Intraoperative Hydromorphone Decreases Post-Operative Pain Who Would Have Thought? An Instrumental Variable Analysis

Abstract

Background A growing body of literature suggests that intraoperative opioid administration can lead to both increased post-operative pain and opioid requirements. However, there has been minimal data regarding the effects of the intraoperative administration of intermediate duration opioids such as hydromorphone on post-operative outcomes. Causal inference using observational studies is often hampered by unmeasured confounding, where classical adjustment techniques, such as multivariable regression, are insufficient. Instrumental variable analysis is able to generate unbiased causal effect estimates in the presence of unmeasured confounding, assuming a valid instrumental variable can be found. We previously demonstrated, using a natural experiment, how hydromorphone presentation dose, i.e. the unit dose provided to the clinician, affects intraoperative administration dose, with the switch from a 2-mg to a 1-mg vial associated with decreased administration. As the change in hydromorphone presentation dose was unrelated to any external factors, presentation dose could serve as an instrumental variable to estimate the effect of intraoperative hydromorphone administration dose on post-operative outcomes.