Precordial vibrations resulting from atrial contraction are normally neither palpable nor audible. Under pathologic conditions, forceful atrial contraction generates a low-frequency sound (S4) just prior to Si (also termed the atrial diastolic gallop or the presystolic gallop). Atrial contraction must be present for production of an S4. It is absent in atrial fibrillation and in other rhythms in which atrial contraction does not precede ventricular contraction. The S4 follows the onset of the P wave of the ECG by approximately 70 ms. Audibility of the S4 depends not only on its intensity and frequency but also on its separation from S1. The degree of this separation is determined primarily by the PR interval, but it is also somewhat influenced by the PS4 and the QSi intervals. A loud Si also may mask the audibility of a preceding softer
S4. The S4 is best heard at the apex impulse with the patient turned in the left lateral position. It varies considerably with respiration, usually being heard best during expiration. A left-sided S4 may radiate to the brachiocephalic and carotid vessel and be best heard in the areas in patients with severe lung disease or who are very obese. A left-sided S3 may do likewise. A left-sided S4 and S3 may also be augmented post-tussively and with sustained handgrip exercise. Both the intensity and timing of the S4 are closely related to the end-diastolic volume of the ventricle. Maneuvers that increase venous return increase the audibility by increasing the intensity of the sound and by causing it to occur earlier, thereby separating it further from Sj. Decreased venous return does the opposite. Audible fourth heart sounds are usually accompanied by a palpable presystolic apical impulse in the absence of obesity, emphysema, etc., but occasionally, palpable presystolic impulses are not audible. The S4 generated by a forceful right atrial contraction is usually heard best at the lower left sternal border. Unlike the left-sided S4, it tends to be accentuated with inspiration. It is also accompanied by prominent a waves in the JVP and is occasionally audible over the right jugular vein.268
As with the S3, both the ventricular origin of the S4 sound due to the abrupt deceleration of the atrial contribution to late diastolic filling and the impact theory have been proposed.270 It is likely that the former is responsible for the sounds recorded within the ventricular cavities or on their epicardial surfaces, whereas the latter mechanism is responsible for the S4 auscultated at the chest wall.
The presence of an S4, particularly when associated with a palpable presystolic apical impulse, is an abnormal finding. Although it is considered to be a normal finding in older subjects by some investigators,269 others feel strongly that a definite S4 in a middle-aged or older person is unlikely to be a normal event.268 Conditions such as obesity, emphysema, or barrel-chest deformity may hinder the clinical detection of both an S4 and an apical presystolic impulse.
The common pathologic conditions in which S4 is heard are listed in Table 10-11. A forceful atrial contraction into a hypertrophied, noncompliant ventricle almost always produces an early and easily audible and recordable S4. The severe LV hypertrophy present in systemic hypertension, severe valvular aortic stenosis, and hypertrophic cardiomyopathy often is responsible for a loud S4 (Fig. 10-76). In each case, the S4 is associated with a prominent apical presystolic impulse and is widely separated from S1.
Figure 10-76: Atrial diastolic (ADG) and ventricular diastolic gallops (VDG) are recorded in an adult with severe calcific aortic stenosis. The ADG is associated with a prominent presystolic apical impulse (a), and the VDG occurs during the rapid filling wave of the ACG. The carotid pulse has a very slow rate of rise and a markedly prolonged LV ejection time. The classic diamond-shaped systolic ejection murmur (SM) is present at the base and apex. Note the higher-frequency composition of the SM at the apex but preservation of the crescendo-decrescendo pattern. (From Shaver JA. Current uses of phonocardiography in clinical practice. In: Rapaport E, ed. Cardiology Update: Reviews for Physicians. New York: Elsevier; 1981:356. Reproduced with permission from the publisher and author. Copyright 1981 by Elsevier Publishing Co., Inc.)
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