A total of 41 subjects were recruited for our study, including 26 AF patients and 15 healthy volunteers. One patient and one healthy participant was excluded due to poor 4D flow data quality. Baseline characteristics including demographics, clinical characteristics, echocardiogram data, and CMR data were listed in Table 1 and Table 2. The mean age of the AF group was 70.0 years [57.0; 75.0], and 17 (68%) were male. The mean age of the healthy participant group was 28.5 years [26.2; 50.0], and 10 (71.4%) were male. The mean CHA2DS2-VaSc score of the AF group was 2.6±1.5. The echocardiogram results revealed an enlarged LA dimension (4.4 cm [3.9; 5.1]) of the AF group compared with the healthy participant group. CMR data showed that the AF group had a higher LA volume than healthy participants (97.6 ml [67.1; 132.0]) vs. 48.1 ml [41.4; 53.3]; P<0.001). There were no significant differences in sex, body mass index, and heart rate between groups.
Changes in flow component
Flow visualizations in the systolic phase, the diastolic phase, and the interval between the diastolic and the systolic phase were shown in Figure 2, demonstrating a distinct flow component pattern in an AF patient compared with a healthy participant. AF patients had a lower proportion of direct flow (33.5% [8.43; 47.5] in AF vs. 69.1% [63.4; 74.4] in healthy). Evidently, a significant difference in delayed ejection (24.1±12% in AF vs. 14.7±6.8% in healthy), retained inflow (32.5% [24.6; 36.9] in AF vs. 14.5% [12.5; 18.2] in healthy) and residual volume (4.7% [1.5; 15.0] in AF vs. 0.62% [0.5; 1.6] in healthy) were observed (Table 2).
Flow component and LV systolic function
According to the particle tracing, the flow component in LV was divided into four parts. Flow EF derived from the flow component was calculated. Pearson analysis revealed that direct flow positively correlated with CMR EF (R=0.81, P<0.001), SV (R=0.61, P<0.001), CO (R=0.44, P=0.005), and echo EF (R=0.60, P<0.001); direct flow negatively correlated with LVEDV (R=-0.39, P= 0.014), LVESV (R=-0.68, P<0.001) and BNP (R=-0.54, P=0.044). Similarly, flow EF positively correlated with CMR EF (R=0.86, P<0.001), SV (R=0.66, P<0.001), CO (R=0.46, P=0.003), and echo EF (R=0.68, P<0.001); Flow EF negatively correlated with LVEDV (R=-0.46, P= 0.003), LVESV (R=-0.78, P<0.001), BNP (R=-0.79, P<0.001), and NT-proBNP (R=-0.50, P=0.015) (Figure 4).
Subgroup analysis was conducted in AF and healthy participants seperately. Pearson analysis was performed to assess the correlation between direct flow, flow EF and CMR EF. For direct flow and CMR EF, a borderline (R=0.52, P=0.058) and significant positively correlation (R=0.72, P<0.001) were observed in healthy volunteers and AF patients respectively. For flow EF and CMR EF, high correlation was both observed in healthy volunteers (R=0.59, P=0.028) and AF patients (0.79, P<0.001).
Flow component and LV diastolic function
Echocardiogram results showed that retained inflow significantly correlated with left ventricular diastolic function parameters (E/e’ (R=0.51, P<0.001), Septal e’ (R=-0.52, P<0.001), tricuspid regurgitation velocity (R=0.34, P=0.003)) (Figure 3). Besides, the association between retained inflow and LA function (LA volume (R=0.46, P= 0.003), LA EF (R=-0.60, P<0.001)), left ventricle function (LVEF (R=-0.75, P<0.001), LVEDV (R=-0.42, P= 0.007), LVESV (R=0.63, P<0.001)) was observed; the association between retained inflow and age reached a borderline P-value (R=0.31, P=0.053) (Figure 3). Multivariable linear regression showed that correlation between retained inflow and E/e’ (β=0.151±0.007, P= 0.0476) remained significant when adjusted for age, HR, LA volume index, and gender in AF patients (Table 3).
Flow component and symptom burden
The symptom burden and quality of life was evaluated with MLHFQ. The mean (SD) MLHFQ score of AF patients was 12.6±8.9 (Table 1). Univariate linear regression analysis showed that direct flow and retained inflow were significantly correlated with MLHFQ (β=-0.143, P=0.045 and β=0.319, P=0.025, respectively). Besides, direct flow and retained inflow remained significantly correlated with MLHFQ when adjusted for age and gender. (β=-0.170, P=0.038 and β=0.350, P=0.024, respectively)