BackgroundTo explore the incidence, predictors, and association of first pass effect (FPE) on clinical outcomes of mechanical thrombectomy (MT) for acute vertebrobasilar artery occlusion (VBAO) in the Asian population.MethodsWe selected patients from the ANGEL-ACT registry. We defined FPE as complete recanalization after one MT attempt without rescue treatment, multiple pass effect (MPE) as complete recanalization after >1 MT attempt or after one MT attempt with rescue treatment, and incomplete recanalization (ICR) as a modified Thrombolysis in Cerebral Ischemia score of 2b, independent of the number of MT attempts. We conducted multivariable logistic analyses to identify the independent predictors of FPE and to compare the outcomes, including favorable outcome, mortality within 90 days, and intracranial hemorrhage (ICH) among the FPE, MPE, and ICR groups.ResultsTwo hundred and seventy-nine patients, 68 with FPE (24.4%), 114 with MPE (40.9%), and 83 with ICR (29.7%), were included. Underlying intracranial atherosclerosis disease (ICAD) (adjusted OR (aOR) 0.16, 95% CI 0.08 to 0.34, P<0.001) was independently associated with a low chance of FPE. Furthermore, FPE was associated with a favorable outcome compared with MPE and ICR (MPE vs FPE, aOR 0.49, 95% CI 0.25 to 0.97, P=0.040; ICR vs FPE, aOR 0.38, 95% CI 0.18 to 0.81, P=0.012), and decreased mortality (MPE vs FPE, aOR 2.57, 95% CI 1.04 to 6.36, P=0.041) compared with MPE but similar mortality to ICR (P=0.374). No difference was found for ICH among the three groups (P>0.05).ConclusionsFPE occurred in 24.4% of patients with VBAO in our cohort, which was associated with improved clinical outcomes compared with MPE and ICR. VBAO with underlying ICAD was less likely to achieve FPE.Trial registration numberNCT03370939.