Hemodynamic mechanisms in patients with atrial functional and structural mitral regurgitation based on 4D flow cardiac MRI

Abstract

Background: Atrial functional mitral regurgitation (AFMR) can cause mitral regurgitation (MR) and tricuspid regurgitation (TR) without structural valve abnormalities, due to atrial enlargement and annular dilatation. On the other hand, atrial fibrillation (Af) is one of the primary causes of AFMR, and increasing in the elderly population residing in industrialized countries. In addition, in recent years, flow energy loss (EL) assessed by four-dimensional (4D) flow magnetic resonance imaging (MRI) is expected to be a novel parameter of cardiac workload. This study aimed to evaluate the pathophysiology of AFMR by comparing parameters between AFMR with Af and structural MR (StMR) with prolapse from a viewpoint of hemodynamic. Methods: This study was a prospective study. Preoperative 4D flow MRI studies were performed in 10 AFMR for Af and 10 StMR for prolapse surgical candidates. Study results were segmented to visualize flow patterns, quantify hemodynamics, and energy dynamics [EL and cardiac output (CO), among other parameters], using iTFlow2 (Cardio Flow Design Inc., Tokyo, Japan). Statistical analysis the Mann-Whitney U test was used to compare the differences between the two groups, and Spearman’s correlation coefficient by the Mann-Whitney U test, and Spearman’s correlation coefficient was performed. Results: In AFMR, CO and cardiac index (CI) were significantly lower [CO: 5.01 (4.53–6.13) vs. 8.54 (7.51–11.11) L/min, P<0.001; CI: 3.54 (3.17–3.98) vs. 5.47 (4.37–6.09) L/min/m2; P=0.007], with lower systemic (left side) EL (ltEL) [3.34 (1.96–6.46) vs. 7.13 (4.75–9.72) mW, P=0.08] and higher pulmonary (right side) EL (rtEL) [2.43 (1.87–4.03) vs. 1.70 (1.18–2.13) mW, P=0.17]. TR fraction was higher [56.1% (39.5–69.2%) vs. 29.0% (20.9–37.9%), P=0.01], while the left ventricular end-diastolic volume (LVEDV) was significantly lower [134.6 (114.1–177.3) vs. 209.0 (190.1–255.6) mL, P=0.01]. EL densities [defined as EL/each chamber volume (mW/mL)] and CO ratios [defined as EL/CO (mW·min·m2/L)] were significantly reduced compared to those of StMR (all P<0.05). In the AFMR group, a positive relationship was observed between ltEL and both ventricular volumes, rtEL, and right ventricular volume (P<0.05). Af duration showed a non-significant but positive correlation with rtEL/CO (P=0.056). Conclusions: 4D flow MRI revealed that AFMR is associated with lower CO, higher pulmonary load and right ventricular enlargement, compressing the left ventricle, and increasing systemic EL. Longer Af duration correlates with higher EL in smaller left ventricles.