Yes, a split S2 that appears during deep inspiration and disappears during expiration is a normal physiologic finding.
Understanding Physiologic Splitting
This respiratory variation in S2 splitting represents normal cardiovascular physiology and requires no further evaluation in an otherwise healthy individual. The phenomenon occurs because inspiration causes the aortic component (A2) to occur earlier while the pulmonic component (P2) is delayed, creating an audible split that disappears with expiration 1.
Mechanism of Normal Splitting
The physiologic basis involves:
- During inspiration: The split widens to approximately 27 msec on average, with the major contributor being a decrease in pulmonary vascular impedance rather than simply increased venous return 1
- During expiration: The two components (A2 and P2) merge back together, making the split inaudible
- The aortic valve normally closes before the pulmonic valve, with A2 preceding P2 1
Critical Distinction: When S2 Splitting is Abnormal
Fixed splitting (present during both inspiration AND expiration) is pathologic and should raise immediate concern for structural heart disease, particularly atrial septal defect 2. The ACC/AHA guidelines explicitly state that "fixed splitting of the second heart sound during inspiration and expiration in a patient with a grade 2/6 midsystolic murmur in the pulmonic area and left sternal border should suggest the possibility of an atrial septal defect" 2.
Clinical Algorithm for S2 Splitting Assessment
If the split:
- Appears with inspiration and disappears with expiration → Normal finding, no workup needed
- Persists through both inspiration and expiration (fixed) → Obtain echocardiography to evaluate for ASD 2
- Shows reversed pattern (wider on expiration) → Evaluate for severe aortic stenosis or left bundle branch block 2
Common Pitfall to Avoid
Do not confuse physiologic (respiratory-variant) splitting with fixed splitting. The key distinguishing feature is the respiratory variation—normal splitting is dynamic and changes with breathing, while pathologic fixed splitting remains constant regardless of respiratory phase 3.