Comprehensive Cardiac Mechanics Assessment

The LVEDV (determined by echocardiography) and PCWP data were used to construct LV end-diastolic pressure-volume curves using the following exponential model, which has been described previously^{5 (link)}: P=P∞ (exp^a(V–V0)−1) where P is PCWP; P∞, pressure asymptote of the curve; V, LVEDV index; V0, equilibrium volume or the volume at which P=0 mm Hg, and a is a constant that characterizes the chamber stiffness. LV end-diastolic transmural pressure-volume curves also were constructed using estimated transmural pressure (PCWP–right atrial pressure).^{18 (link)} The PCWP and stroke volume (SV) data obtained by the acetelyene rebreathing method were used to construct Frank-Starling curves. The LVEDV, SV, and mean atrial pressure data were used to construct preload recruitable stroke work(PRSW) relationships. Circumferential LV wall stress (σ_c) and strain were determined as previously described^{5 (link)} by use of the modified Laplace relation: σ_c=Pb/h[1–(h/2b)][1–(hb/2a2)], where P is estimated transmural pressure; h, LV midwall thickness; a, major semiaxis; and b, minor semiaxis. The LV midwall thickness and semiaxis measurements were calculated from the transthoracic echocardiographic images. Ventricular strain was calculated as follows: strain=(V–V_min)/V_min, where the smallest end-diastolic volume measured during cardiac unloading (V_min) was determined. This value was subtracted from the end-diastolic volume at each loading and unloading condition (V–V_min). The resulting data were used to construct stress-strain plots, which were modeled by an exponential equation (y=ae^bx). Total arterial compliance was estimated by the ratio between the acetylene rebreathing-derived SV and pulse pressure.^{19 (link)} Effective arterial elastance was estimated as the LV end-systolic pressure divided by SV, where LV end-systolic pressure was estimated as 0.9×systolic blood pressure.^{20 (link),21 (link)}