As the ventricle fills with blood, the pressure and volume that result from filling are determined by the compliance of the ventricle. Normally, compliance curves are plotted as the change in volume (ΔV) over the change in pressure (ΔP). For the ventricle, however, it is common to plot ΔP versus ΔV (see Figure). Therefore, the slope of the relationship is the reciprocal of the compliance, which is sometimes referred to as ventricular "stiffness."
As the ventricle fills with blood and its volume increases, the pressure within the ventricular chamber passively increases (see the Normal filling curve in the figure). The relationship is not linear, particularly at higher volumes, because the compliance of the ventricular wall decreases ("stiffness" increases) the more the ventricular wall is stretched. This occurs in most biological tissues.
The compliance of the ventricle is determined by the structural properties of the cardiac muscle (e.g., muscle fibers and their orientation, and connective tissue) as well as by the state of ventricular contraction and relaxation. For example, in ventricular hypertrophy the ventricular compliance is decreased (i.e., the ventricle is "stiffer") because the thickness of the ventricular wall increases; therefore, ventricular end-diastolic pressure (EDP) is higher at any given end-diastolic volume (EDV) (see Figure). Other cardiac disease states such as different forms of restrictive cardiomyopathy also lead to a decrease in ventricular compliance. Alternatively, at a given EDP, a less compliant ventricle would have a smaller EDV (i.e., filling will be impaired). If ventricular relaxation is impaired (as occurs in some forms of diastolic heart failure), the functional ventricular compliance is also reduced (because of residual active tension), which will also impair ventricular filling. In a disease state such as dilated cardiomyopathy, the ventricle becomes very dilated without appreciable thickening of the wall. This dilated ventricle will have increased compliance as shown in the figure; therefore, although the EDV may be very high, the EDP may not be greatly elevated.
The following mini-lecture (9 minutes) describes normal and abnormal ventricular compliance: