The pressure-volume loop begins with contraction and the isovolumic phase of the cardiac cycle. Ventricular ejection begins once ventricular pressure rises above aortic pressure. Peak LV pressure is similar to peak aortic pressure or systolic blood pressure, which is followed by the end-systolic pressure (Pes). This is followed by the isovolumic relaxation phase and then ventricular filling phase of the cardiac cycle once atrial pressure exceeds ventricular pressure. The difference between end-systolic volume (ESV) and end-diastolic volume (EDV) is stroke volume (SV).
The EDPRV is established by altering ventricular loading conditions, generating a series of PVLs, and connecting the end-diastolic pressure–volume points of each loop. The EDPVR is curvilinear in nature and is indicative of ventricular passive viscoelastic properties or stiffness at the point of complete myofilament inactivation. Chamber stiffness, or its reciprocal, chamber compliance, relates changes in ventricular volume in response to changes in diastolic transmural pressure. An increase in ventricular stiffness is reflected in a shift of the EDPVR up and to the left (more pressure for the same volume), and a decrease in stiffness is reflected in a shift of the EDPVR down and to the right (less pressure for the same volume). For a given EDPVR, as volume increases, the pressure begins to rise, and with large volume, the pressure increases disproportionately. That is to say that operating chamber stiffness increases at higher volumes (volume-dependent changes in chamber stiffness). The maximal volume along the EDPVR at which the pressure remains zero is referred to as the unstressed volume, Vo.
The ESPVR reflects the stiffness of the ventricular chamber at the point of maximum myofilament activation. It is examined by altering ventricular loading conditions, generating a series of PVLs, and connecting the end-systolic pressure–volume points of each loop. The relationship between ventricular pressure and volume at end-systole was demonstrated to be reasonably linear over the working pressure–volume range of the heart.
Arterial elastance is a concept used to describe the stiffness of the arterial system and its ability to stretch in response to pressure. In the context of a pressure-volume loop, arterial elastance can be represented as the slope of a line connecting the end-systolic pressure and end-diastolic volume.
In simpler terms, think of arterial elastance as a measure of how much the pressure in the arteries increases for a given increase in volume. A steeper slope on the pressure-volume loop at end-systole (when the ventricle finishes contracting) indicates higher arterial elastance, or stiffer arteries. This means that for any given volume of blood the heart ejects, the pressure rise in the arteries is greater if the arteries are stiffer.
This concept is crucial in understanding how the heart and arteries interact, especially in conditions like hypertension (high blood pressure) where increased arterial stiffness demands more work from the heart to pump blood.
