The impact of surgical time of day and surgical site infection.

Disruption to the circadian clock has impacts on health and wellbeing, including impaired immune function and wound healing. It is plausible that the timing of surgical procedures could impact the body's response to trauma and susceptibility to infections. Using the Health Quality & Safety Commission data on surgical site infections (SSI) we sought to investigate any possible time of day effects on the incidents of recorded infections. All recorded data from June 2017 to July 2021 were extracted for cardiac and orthopaedic operations in New Zealand. The incidence of SSI was calculated for each 4-hour bin of time across the day. There was a total of 87,034 cases in the analysis, of which 1,327 (1.5%) had an infection. Univariately, there was a significant time of day effect, with operations after 8 pm being 3.91 times more likely to have an infection (p<0.001), and operations between 4 pm and 8 pm being 0.71 times more likely (p=0.03) to have an infection compared to operations conducted between 12 pm and 4 pm. However, when age, acuity and American Society of Anesthesiologists score were included in the model, there were no differences in the rate of infection by time of day. This preliminary work suggests that surgical time of day does not significantly impact incidence of surgical infection when other variables are controlled for. However, this work has not considered other factors that may also be important. We plan to link the SSI data with the National Minimum Dataset to investigate these other factors and conduct a more comprehensive analysis.