Closure of the valves and rapid movement of blood within the heart give rise to sounds that can be heard at the chest wall. The first heart sound, S1 in Fig. 3, accompanies the start of the isovolumetric contraction phase and is caused primarily by tensing of the atrioventricular valves as they close and the redistribution of blood within the ventricles. Note that antegrade flow through the semilunar valves does not contribute to S1. As atrial systole forces blood through the AV valves, the Bernoulli forces tend to keep the valve cusps close to each other. Thus, as ventricular contraction occurs, very little blood regurgitates back into the atrium before the AV valves close. If ventricular contraction is delayed, however, as in first-degree heart block, then the cusps may have moved away from each other before ventricular systole and an exaggerated first heart sound will result because the valves must stop a considerable regurgitant flow. S1's pitch is low because the large atrioventricular valves and associated ventricular structures have a low resonant frequency. S1 sounds like the word lub. Because the vibrations involve ventricular structures, the first heart sound is heard best low in the chest near the apex of the heart. The second heart sound, S2 in Fig. 3, is associated with the closing of the aortic and pulmonic valves and is caused by the resulting vibration of the valves and the large arteries near the valves as blood is abruptly decelerated in the outflow tract. The second heart sound occurs at the beginning of the isovolumetric relaxation phase. The semilunar valves, the aortic and the pulmonic, are small and have a higher resonant frequency. The second heart sound sounds like the word dub. Because the second heart sound involves vibrations of the aorta and pulmonary artery, it is best heard high in the chest.
The third heart sound (S3) results from turbulence associated with the rapid filling of the ventricles; the fourth heart sound (S4) results from the movement of blood during atrial systole. The third and fourth heart sounds are not usually audible with a stethoscope, but can be seen when the sound waves are recorded electronically and displayed as in Fig. 3. They are best appreciated when the stethoscope is placed near the apex of the heart. When S3 or S4 is heard, the sound is referred to as a gallop because its cadence in conjunction with S1 and S2 sounds like a galloping horse. S3 is normally audible in children and some narrow-chested adults. In certain disease states, the third or fourth heart sound may be significantly augmented. The third heart sound is most commonly audible when the ventricle is dilated and atrial pressure is high as in heart failure. An audible fourth heart sound occurs most commonly in hearts in which the ventricle is very stiff, as is the case in a hypertrophied heart.
Although the mitral valve normally closes slightly before the tricuspid, the separate components are not easily discerned. Because the right ventricle is normally weaker than the left ventricle, the pulmonic valve closes significantly later than the aortic, causing splitting of the second heart sound. If the venous return is selectively increased to the right ventricle by a deep inspiration, pulmonic valve closure will be further delayed and the splitting will be accentuated. We call this response physiologic splitting. This is of diagnostic significance because a failing left ventricle, which has abnormally low contractility, will eject blood more slowly and valve closure will be delayed. If the left ventricle were so depressed that the aortic valve closed after the pulmonic valve, then a deep inspiration would reduce the splitting rather than accentuate it. This is referred to as paradoxical splitting and it may occasionally be heard in patients with left ventricular failure. Left bundle branch block can also delay aortic valve closure and produce paradoxical splitting of the second heart sound.
Was this article helpful?