12-lead electrocardiogram (ECG) of a midseptal accessory AV pathway

Electrocardiogram from a patient with Wolff-Parkinson-White syndrome. The ECG criteria for a midseptal pathway are intermediate between those for posteroseptal and anteroseptal connections within the triangle of Koch. These pathways have been difficult to predict from the surface ECG and have usually been grouped as septal pathways. The ECG shows positive delta waves in the lateral leads I, aVL, and V6, with early transition and a tall R wave in lead V2. The delta wave is positive in leads II and aVF and negative in lead III, and the QRS axis is <0°, localizing the pathway in the midseptum.