Detecting the gallop: the third heart sound and its significance

Good technique and a reflective approach assist clinicians to identify an easily missed indicator of ventricular dysfunction

The term “gallop rhythm” was first coined in 1847 by Jean-Baptiste Bouillaud to describe the cadence of three heart sounds occurring in rapid succession. It was further described by his pupil Pierre Potain as follows:

Third heart sound (S3) and fourth heart sound gallops are indicators of underlying ventricular dysfunction; however, the term gallop is only used when the sounds are pathological.

The S3 is a mid-diastolic, low pitched sound (Box 1). In early diastole, the ventricular pressure falls below atrial pressure, and the atrioventricular valves open initiating rapid ventricular filling. Filling slows as the ventricles reach the limit of their elasticity. It is thought that the S3 occurs in the presence of volume overload or ventricular dysfunction when the rapid filling phase ends abruptly.2 It is not completely clear whether the sound arises from vibrations in the valve cusps or tautening of the papillary muscles. Factors that affect S3 intensity include age, atrial pressure, unobstructed flow across the atrioventricular valve, rapidity of early diastolic filling, compliance of the ventricle, blood volume, ventricular cavity size, and patient positioning.

The sound is easily missed, as the level of clinical experience correlates with the ability to detect it.3 The S3 is a low frequency sound in the range of 20 to 70 Hz, with a thudding quality, easily masked by respiratory or environmental sounds.2 It is localised to a small area of the precordium, does not radiate and can be completely missed unless the bell or its equivalent is used during auscultation. An S3 can originate from the left or right ventricle. The differential diagnosis includes splitting of the second heart sound, opening snap of the mitral or tricuspid valve, tumour “plop” from atrial myxoma, or pericardial knock. An S3 is best detected when it is anticipated based on valuable clinical clues of heart failure or valvular pathology (Box 2).

While the sound challenges many clinicians, there are several strategies that can be effective in its detection. It is essential to tune out all other sounds, including systole, and focus on diastole alone (Box 3). As the sound itself may not be distinct or sharp like the first or second heart sounds, clinicians should become accustomed to recognising the cadence which could indicate its presence. An S3 originating in the left ventricle is best heard in the left lateral decubitus position (a position that brings the apex closest to the stethoscope), in held end expiration, and by using the bell or light pressure. It can be augmented by mild exertion which worsens left ventricular dysfunction. An S3 originating in the right ventricle is best heard along the left lower sternal border, sometimes in the epigastrium, and augmented during inspiration and manoeuvres that increase venous return.

The S3 is often physiological in asymptomatic children and young adults, but usually pathological in people over 40 years of age. It correlates with ventricular dysfunction or volume overload. Typical causes of ventricular dysfunction include ischaemic heart disease, cardiomyopathy, myocarditis and cor pulmonale. The S3 can also be heard in aortic and mitral regurgitation, and high output heart failure associated with anaemia, pregnancy, arteriovenous fistulae, thyrotoxicosis and left-to-right shunts.

The S3 can serve as a vital clue in the detection of patients with congestive cardiac failure as well as their risk stratification. The acute decompensated heart failure syndromes (ATTEND) registry study, a multivariate analysis performed in 4107 patients hospitalised with acute heart failure, suggested that the presence of the S3 was independently associated with increased in-hospital all-cause death (adjusted odds ratio [OR], 1.69; 95% CI, 1.19–2.41) and cardiac death (adjusted OR, 1.66; 95% CI, 1.08–2.54).4 Box 4 lists the haemodynamic significance of the presence of the S3.3,5,6

As elusive as the S3 might seem, good technique and reflective practice can help clinicians in eliciting the sound — a finding that provides clues to important cardiac pathology as well as severity of disease.

Box 1 – Phonocardiogram from an abnormal heart, showing the third heart sound

Box 3 – Normal heart sounds (A) and abnormal diastolic heart sounds showing the third and fourth sounds (B)