Heart Sounds and Murmurs
The first heart sound (S1) represents closure of the atrioventricular (mitral and tricuspid) valves as the ventricular pressure exceeds atrial pressure at the beginning of systole. S1 is normally a single sound because mitral and tricuspid valve closure occurs almost simultaneously.
The second heart sound (S2)represents closure of the aortic and pulmonary valves. S2 is normally split. The more muscular, and less compliant left ventricle (LV) empties earlier than the right ventricle, so the aortic component (A2) precedes the pulmonary component (P2) by a discernible interval. Venous return to the right ventricle (RV) increases during inspiration and P2 is even more delayed, so it is normal for the split of the second heart sound to widen during inspiration and narrow during expiration.
Abnormally wide splitting of S2 may be found in conditions associated with:
- RV volume overload, such as atrial septal defect (ASD). In the presence of an ASD, the split may be "fixed" with no change during inspiration and expiration.
- RV outflow obstruction, such as pulmonary stenosis.
- Delayed RV depolarization such as complete right bundle branch block.
S2 is single if one of the semilunar valves is missing, as in pulmonary and aortic atresia or truncus arteriosus, or if both valves close simultaneously, as in pulmonary hypertension with equal ventricular pressures.
The third heart sound (S3) represents a transition from rapid to slow ventricular filling in early diastole. S3 may be heard in normal children.
The fourth heart sound (S4)is an abnormal late diastolic sound caused by forcible atrial contraction in the presence of decreased ventricular compliance.