This ECG demonstrates the features of ventricular pre-excitation 1. The PR interval is less than 0.12 seconds (0.09 seconds). 2. The initial portion of the QRS complex is slurred (arrows) due to slow conduction in the pre-excited portion of the ventricle. This is the “delta wave”. The remainder of the QRS complex is narrow, representing normal ventricular activation via the His-Purkinje system. 3. The duration of the QRS complex is prolonged to 0.12 seconds. 4. The abnormally flat, or inverted T waves in leads I, V3-V6 are secondary T wave changes.