Pressure-Volume Relationships

The cardiac pressure-volume (PV) loop is a graphical representation of the mechanical work performed by the ventricle during a single cardiac cycle, plotting left ventricular pressure (y-axis) against left ventricular volume (x-axis). The loop traces four phases: (1) isovolumetric contraction (mitral valve closes, aortic valve still closed — pressure rises at constant volume), (2) ejection (aortic valve opens when LV pressure exceeds aortic diastolic pressure — volume decreases as blood is ejected), (3) isovolumetric relaxation (aortic valve closes, mitral valve still closed — pressure falls at constant volume), and (4) filling (mitral valve opens when LV pressure falls below left atrial pressure — volume increases). The width of the PV loop represents stroke volume (SV = end-diastolic volume minus end-systolic volume), while the area enclosed represents stroke work. The Frank-Starling mechanism describes how increasing preload (end-diastolic volume) stretches sarcomeres toward their optimal length (~2.2 micrometers), increasing the number of actin-myosin cross-bridge interactions and thus contractile force — this is represented on the PV loop as a rightward shift along the end-diastolic pressure-volume relationship (EDPVR). The EDPVR slope reflects ventricular compliance...