(See also Heart Failure in Children.) Physiology of Heart Failure Cardiac contractility (force and velocity of contraction), ventricular performance, and myocardial oxygen requirements are determined by Preload Afterload Substrate availability (eg, oxygen, fatty acids, glucose) Heart rate and rhythm Amount of viable myocardium Cardiac output (CO) is the product of stroke volume and heart rate; it is also affected by venous return, peripheral vascular tone, and neurohumoral factors.