A higher venous pressure (CVP) and/or a higher pulmonary capillary wedge pressure (PCWP) is still often regarded as an indicator of higher preload (CVP for the right heart, PCWP for the left heart). However, many studies have shown that CVP and PCWP are not reliable indicators for this purpose. This is mainly due to the limitation that pressure cannot directly be transferred into volume. So any volumetric parameter assessing the filling of the ventricle at the end of diastole reflects more precisely the actual preload.
The Frank-Starling law states that the greater the volume of blood entering the ventricle during diastole (end-diastolic volume), the greater the volume of blood ejected during systolic contraction (stroke volume) and vice-versa. This is an adaptive mechanism of the organism to compensate for slight changes in the ventricular filling.
However, it can also be used to increase stroke volume by volume administration for therapeutic reasons. The force that any single cardiac muscle fibre generates is proportional to the initial sarcomere length (known as preload), and the stretch on the individual fibres is related to the end-diastolic volume of the ventricles.
An increase in preload will, to a certain extent, lead to an increase in stroke volume (SV), based on optimal myocardial muscle fibre pre-stretching.
Up to a certain limit, the more the sarcomeres of the muscle cells are stretched the greater the contraction. On the other hand, contractility may decrease in conditions of volume overload.
The power of the heart muscle depends on its initial load before the start of contraction.
Fig. Schematic Frank-Starling curve for verification of the preload status