An “extra” sound is auscultated in between closure of the AV valves (S1) and closure of the semilunar valves (S2), along with an intermittent grade 2/6 systolic murmur (over the left apex) that is plateau shaped and caused by mild mitral insufficiency.
It is often hard to differentiate this from a gallop sound at first auscultation, but the sound is a click that occurs during systole in between the S1 and S2 sounds. A gallop sound would be heard during diastole.
The genesis of the sound in dogs is uncertain, but it is likely caused by the sudden tensing of redundant valve leaflets or elongated chordae tendineae as they buckle into the left atrium. The systolic click needs to be differentiated from a split or gallop heart sound, utilizing the timing the additional sound as the differentiating factor.