Figure S1. Sample ECGs (limb leads only) pre- and postablation of a left lateral AP. The QRS complexes in the inferior leads change markedly after abolition of AP conduction. Despite the complete reversal of dominant lead III polarity in both examples, the typical II > III relationship is maintained with preexcitation from a left anterolateral AP.

Figure S2. (A) Twelve-lead ECG with minimal preexcitation incorrectly identified as LAL-AP, with lead II/III ratio > 1. (B) During rapid atrial pacing and maximal preexcitation, the QRS polarity becomes negative in lead III, resulting in a lead II/III ratio < 1.

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