Which S2 variant is commonly seen with left bundle branch block and tends to be heard on expiration and disappear on inspiration?

• A. Physiologic S2
• B. Fixed splitting S2
• C. Paradoxical (reversed) S2
• D. Splitting S2 on inspiration

Answer: (C)

Paradoxical (reversed) S2 is the variant seen with left bundle branch block. In LBBB, left ventricular activation is delayed, which prolongs the LV ejection phase and shifts the aortic valve closure (A2) later in the cycle. The pulmonic valve closure (P2) is not delayed in the same way, so P2 tends to occur before A2. That reverses the normal order of the two components and creates a split that is best heard on expiration. During inspiration, P2 is further delayed normally, which reduces the audible gap and can make the split disappear. So you hear a split mainly when the patient exhales, with the sounds reversed, which is characteristic of paradoxical S2.

Physiologic S2 splitting is normal and broadens with inspiration. Fixed splitting remains constant with respiration and is seen with an atrial septal defect. Splitting only on inspiration describes the normal inspiratory widening rather than a reversed pattern.