Which statement best defines preload?

• A. End diastolic volume
• B. The pressure the ventricle must overcome to eject blood
• C. Ejection fraction
• D. Cardiac Output

Answer: (D)

Preload is the initial stretch of the ventricular myocardium right before contraction, determined by how much the ventricle fills during diastole. The amount of filling is best represented by the end-diastolic volume, because more filling stretches the muscle fibers more and sets the stage for the force of the next contraction (the Frank-Starling relationship). This concept is distinct from afterload, which is the pressure the ventricle must overcome to eject blood; it’s also different from ejection fraction, which is the percentage of EDV that is actually pumped out, and from cardiac output, which is heart rate times stroke volume. So preload is best understood as the load from filling, closely tied to end-diastolic volume (and, in practice, can be estimated by end-diastolic pressure as well).