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. In other words, it is the initial stretching of the
cardiomyocytes prior to contraction; therefore, it is related to the sarcomere length at the end of diastole. Parameters such as ventricular end diastolic volume or pressure are used to measure preload since the ideal length of the cardiac sarcomere cannot be measured. Passive filling of the (heart)
ventricle and subsequent atrial contraction thus allows an echocardiographically volumetric measurement. Preload is theoretically most accurately described as the initial stretching of a single
cardiomyocyte prior to contraction. This cannot be measured
in vivo and therefore other measurements are used as estimates. Estimation may be inaccurate, for example in a chronically dilated ventricle new
sarcomeres may have formed in the heart muscle allowing the relaxed ventricle to appear enlarged. The term
end-diastolic volume is better suited to the clinic, although not exactly equivalent to the strict definition of
preload. Atrial pressure is a surrogate for preload.