| Summary: | Aim: To evaluate the role of baseline 12‑lead ECG in predicting the syncope mechanism during continuous monitoring using an implantable loop recorder (ILR). Methods: Consecutive patients with syncope implanted with an ILR were enrolled. Baseline 12‑lead ECG were related to ECG diagnosis derived from ILR tracings recorded at the time of syncope recurrence. Results: In total 300 patients with a mean age of 66 ± 16 years were included, 49% (146/300) received an ILR-guided diagnosis during follow-up. Patients with abnormal baseline ECG more frequently received an ILR-guided diagnosis compared to those with normal baseline ECG 59% vs. 44%, p = 0.018. For a diagnosis of arrhythmic syncope, the corresponding frequencies were 45% vs. 26%, p = 0.001.Patients with bifascicular block significantly more common received an ILR-guided diagnosis 76% (25/33) compared to those with normal baseline ECG 44% (90/205), p ≪ 0.001. In this subgroup, 96% (24/25) were diagnosed with arrhythmic syncope, 23 of which were due to bradyarrhythmia. Bifascicular block occurred almost exclusively among those ≥60 years (31/33). After logistic regression the adjusted OR for arrhythmic syncope was significant for bifascicular block 5.5 (95%CI 2.3–13.2), p ≪ 0.001. PPV for bifascicular block in predicting arrhythmic syncope was 73% and NPV 73%. Conclusion: A baseline 12‑lead ECG with bifascicular block was a strong predictor for syncope during follow-up, most often due to bradyarrhythmia caused by intermittent complete heart block. No other ECG findings were associated with the ILR outcome. We find it reasonable to consider permanent pacing instead of an ILR for patients with bifascicular block and unexplained syncope. Keywords: Syncope, Implantable loop recorder, Arrhythmia, Bifascicular block
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