However, the instrumental variable analysis found no statistically significant difference between groups (OR 1.03, 95% CI 0.99 to 1.07, P = 0.141). Therefore, while a weak association was found between general anaesthesia exposure and postoperative delirium, an instrumental variable analysis to compensate for unmeasured confounding showed no causal association between general anaesthesia and postoperative delirium.