A i.e Q wave and S2 (Q-S2 Interval)Cardiac Cycle : Pecualirities- Cardiac muscle contract & repolarize faster at higher hea rates (HR) and the duration of systole decreases mainly d/t a decrease in the duration of systolic ejection.- However, the duration of systole is much more fixed and the diastole is shoened to a much greater degree- Only during diastole, coronary blood flows to the endocardial poion of left ventricle and hea muscles rest. Most of the ventricular filling occurs in diastole. So at very high hea rates (>180/min) filling may be compromised decreasing cardiac output per minute.- Because of prolonged action potential, cardiac muscles cannot contract in response to a second stimulus until near the end of initial contraction. That's why cardiac muscles cannot be tetanized like skeletal muscles. The highest rate atwhich hea can contract is 400/min (theoretically) but AV node will not conduct >230 impulses/min b/o its long refractory period. Hence ventricular rate > 230/min is onlyseen in paroxysmal ventricular tachycardia.- Exact clinical measurement of isovolumetric ventricular contraction is difficult but simultaneous recordings of ECG, phonocardiogram and carotid pulse can easily measureTotal Electromechanical systole (Q-S2 interval)It is period from onset of ventricular depolarization (activation) or QRS complex to the closure of aoic valves, as determined by the onset on second hea sound (S2)Pre-ejection Period (PEP)It is difference between QS2 and LVET and represents the electrical and mechanical events preceding systolic ejectionLeft Ventricular Ejection Time (LVET)It is the period between the begining of carotid pressure rise to dicrotic notch (incisura)- Normal PEP/ LVET ratio is 0.35; which increases without change in QS2 when left ventricular function is compromized