BACKGROUND: Surgical ablation(SA) at the time of isolated mitral valve surgery(MVS) is recommended in patients with preexisting atrial fibrillation(AF). However, SA remains infrequently utilized during MVS with a poorly quantified impact on stroke and survival. METHODS: Medicare claims(2008-2019) were queried to identify beneficiaries with preexisting AF undergoing MVS. All-cause mortality and the post-operative incidence of stroke/transient ischemic attack(TIA) were evaluated as separate endpoints. Overlap propensity score weighting was used to risk-adjust for measured confounding variables. Analyses were repeated using surgeon frequency of SA as an instrumental variable to adjust for unmeasured confounding variables. RESULTS: From 2008-2019, 41,795 Medicare beneficiaries with a preexisting diagnosis of AF underwent MVS. Surgeons were categorized, with 1,326 infrequently(bottom quartile) performing SA(<
30%
10,364 beneficiaries) and 740 frequently(top quartile) performing SA(≥62%
10,476 beneficiaries) during MVS. Beneficiaries undergoing MVS with SA("as-treated" analysis) had a risk-adjusted median survival advantage of 0.56[0.33-0.81] years (8.85[8.64-9.04] vs 8.29[8.11-8.47] years, P<
0.001 for risk-adjusted survival comparison) compared to those without. Beneficiaries undergoing MVS by frequent SA surgeons("surgeon-preference" analysis) had a risk-adjusted median survival advantage of 0.35[0.05-0.71] years (8.59[8.40-8.85] vs 8.24[7.97-8.40] years, P=0.0015 for risk-adjusted survival comparison) compared to surgeons who infrequently performed SA. CONCLUSIONS: In Medicare beneficiaries with preexisting AF, concomitant SA during MVS is associated with improved survival, as is undergoing surgery by a frequent SA surgeon. When analyzed based on surgeon preference for SA, the magnitude and time-dependent nature of the treatment effect of SA were substantially different compared to the "as-treated" analysis, suggesting that "as-treated" analyses may be subject to bias from unmeasured confounding variables.