ABSTRACT
An accurate method of assessing intravascular volume and preload of the heart is an essential component in the successful management of patients in the operating room and intensive care unit. Left ventricular (LV) preload represents the force or load acting to stretch ventricular muscle fibers at end-diastole. As sarcomeres in the mid-wall of the myocardium are stretched from 1.8 to 2.25mm, the ascending limb of Starling’s law of the heart is defined. This relationship cannot be adequately explained as originally thought by a sliding filament theory with corresponding changes in actin and myosin overlap at greater sarcomere lengths. Rather, the steepness of the ascending limb appears secondary to length-dependent activation of myofibrils (1), perhaps from greater calcium affinity by troponin C or from enhanced release of calcium from the sarcoplasmic reticulum at greater sarcomere lengths (2). In the intact heart, sarcomere length and end-diastolic volume are closely related. As end-diastolic volume is increased, stroke volume is augmented in a linear manner (3).
