A paradoxically split S2 describes the presence of a split S2 during expiration only (the opposite of a physiologically split S2). This is caused by any condition that delays aortic valve closure. During expiration, the aortic valve closes after the pulmonary valve, creating a split. During inspiration, the increase in RV filling causes pulmonic valve closure to become “as delayed as” aortic valve closure, such that the two sounds become single. The most common cause of paradoxically split S2 is a left bundle branch block. Other causes of delayed aortic valve closure include aortic stenosis, systemic hypertension, aortic regurgitation, and LV failure.