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©2013 UpToDate ® Print Email 12-lead electrocardiogram (ECG) of a left posteroseptal accessory AV pathway Electrocardiogram during right atrial pacing in a patient with Wolff-Parkinson-White syndrome shows a long stimulus to the P wave with a short PR interval, the appearance of both right and left bundle branch block morphologies, and a negative QRS morphology and delta wave in the inferior leads. This localizes the pathway to the posteroseptal area. The presence of a positive delta wave in lead I, RS ratio in lead V1 of approximately 1, and a negative delta wave in lead II places the pathway on the left side, inside the coronary sinus os or allong the middle cardiac vein. This type of accessory pathway may be related to cardiac venous anomalies and aneurysms.