Left ventricular geometry during unloading and the end-systolic pressure volume relationship: Measurement with a modified real-time MRI-based method in normal sheep.

Abstract

The left ventricular (LV) end-systolic pressure volume relationship (ES; ESPVR) is the cornerstone of systolic LV function analysis. Recently, it became possible to measure 2D LV chamber shape during vena cava occlusion (VCO) with MRI. We used an improved level-set semi-automatic segmentation method (LSSM) to determine the effect of VCO on LV geometry, ES pressure area (PA) and ESPVR. 10 healthy adult sheep were anesthetized. LV pressure transducer and inferior vena cava (IVC) balloon catheter were percutaneously inserted. Ferumoxytol (0.125 ml/kg iv; AMAG Pharmaceuticals, Waltham, MA) was given to enhance blood pool contrast. LV pressure and 2D retrospectively-gated cine MRI of LV cross sections 25 (Apex), 50 (Mid) and 75% (Base) of the distance from the apex to the base of the LV were obtained during separate IVC balloon inflations (VCO). LV pressure was digitally filtered and LV chamber segmented with the LSSM. Cross sectional area, major and minor axes, major axis orientation, ESPAR and ESPVR were calculated. The LSSM had excellent reliability. All cross sections became more elliptical during VCO. The orientation (angle) of each major axis relative to the anterior RV insertion shifted during VCO. However, the orientation remained toward the septum. There was chamber collapse (LV area < 0.25 cm2) at the apical level during VCO (7 cases). ESPAR was non-linear at all levels. ESPVR was non-linear because of apical collapse. In conclusion, MRI-based measurement of LV geometry, ESPAR and ESPVR during VCO is a valuable method that may lead to improved understanding of systolic LV function. New and NoteworthyReal-time MRI was used to continuously measure the LV PA relationship as loading conditions were transiently varied in anesthetized sheep. All three examined cross-sections became more elliptical during VCO. The ESPAR were non-linear at all three cross-sections. Chamber collapse at the apical level during VCO resulted in a non-linear ESPVR. The heart contracted in a non-concentric manner during VCO which could inform modeling studies and elucidate mechanisms underlying LV adaptations to sudden load changes.