Two normally heard heart sounds are the first heart sound (S1) and second heart sound (S2). Second heart sound has two components – aortic (A2) and pulmonary (P2). Normal A2 is heard in all auscultatory areas while normal P2 is heard only in the pulmonary area. If it is heard at the apex, it can be considered as loud P2. Normal split of second heart sound closes in expiration and is audible only in inspiration. If it is audible in both inspiration and expiration, it is called wide split. If the split increases in expiration and closes in inspiration, it is paradoxical split. Wide fixed split is audible in both inspiration and expiration, without any change in the A2-P2 interval. Third (S3) and fourth (S4) heart sounds are usually heard in pathological conditions. But an S3 may be heard in normal children and in pregnancy. An S4 may be heard in the elderly.
When S3 or S4 is associated with tachycardia, it is called a gallop rhythm (S3 or S4 gallop). S3 gallop is also known as protodiastolic gallop. S4 gallop is otherwise known as presystolic gallop. In tachycardia, S3 may fuse with S4 and produce a summation gallop (sometimes called S7 gallop!).
- First heart sound: First heart sound is a high pitched sound and is heard best with the diaphragm of the stethoscope. It is best heard at the cardiac apex. First heart sound is timed in relation to the carotid pulse as it occurs at the onset of systole. Interval between the first and second heart sounds constitutes the clinical systole and that from second heart sound to the next S1 the clinical diastole. A loud S1 is heard in mitral stenosis while a soft S1 is heard in mitral regurgitation. Intensity of S1 decreases with decreasing left ventricular systolic function. In severe calcific mitral stenosis, when the valve becomes very stiff, intensity of S1 may decrease. Intensity of S1 increases in tachycardia when the diastolic period is shortened. Split of S1 is not audible usually, but may be heard in right bundle branch block and Ebstein’s anomaly of the tricuspid valve. In the latter condition, the loud tricuspid component (T1) is known as sail sound and it is due to the closure of the large anterior tricuspid leaflet.
- Second heart sound: Second heart sound is also a high pitched sound, better heard with the diaphragm of the stethoscope. A2 is loud in systemic hypertension and soft in aortic stenosis. P2 is loud in pulmonary hypertension and soft in pulmonary stenosis. Wide fixed split of S2 is a feature of atrial septal defect. Wide variable split may be heard in right bundle branch block due to delayed closure of the pulmonary valve. Paradoxical split can occur in left bundle branch block and aortic stenosis. But it is often difficult to appreciate in the latter condition due to the preceding loud murmur.
- Third heart sound: Third heart sound is a low pitched sound and hence better heard with the bell of the stethoscope. Left ventricular S3 is usually a feature of left ventricular failure and best heard at the apex. S3 in mitral regurgitation need not imply left ventricular failure while an S3 in aortic regurgitation implies left ventricular failure. A right ventricular S3 may be heard medial to the apex beat in the tricuspid area in right ventricular failure.
- Fourth heart sound: Fourth heart sound occurs when there is a forceful atrial contraction and is common in the elderly with a stiff left ventricle. S4 is designated as an atrial sound, though it possibly originates in the ventricle. It can occur in acute ischemia of the myocardium when the relaxation of the left ventricle is impaired. Being a low pitched sound, it is better heard with the bell of the stethoscope.