Split S2 Heart Sounds

Amara Estrada, DVM, DACVIM (Cardiology) University of Florida

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A split S2 sound is appreciated in this recording.

The second heart sound is associated with closure of the semilunar valves (aortic and pulmonic) at the end of systole, following the T wave on ECG. In dogs and cats, pulmonic valve (P2) closure follows aortic valve (A2) closure by a very short interval; this causes S2 to be heard as a single sound.

On occasion, an audible split second heart sound may be auscultated in healthy, large-breed dogs during inspiration because of a longer right ventricular ejection period. Pathologic splitting of S2 resulting from delayed closure of the pulmonic valve is most commonly auscultated in dogs with pulmonary hypertension (typically heartworm disease, as in these two cases). 

Additional causes of split S2 from the delayed closure of P2 include left-to right intracardiac shunts (atrial septal defects), pulmonic stenosis, right bundle branch blocks, ectopic beats, and ventricular pacing. Paradoxic splitting of S2 results from delayed closure of the aortic valve sometimes noted in left bundle branch block, ectopic beats, aortic stenosis, and possibly systemic hypertension (as seen in humans).

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