The Second Heart Sound (S2)

Abstract

Definition The second heart sound (S 2) is a short burst of auditory vibrations of varying intensity, frequency, quality, and duration . It has two audible components, the aortic closure sound (A2) and the pulmonic closure sound (P2), which are normally split on inspiration and virtually single on expiration . S 2 is produced in part by hemodynamic events immediately following closure of the aortic and pulmonic valves. The vibrations of the second heart sound occur at the end of ventricular contraction and identify the onset of ven-tricular diastole and the end of mechanical systole. Technique The examination should be performed in a warm, quiet room in a manner identical to that described in Chapter 22, The First Heart Sound. Clinical assessment of S 2 is best performed with the patient lying comfortably in the supine position and breathing normally. First, attempt to palpate the aortic and pulmonic components of the second heart sound in the second right and second or third left intercostal spaces (ICS), respectively. Then begin cardiac auscultation with the stethoscope placed at the second right ICS. The second sound, like the first, is evaluated by sequentially auscultating over the second left ICS, the fourth left ICS along the left sternal border, and the cardiac apex. When listening to the heart sounds, it is essential simultaneously to palpate either the carotid artery or apex impulse to determine the onset of systole. The second heart sound is of shorter duration and higher frequency than the first heart sound. It has two audible components, the aortic closure sound (A 2) and the pulmonic closure sound (P 2), which must be separated by more than 20 msec (0 .20 sec) in order to be differentiated and heard as two distinct sounds. It is clinically very important to determine the presence and degree of respiratory splitting and the relative intensities of A 2 and P 2. Splitting is best identified in the second or third left ICS, since the softer P2 normally is confined to that area, whereas the louder A 2 is heard over the entire precordium, including the apex. In order to appreciate splitting of S 2 , it may be useful to gradually move the stethoscope ("inching") from the second right ICS to the fourth left ICS. The influence of respiration on the second sound is extremely important. The examiner will wish to note respiratory variation both during quiet breathing and at times during exaggerated breathing. Slow, regular respirations are best for auscultation because a long deep breath may attentuate P2 by interposing lung tissue over the stethoscope , and only a single sound will be heard. The interval between the two audible components of the second heart sound normally increases on inspiration and virtually disappears on expiration. The patient's age must be taken into 122 consideration when assessing splitting of the second sound, since the likelihood of hearing a single S2 during both respiratory phases increases with advancing age. As with the first sound, the patient should be examined in several positions . The supine position in young patients, for example, may yield an erroneous impression of abnormally wide S 2 splitting, which can be avoided by reexamining the patient in the sitting or standing position. The Valsalva maneuver may also be used to exaggerate splitting of the second sound. When the second sound is split and both components can be heard and identified, a reliable judgment about the relative loudness (intensity) of each component can be made. Basic Science Rouanet, more than 140 years ago, attributed the second heart sound to closure of the aortic and pulmonic valves. Although this explanation has been generally accepted to the present time, several theories have been proposed to explain the genesis of the second heart sound. The most tenable to date suggests that closure of the aortic and pul-monic valves initiates the series of events that produces the second heart sound. The main audible components, however , result from vibrations of the cardiac structures after valve closure. Using high-fidelity, catheter-tipped micro-manometers and echophonocardiography, it has been shown that the aortic and pulmonic valves close silently and that coaptation of the aortic valve cusps precedes the onset of the second sound by a few milliseconds. The second sound therefore originates from after-vibrations in the cusps and in the walls and blood columns of the great vessels and their respective ventricles. The energy from these oscillations comes from sudden deceleration of retrograde flow of the column of blood in the aorta and pulmonary artery when the elastic limits of the tensed valve leaflets are met. This abrupt deceleration sets the whole cardiohemic system into vibration. In order to understand splitting of the second heart sound, knowledge of its relationship to the cardiac cycle is essential. A2 and P2 are coincident with the incisurae of the aorta and pulmonary artery pressure curves, respectively, and terminate left and right ventricular ejection periods. Right ventricular ejection begins prior to left ventricular ejection, has a slightly longer duration, and terminates after left ventricular ejection, resulting in P2 normally occurring after A2. The differences between the aortic and pulmonary artery vascular impedance characteristics are also essential to understanding the effects of respiration on splitting of S 2. When the pressure curves of the pulmonary artery and right ventricle are recorded simultaneously, the pulmonary artery curve at the level of the incisura (dicrotic notch) lags behind the right ventricular curve, or "hangs out" after it. The duration of the "hangout interval" is a measure of