Heart Sounds:

The vibration of the blood against the ventricular walls as the AV valves close creates the first heart sound. This "loud" sound extends throughout the short interval of isovolumic contraction. The QRS complex of the EKG precedes the first heart sound slightly. S1 is heard best at the apex of the heart (fifth intercostal space); S1 is louder and of slightly lower pitch (mimicked as"lub") than the second heart sound (S2, mimicked as "dup").

The sudden closure of the semilunar valves creates the second heart sound (S2). The closure of the aortic and pulmonic valves occur almost simultaneously at the end of expiration, but during inspiration the aortic slightly precedes the pulmonic, producing a slight (scarcely audible) splitting of S2. S2 marks the end of the interval of ejection and the beginning of the interval of isovolumic relaxation. It is heard best at the second intercostal space, slightly to the right (aortic component) or the left (pulmonic component) of the sternum.

In most normal subjects, S2 is heard through the stethoscope as a single sound. However, the aortic valve may close slightly (producing A2) before the pulmonic valve, especially during inspiration. This is because the intrapleural (intrathoracic) pressure becomes more negative during inspiration. This negative pressure transiently increases the capacitance (or decreases impedance or resistance) of the intrathoracic pulmonary vessels. As a result, a temporary delay is produced in the diastolic “back pressure” of the pulmonary artery responsible for closure of the pulmonary valves. Therefore, the pulmonic component of S2 (P2) is delayed. Also during inspiration, the negative intrathoracic pressure decreases return of blood to the left heart. This slightly decreases left ventricular stroke volume and shortens the time required for emptying, causing the aortic valve to close slightly sooner, hence advancing the aortic component (A2) of S2. The slight advance of aortic valve sound along with the slight delay of pulmonic valve sound produces the splitting of S2 during inspiration. This physiological splitting of S2 is normal if you are a breathing person.

The third heart sound (S3) is normally not heard with stethoscope, but can be heard in patients with left ventricular failure. The sound is due diastolic inflow through the mitral valve as the ventricles expand rapidly. The fourth heart sound (S4) is due to atrial contraction-induced oscillation of blood, and has very low frequency and intensity. S4 is heart just before S1.

Murmurs

Murmurs result from turbulent blood flow. When normal heart valves are open, their wide diameter allows rapid blood flow without rapid blood velocity and no appreciable turbulence develops. Stenotic valves have smaller orifices, causing more rapid blood velocities for the same blood flow. Valves that do not close properly (incompetent or insufficient valves) provide a small orifice through which blood regurgitates and the rapid velocity produces turbulence and the murmur. Distortion of the normally smooth surfaces of the valves by rheumatic heart disease may also contribute to the development of turbulence. Often a valve that is damaged by rheumatic disease is both stenotic and incompetent.

Murmurs can be heard with the aid of the stethoscope, and they can be recorded with a phonocardiograph. Phonocardiograms that are characteristically recorded for murmurs of the valves of the left heart are shown below.

(Reprinted from Circulatory Physiology-the essentials 2nd ed., (1984) by J.J. Smith & J.P. Kampine, Figure 3.6, page 44, Lippincott, Williams & Wilkins.)

A stenotic aortic valve results in a loud harsh murmur during systole. These murmurs are sometimes so intense that they can be heard without the aid of a stethoscope and they sometimes produce a palpable vibration referred to as a "thrill". A major feature of aortic stenosis is the high left ventricular pressures generated during systole - much higher than the aortic pressure. This increased afterload induces left heart hypertrophy, and eventually the left heart fails.

A stenotic mitral valve results in a softer lower pitched murmur than that of aortic stenosis and it is heard during diastole. A major feature of mitral stenosis is the increased left atrial pressure that develops. This pressure 'backs up' into the pulmonary vein, pulmonary capillaries and pulmonary artery (leading to a "congested lung" and pulmonary edema).

An incompetent aortic valve allows aortic blood to regurgitate into the left ventricle during diastole. The sound of aortic regurgitation is described as a "blowing" diastolic murmur, less loud than that caused by a stenotic aortic valve. A major feature of aortic regurgitation is that the end-diastolic volume (preload) of the left ventricle is greatly increased.

An incompetent mitral valve allows blood to regurgitate through the mitral valve (and into the left atrium and pulmonary vein) during systole in addition to being pumped properly through the open aortic valve. The sound of mitral regurgitation is similar to that of aortic regurgitation. However, the latter occurs during diastole. Mitral regurgitation also produces increased pressure in the pulmonary circuit that may eventually result in pulmonary congestion.

If severe, any of the valvular problems described above will decrease cardiac output and lead to a cascade of complications, some helpful and some deleterious to cardiac function.