S1 is 1st heart sound (closing of AV valves)
S2 is 2nd heart sound (closing of semi-lunar valves).
PEP (pre-ejection period; period when ventricle is electrically activated & contracting isovolumetrically)
LVET (left ventricular ejection time)
Q to S1 is due to the delay between electrical and mechanical events
Note: In healthy subjects PEP/LVET = 0.35 +/- 0.04. An elevated ratio is an index of pathology (i.e. a negative inotropic or dromotropic action)
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