In cardiac physiology, preload is the end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demand. It is defined as the length of the heart muscle just before it starts to pump and is measured clinically by the volume of blood in the left ventricle, the main pumping chamber, when it is most relaxed. Preload failure occurs when the maximal exercise-induced CO is decreased without a concomitant increase in right heart or pulmonary vascular pressures.
Preload can be influenced by blood volume, heart rhythm, left ventricular relaxation, and alterations in afterload.
Preload failure is associated with exercise intolerance in myalgic encephalomyelitis, chronic fatigue syndrome and postural orthostatic tachycardia. David Systrom at Brigham Women's Hospital in Boston is currently trialing the use of Mestinon in patients with preload failure.Template:Fix/category (see discussion page for citations)