**4. Preload**

This concept suggests that the length of ventricular muscle fiber determines the magnitude of contraction. The length of the LV muscle fiber in turn is dependent upon the left ventricular end diastolic volume (LVEDP). In other words increase in left atrial filling would increase the magnitude of LV contraction. The ability of the LV to vary the strength of its contraction as a function of the LVEDP and end diastolic muscle fiber length is defined as Frank Starling law (fig 3). This gain in contractility is impaired when the stretch goes beyond physiological limits. In clinical practice the length of the muscle fiber is proportional to the LVEDP 'which is measured indirectly as the pulmonary artery wedge pressure (PAWP) by means of a Swan Ganz catheter. In a normal LV, very slight changes in PAWP or LVEDP, produces significant increases in stroke volume.
