Timing of insertable cardiac monitor implantation after embolic stroke of undetermined source and its impact on atrial fibrillation detection: A target trial emulation analysis

Background: A substantial proportion of ischemic strokes remain classified as embolic stroke of undetermined source (ESUS) despite standard diagnostic evaluation. Prolonged cardiac monitoring with implantable cardiac monitors (ICMs) increases atrial fibrillation (AF) detection, but the optimal timing of ICM implantation after ESUS remains uncertain. Aims: To evaluate whether early versus delayed ICM implantation after ESUS influences AF detection and time to diagnosis. Methods: We conducted a multicenter observational cohort study emulating a target trial. Consecutive ESUS patients undergoing ICM implantation were classified as ICMEARLY (⩽30 days) or ICMDELAYED (31–365 days) implantation after the index event. Inverse probability weighting was applied to adjust for baseline confounding. Primary and secondary outcomes included AF detection within 30, 90, and 120 days after implantation, assessed using weighted logistic regression, Poisson models for detection rates per person-time, Cox proportional hazards models, and restricted mean survival time (RMST). Sensitivity analyses included center-level clustering and competing-risk models. Results: Among 333 patients (90 ICMEARLY and 243 ICMDELAYED), early implantation was associated with significantly higher AF detection within 30 days (7.8% vs 1.6%; odds ratio (OR) = 4.49, 95% confidence interval (CI) = 1.17–17.27; p = 0.028) and higher detection rates per person-time (incidence rate ratio (IRR) = 4.26, 95% CI = 1.16–15.60; p = 0.029). Consistent associations were observed at 90 and 120 days. Time-to-event analyses showed higher hazards of AF detection with early implantation (hazard ratio (HR) = 4.29 at 30 days; HR = 2.97 at 90 days; HR = 2.77 at 120 days; all p < 0.01). RMST analyses demonstrated progressively shorter time to AF diagnosis in the ICMEARLY group across multiple time horizons. Results were robust across sensitivity analyses. Conclusion: Early ICM implantation after ESUS is associated with higher and faster AF detection compared with delayed implantation. When ICM monitoring is indicated, avoiding unnecessary delays may substantially enhance diagnostic yield.