During inspiration, the physiologic split of S2 widens due to delayed closure of which valve, and what is the primary hemodynamic mechanism responsible for this delay?

## Physiologic Splitting of S2 During Inspiration **Key Point:** During inspiration, the A2–P2 split widens because P2 (pulmonary valve closure) is delayed. The primary mechanism is **increased venous return to the RV**, which increases RV stroke volume and prolongs RV ejection time. ### Hemodynamic Mechanism ```mermaid flowchart TD A[Inspiration: intrathoracic pressure becomes more negative]:::action A --> B[Increased venous return to right atrium]:::action B --> C[Increased RV preload and stroke volume]:::action C --> D[Prolonged RV ejection time]:::action D --> E[Delayed pulmonary valve closure<br/>P2 moves later in diastole]:::outcome E --> F[A2-P2 split widens<br/>becomes audible]:::outcome ``` ### Physiologic Details | Phase | Aortic Valve (A2) | Pulmonary Valve (P2) | Split | | --- | --- | --- | --- | | **Expiration** | Closes at ~80 ms | Closes at ~90 ms | 10–20 ms (usually inaudible) | | **Inspiration** | Closes at ~80 ms (unchanged) | Closes at ~110 ms (delayed) | 40–60 ms (audible) | **High-Yield:** The widened split during inspiration is a **normal finding** and indicates normal RV function and pulmonary compliance. A fixed (non-splitting) S2 suggests atrial septal defect (ASD) or other right-to-left shunt. **Mnemonic:** **RILS** — **R**ight side receives more blood during **I**nspiration, **L**engthens RV ejection, **S**plit widens. **Clinical Pearl:** The physiologic split disappears in expiration because venous return to the RV decreases, RV stroke volume normalizes, and P2 moves closer to A2. This is an important bedside finding for confirming normal cardiac physiology.