First Heart Sound

Introduction

The heart sounds (S1, S2, S3, and S4) are discrete short auditory events caused due to vibrations that are generated during the closure of the valve leaflets or tensing of the chorda tendineae.1 They are distinct from murmurs that are longer and are caused by turbulent blood flow. They are identified by the timing in the cardiac cycle, location, intensity, pitch, and quality. The first heart sound is conventionally referred to as S1 and has multiple high-frequency components. But only the first two sets (temporally related to the closure of mitral and tricuspid valves) are identified as two components—the mitral (M1) and tricuspid (T2); M1 always precedes T1. These two components are not easily appreciated separately in health as the separation interval is < 30 ms. Being a high-pitched sound, it is best heard with the diaphragm, which may have to be applied with sufficient pressure to listen to the S1. The quality is described as booming, duller, and longer than S2. The variations in the first sound such as split, intensity, and quality can suggest the underlying pathology to some extent. Mitral component is best heard at the apex and is louder than the tricuspid component due to the higher pressures in the left side of the heart; it can be widely heard in other areas of auscultation as well.2 3

Genesis of S1

The first heart sound is generated at the end of diastole when the atrioventricular (AV) valves reach their greatest excursion, and the blood volume is suddenly halted. A stretch recoil and subsequent vibrations in the cardio-hemic system, including tensing of chordae tendineae, results in the audible first heart sound. The S1 precedes the upstroke of carotid pulsation.4

Auscultation of S1

High pitched sounds (S1 and S2) are best heard with the stethoscope. An experienced observer can realize the time relation of S1 and S2 and the rhythmic pairing to identify the S1 and S2 (Table 1). An examination is best conducted in a warm and quiet room.

• Explain to the patient what is going to be done.

• Ensure supine position with adequate exposure of the area to be examined.

• Ensure your stethoscope has recommended features for good auscultation.

• Keep your hands and stethoscope clean and warm.

• Better complete inspection and palpation of the whole precordium first.

• It is common practice to begin the auscultation at aortic area; inching to pulmonary, tricuspid and mitral areas.

• At times turning to the left or bending forward with breath held at maximum expiration make it more obvious.

• Try to concentrate on one sound at a time. Listen to several cycles before concluding about intensity and quality of any sound.

• First heart sound is best heard with the diaphragm.

• A look at apex for the impulse (or the movement of a stethoscope with each beat) or the upstroke of carotid have additional clues to identify the S1. One may place a finger on carotid for accurate timing.

• Analysis of any acoustic event has to be coordinated with the venous, arterial, and chest wall pulsations.

Split S1

As M1 precedes TI, a split may be heard more so in tricuspid area, especially in deep inspiration, which can increase venous return to right heart and delay the tricuspid valve closure. Common causes of abnormal split first heart sound include right bundle branch block (RBBB), atrial septal defect (ASD), premature ventricular contraction (PVCs) (left ventricle [LV] origin), LV pacing, Ebstein's anomaly, or ventricular tachycardia.5 In RBBB, the LV is stimulated earlier, and it contracts faster, and M1 occurs earlier. Left bundle branch block (LBBB) has an opposite effect with no split of S1 heard (Fig. 1). Reverse S1 is a theoretical possibility when M1 can follow T1 in conditions such as LBBB, right ventricle pacing, severe mitral stenosis (MS), and left atrial myxoma; but it is very difficult to appreciate on clinical grounds.

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Fig. 1 Relationship between M1 and T1 in normal, RBBB, and LBBB conduction.

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Fig. 1 Relationship between M1 and T1 in normal, RBBB, and LBBB conduction.

Differential Diagnosis of Split S1

Three important confusing situations are possible with a split S1—(1) S4-S1, (2) S1-ejection sound, and (3) S1 and early ejection click (of mitral valve prolapse syndrome [MVPS]). S4 is a low-pitched late diastolic sound. It is best heard with the bell of stethoscope when lightly pressed; on further pressure of the bell, it may be attenuated. When left sided, it is best heard at apex and is at times palpable. On standing, S4 fades off due to lessened venous return. Ejection sound is a high-pitched early systolic sound, best heard at the base, a place where a split S1 is not well heard. Many times, it may not be easy to differentiate these two auscultatory events. Ejection click in MVPS is a late systolic event and better heard on standing.

Unusual Findings

Loud S1—Though it is common to expect a soft S1 in the presence of MR, in MVPS it may be surprisingly loud.

Soft S1—In severe MS, the S1 can be soft if there is heavy calcification, associated MR, myocarditis, or cardiac shock.

Variable Intensity of S1

Mobitz type-1 heart block, electrical alternans, digitalis toxicity, atrial fibrillation, pulsus alternans, and ventricular tachycardia with AV dissociation cause variation in the intensity of first heart sound either due to variable forces of ventricular contractions or change in PR interval.6

Conclusion

Accurate clinical identification of the first heart sound has to be practiced by all medicos and clinicians. The variations in its characteristics have to be interpreted along with other auscultatory events; not in isolation.