Left Ventricular Afterload
Left ventricular afterload is most commonly defined as either left ventricular wall stress during systole (which, by Laplace’s law, is equal to [Pr/2h]) or as aortic input impedance (a biphasic descriptor [modulus and phase] of the forces which oppose pulsatile flow, primarily vascular resistance, aortic compliance, and wave reflections). Proponents of wall stress point out that myocardial VO2 is related to wall stress, whereas proponents of impedance point out that wall stress is to some degree determined by the ventricle itself (since S = Pr/2h) and thus cannot represent an external load (which is what afterload is supposed to be) [Nichols WW, Pepine CJ. Prog Cardiovasc Dis 24: 293, 1982]. SVR is a drastic oversimplification of afterload and has been shown to be poorly related to wall stress [Lang RM et al. Circulation 74: 1114, 1986].