Longer collapse-to-first-medical-contact interval is independently associated with ROSC in out-of-hospital cardiac arrest: a paradoxical finding that may reflect case-mix differences

Background Shortening prehospital time intervals is a cornerstone of out-of-hospital cardiac arrest (OHCA) management, with earlier medical contact generally assumed to improve survival. However, this assumption has rarely been scrutinized at the level of individual time metrics, particularly the interval from witnessed collapse to first medical contact (collapse-to-FMC), whose relationship with resuscitation outcomes independent of case-mix remains unclear. This study aimed to examine the association between core prehospital time intervals and the achievement of prehospital return of spontaneous circulation (ROSC). Patients and methods This retrospective cohort study included 152 consecutive OHCA patients managed by the Cixi Emergency Center between April 2023 and March 2026. Time intervals were derived from synchronized dispatch center logs and onboard system timestamps, including dispatch-to-scene arrival, collapse-to-first medical contact, and collapse-to-CPR initiation intervals. Candidate variables for multivariable modeling were selected based on a prespecified causal rationale and clinical relevance, rather than solely on univariate statistical significance. Univariate logistic regression was performed for descriptive purposes to examine crude associations with prehospital ROSC. The main model included the two core time intervals (collapse-to-FMC and collapse-to-CPR initiation). Model 1 further adjusted for age, a clinically important confounder that did not reach statistical significance in univariate analysis. Model 2 additionally adjusted for etiology of arrest and presence of a shockable rhythm, which were selected a priori as pre-exposure confounders based on established prognostic importance in OHCA research. This approach ensured that clinically important confounders were retained regardless of their univariate P values. A main model included only time variables, and an adjusted model further incorporated clinical covariates. Multicollinearity was assessed using variance inflation factors, and model calibration was evaluated with the Hosmer–Lemeshow test. Discriminative ability was assessed by receiver operating characteristic curve analysis. Subgroup analyses were performed by age (<65 vs. ≥65 years) and etiology (cardiac vs. non-cardiac). Results Among 152 patients, 52 (34.2%) achieved prehospital ROSC. Conventional dispatch-related intervals did not differ between groups. The collapse-to-first medical contact interval was significantly longer in the ROSC group (20.0 vs. 13.1 min, P < 0.001) and remained independently associated with ROSC after adjustment for prespecified pre-exposure confounders, including age, etiology of arrest, and presence of a shockable rhythm (OR 1.188, 95% CI: 1.079–1.307, P < 0.001). Bootstrap internal validation yielded an optimism-corrected AUC of 0.882 (95% CI: 0.827–0.936), with a calibration intercept of −0.063, a calibration slope of 0.972, and a Brier score of 0.132, indicating adequate calibration. In subgroup analyses, this association was predominantly observed in patients younger than 65 years (adjusted P = 0.037), and the interval from collapse to CPR initiation became significant in the cardiac etiology subgroup after adjustment (OR 1.567, 95% CI: 1.054–2.329, P = 0.026). Conclusion In this cohort of OHCA patients, a longer collapse-to-first medical contact interval was paradoxically associated with increased odds of prehospital ROSC, independent of prespecified pre-exposure confounders. This counterintuitive finding suggests that the interval may reflect patient salvageability and unmeasured case-mix characteristics rather than treatment delay alone. These results challenge the conventional premise that shorter time to medical contact uniformly translates to better outcomes, and underscore the need for a more nuanced understanding of how patient characteristics shape the relationship between prehospital timing and resuscitation success.