Listening for Extra Heart Sounds: What Do They Mean?

Adapted from a veterinary cardiology discussion by Dr Janet Olson, DVM, DACVIM (Cardiology).

Lub-dub, lub-dub, click. In dogs and cats, every heartbeat normally produces two audible sounds, lub and dub, marking the closure of the AV valves (mitral and tricuspid) and the semilunar valves (aortic and pulmonic) respectively. Occasionally, though, a third sound creeps in. The three most common extra heart sounds are gallops, mid-systolic clicks, and a split S2.

Gallop sounds occur during diastole, produced as blood rushes into a stiff, non-compliant left ventricle. They’re most often heard in cats with hypertrophic cardiomyopathy (HCM) or dogs with dilated cardiomyopathy (DCM), and tend to become more audible at higher heart rates. It’s worth noting the correct terminology is a gallop sound, not a gallop rhythm, arrhythmias show up on an ECG, whereas extra heart sounds don’t.

Mid-systolic clicks and split S2 sounds both occur during systole, the heart’s contraction phase. Mid-systolic clicks reflect early chronic degenerative valve disease with valve prolapse. They’re heard relatively rarely, since affected dogs usually also develop a heart murmur that masks the click, but in the very earliest stages, before a murmur develops, the click can be detected on its own.

The crisp S2 (dub) sound normally results from the pulmonic and aortic valves closing at essentially the same moment. When that timing is off, the sound splits into two distinct components. The most common cause is pulmonary hypertension, which increases resistance in the pulmonary artery and delays pulmonic valve closure, producing the split.

Telling the sounds apart

Working out whether an extra sound is a gallop, a click, or a split S2 partly comes down to timing, diastole versus systole, though this becomes genuinely difficult at higher heart rates. Breed can offer a useful clue. In cats, extra heart sounds are typically gallops linked to hypertrophic cardiomyopathy. In large and giant breed dogs, they’re also usually gallops, but linked to dilated cardiomyopathy instead.

In small breed dogs with no other clinical signs, an extra sound is typically a mid-systolic click tied to early degenerative valve disease. In small breed dogs showing respiratory distress, West Highland White Terriers particularly, an extra sound is more often a split S2 linked to pulmonary hypertension.

What tests are appropriate?

Any patient with a detectable extra heart sound should have thoracic radiographs, a six-lead ECG, and an echocardiogram, giving the best chance of a definitive diagnosis and appropriate ongoing monitoring or treatment.

Extra heart sounds are well worth paying attention to, they often reveal a great deal about what’s really going on with the heart.

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