ECG Diagnosis: The ECG diagnosis is sinus rhythm (80 bpm), with an intraventricular conduction disturbance, in this case, a partial right bundle branch block. A sinus rhythm is present based on upright P waves in Lead II (arrows on annotated version). The P waves are conducted normally (P-Q interval: 100 ms) to wide QRS complexes with prominent S waves. Because the wide QRS complexes are consistently related to sinus P waves, their abnormal shape likely reflects an intraventricular conduction disturbance. As the QRS duration is at the upper limit of normal (~80ms) and the S wave is slightly more than 50% of the QRS duration, an incomplete or partial right bundle branch block is the most likely cause of this conduction disturbance. The S waves suggest the current is moving away from Lead II, which means it is moving to the right at the end of ventricular depolarization; this occurs because the normal rapid conduction through the right bundle to the right heart is blocked. Fuzzy’s right bundle branch block was not present prior to his balloon pulmonary valvuloplasty. Damage to the right bundle branch may occur due to local trauma caused by the balloon inflation and/or the catheter passing through the right heart. View an annotated image of Fuzzy’s ECG highlighting the relevant findings.