Left ventricular compliance and elastance

The heart fills during the diastole by venous return, augmented, if the heart is in sinus rhythm, by atrial contribution. The pressure–volume relationship in an object being filled is called compliance (dV/dp) – in this case the left ventricular compliance, left ventricular end-diastolic volume and left ventricular end-diastolic pressure.

In practice, ventricular pressures are more easily measured than ventricular volumes, and therefore the inverse of compliance, i.e. elastance or stiffness (dp/dV), is of interest. In Figure 91, the left ventricular pressure is chosen as the dependent variable, and the slope of the relationship between LVEDP and LVEDV corresponds to the stiffness of the left ventricle: the steeper the slope, the stiffer (less compliant, more thick walled or restricted) is the left ventricle. The relationship is linear up to a certain point of maximum stretch, beyond which the LVEDP rises steeply with any further increment of volume. Under physiological conditions, the heart operates on the linear part of the curve, and therefore it is possible to assume that LVEDP will correspond to LVEDV.

Note that the left ventricular end-diastolic volume corresponds to the initial fibre length of the Frank–Starling relationship. When measuring the pulmonary artery occlusion pressure (PAOP), we estimate the left ventricular filling from a pressure measurement inside a small pulmonary artery. The relationship between the PAOP and the LVEDV is complex (see more in chapter Control of cardiac output); ventricular compliance (or stiffness) is only one of the factors which can alter this relationship: a stiffer ventricle will achieve a smaller filling volume before the ventricle reaches the maximum capacity point, or conversely higher end-diastolic pressures are needed in order to achieve a comparable end-diastolic volume.