Left ventricular pressure-volume (PV) loops are derived from pressure and volume information found in the cardiac cycle diagram (upper panel of figure). To generate a PV loop for the left ventricle, the left ventricular pressure (LVP) is plotted against left ventricular (LV) volume at multiple time points during a complete cardiac cycle. When this is done, a PV loop is generated (bottom panel of figure and animated figure).
To illustrate the pressure-volume relationship for a single cardiac cycle, the cycle can be divided into four basic phases: ventricular filling (phase a; diastole), isovolumetric contraction (phase b; systole) , ejection (phase c; systole), and isovolumetric relaxation (phase d; diastole) . Point 1 on the PV loop is the pressure and volume at the end of ventricular filling (diastole), and therefore represents the end-diastolic pressure and end-diastolic volume (EDV) for the ventricle. As the ventricle begins to contract isovolumetrically (phase b), the mitral valve closes and the LVP increases, but the LV volume remains the same, therefore resulting in a vertical line (all valves are closed). Once LVP exceeds aortic diastolic pressure, the aortic valve opens (point 2) and ejection (phase c) begins. During this phase the LV volume decreases as LVP increases to a peak value (peak systolic pressure) and then decreases as the ventricle begins to relax. When the aortic valve closes (point 3), ejection ceases and the ventricle relaxes isovolumetrically - that is, the LVP falls but the LV volume remains unchanged, therefore the line is vertical (all valves are closed). The LV volume at this time is the end-systolic (i.e., residual) volume (ESV). When the LVP falls below left atrial pressure, the mitral valve opens (point 4) and the ventricle begins to fill. Initially, the LVP continues to fall as the ventricle fills because the ventricle is still relaxing. However, once the ventricle is fully relaxed, the LVP gradually increases as the LV volume increases. The width of the loop represents the difference between EDV and ESV, which is by definition the stroke volume (SV). The area within the loop is the ventricular stroke work.
The filling phase moves along the end-diastolic pressure-volume relationship (EDPVR), or passive filling curve for the ventricle. The slope of the EDPVR is the reciprocal of ventricular compliance. The maximal pressure that can be developed by the ventricle at any given left ventricular volume is defined by the end-systolic pressure-volume relationship (ESPVR), which represents the inotropic state of the ventricle. The pressure-volume loop, therefore, cannot cross over the ESPVR because that relationship defines the maximal pressure that can be generated under a given inotropic state. The end-diastolic and end-systolic pressure-volume relationships are analogous to the passive and total tension curves used to analyze muscle function.
The PV loop changes when the preload, afterload and inotropic state of the heart change. To see how these affect PV loops, CLICK HERE.
Click below to see how the following affect PV loops: