9.6 VENTRICULAR VOLUME AND ARTERIAL FLOW DURING PRELOAD REDUCTION: AN MRI STUDY

Abstract

Lower body negative pressure (LBNP) has been used to assess the cardiovascular effect of preload reduction. We are the first to use MRI to investigate ventricular volumes and great vessel flow during LBNP. 13 volunteers (23–47 years) underwent LBNP at 0, −5 and −20mmHg. We acquired contiguous short axis steady state free precession cine images (8mm slices) of both ventricles during relaxed expiratory breath hold, and flow images with free breathing phase contrast MR angiography of the ascending aorta (Ao) and main pulmonary trunk (MPA). Analysis was performed using Argus software (Siemens Medical Solutions), statistical assessment by one-way ANOVA and Bonferroni post hoc tests with p-values adjusted for multiple comparisons. At 5mmHg, no change in Ao flow, velocity or left ventricular (LV) volumes was seen. Diastolic blood pressure (DBP) increased (p=0.04). Right ventricular (RV) output (p=0.01) and MPA flow (p=0.03) was decreased. At 20mmHg, Ao flow (p<0.0001) and velocity (p=0.0005) were decreased. Ao retrograde flow increased (p=0.04). LV stroke volume (SV, p=0.0005), ejection fraction (EF, p=0.02) and end diastolic volume (EDV, p<0.0001) decreased. DBP increased (p=0.02). MPA flow (p<0.0001) and velocity (p<0.0001) decreased, with no change in retrograde flow. RV EDV (p<0.0001) and ESV (p=0.02) reduced. Our data implies (1) that at 5 mmHg LBNP there is an increased left to left shunt likely via the bronchial circulation to explain the different LV/Ao and RV/MPA response (2) different vasoconstrictive response in the systemic vs. pulmonary circulation to explain the differences in retrograde flow.