Q marks the beginning of depolarization of the ventricle

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)

= (Q - S2) - (LVET)

= Q - aortic valve opening*

ICT (isovolumetric contraction time)

= (S1 - S2) - (LVET)

LVET (left ventricular ejection time)

= (S1 - S2) - ICT or

= (Q - S2) - PEP

Q to S1 is due to the delay between electrical and mechanical events

= (Q - S2) - (S1 - S2)

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)