Most impoant factor determining myocardial 02 consumption: March 2005

Ans. A: Myocardial fibre tension Tension: One of the most impoant parameter influencing myocardial 02 consumption. Preload: Is defined as the previous stretching of ventricular myocardial fibrils just about to begin a new systole. It is equal to the ventricular wall stress at end-diastole. In clinical practice, ventricular volumes appear to most closely approximate muscle fiber length. Afterload: Is the impedance that the ventricles are submitted at the onset of contraction. Impedance is considered to be the sum of extracardiac forces present in the systemic circulation opposing ventricular ejection and pulsating flow. Clinically, it is assumed to be the systemic vascular resistance, if there is no obstruction between the LV and aoa. Contractility: This is an intrinsic propey of the cardiac cell. It defines the work that the hea can perform at a given load. Contractility is primarily determined by the availability of intracellular calcium. Hea rate: Tachycardia reduces diastole period and consequently, the subendocardial blood flow, thereby reducing myocardial oxygen supply.