## Mechanism of Physiological S2 Split ### Normal S2 Closure The second heart sound (S2) is composed of aortic valve closure (A2) followed by pulmonary valve closure (P2). Normally, these occur nearly simultaneously, but during inspiration, they separate audibly. ### Inspiratory Physiology **Key Point:** During inspiration, intrathoracic pressure becomes more negative, which: 1. Increases venous return to the right heart 2. Increases right ventricular stroke volume 3. Prolongs right ventricular ejection time 4. Delays pulmonary valve closure (P2) Simultaneously, the negative intrathoracic pressure pools blood in the expanded pulmonary vessels, reducing left ventricular preload and shortening left ventricular ejection time, causing earlier aortic valve closure (A2). ### Result The interval between A2 and P2 widens, creating an audible split during inspiration that disappears during expiration. **High-Yield:** The split is **widest at peak inspiration** and **narrows during expiration**. This is a normal finding in healthy individuals, especially children and young adults. **Mnemonic:** **RILE** — Right heart volume increases, Left heart volume decreases during inspiration, creating the split. ### Clinical Pearl An **absent or fixed split S2** suggests pathology: - Fixed split: atrial septal defect (ASD) - Paradoxical split (P2 before A2): left bundle branch block, severe aortic stenosis - Absent split: pulmonary hypertension, mitral stenosis
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