Abstract Body (Do not enter title and authors here): Introduction: Observational studies have consistently demonstrated a strong association between longer door-to-balloon (DTB) times and increased mortality in patients presenting with ST-elevation myocardial infarction (STEMI). However, the extent to which this association is attributable to unmeasured differences between patients with different DTB times (i.e., confounding), as opposed to a causal impact of longer DTBs is unclear.

Aim: In this study, we exploit a natural experiment using an instrumental variable (IV) approach based on weekday vs weekend presentation to evaluate whether incremental delays in DTB times have a causal effect on patient outcomes.

Methods: We performed a retrospective analysis of patients undergoing immediate percutaneous coronary intervention (PCI) for STEMI in the CathPCI Registry from 1/2010 to 12/2021 at 1,422 sites. Time of presentation with STEMI (weekday daytime vs weekend daytime) was used as an IV to address potential confounding. A 2-stage IV analysis was used. In-hospital all-cause mortality was the primary outcome.

Results: A total of 447,355 patients presented with STEMI during the study period. The average difference in weekend (N=125,787) minus weekday (N=321,568) daytime DTB times was 10.5 minutes. Patient, procedural, and hospital characteristics were well balanced between groups. Overall, in-hospital mortality was 3.5%. In the IV analysis, delays in DTB time were not associated with increased odds of in-hospital mortality (OR 0.99, 95% CI 0.95 to 1.03, P = 0.70). This null association was also observed when limited to hospitals with larger differences (19.5 minutes) between weekend and weekday DTB time (OR 1.00, 95% CI 0.96 to 1.03, P = 0.85) and among patients predicted to have the longest (mean 70.8 minutes) DTB times (OR 1.00, 95% CI 0.94 to 1.07, P = 0.96).

Conclusions: Contrary to prior findings, incremental delays in DTB time for patients presenting with STEMI are not associated with increased in-hospital mortality when analyzed using quasi-experimental methods less likely to be influenced by unmeasured confounding. Efforts to further reduce DTB times in patients presenting to hospitals with STEMI are unlikely to improve outcomes. In the current era, policies intended to incentivize better care for STEMI patients should prioritize other measures of quality beyond DTB.