What is Preload?

A debate exists over whether end-diastolic pressure or end-diastolic volume should be used as the indicator of preload. While both are critical factors, relying solely on pressure or volume does not provide a complete picture. Pressure can vary significantly with changes in ventricular compliance, while volume alone does not account for the actual stress experienced by the ventricular wall.
In fact, preload is more accurately defined as the left ventricular wall stress at end of diastole, which can be calculated using the principles of the Law of Laplace, which relates the wall stress in a spherical or cylindrical vessel to the pressure, radius, and thickness of the wall. The formula for left ventricular wall stress (σ) is generally expressed as: σ equates to the intraventricular pressure (P) multiplied by the radius of the ventricle (r) divided by 2 X the wall thickness (w).