Ans. A: Isovolumic relaxation phaseMaximum coronary blood flow occurs during the phase of isovolumetric ventricular relaxation phaseIsovolumetric/isometric relaxation time/IVAn interval in the cardiac cycle, from the aoic component of the second hea sound, that is, closure of the aoic valve, to onset of filling by opening of the mitral valve.Ventricular pressure decreases to zero rapidly while aoic pressure decreases only to 80 mm Hg i.e. it remains fairly high.Therefore, intra myocardial compression of blood vessels is minimal and perfusion pressure is maintained fairly high.So coronary blood flow rises sharplyMaximum coronary blood flow occurs during this phaseIt can be used as an indicator of diastolic dysfunction.Prolonged IV indicates poor myocardial relaxation.A normal IV is about 70 +- 12 ms, and approximately 10ms longer in people over foy years.In abnormal relaxation, IV is usually in excess of 110ms.With restrictive ventricular filling, it is usually under 60 ms