Elevated energy loss in diastolic left ventricular inflow corresponds to an increase in kinetic energy in patients with a repaired atrioventricular septal defect: Quantification from 4D Flow MRI

23 AVSD-repaired patients with NYHA class 1 and 2 (age: 20±8 years) and 23 healthy controls (age: 19±8 years) were included. All subjects (Table 1) underwent whole-heart 4D Flow MRI at 3T with free breathing, three-directional velocity-encoding of 150cm/s in all directions, spatial resolution 2.3×2.3×3.0-4.2mm³ and 30 retrospectively-gated phases reconstructed over one cardiac cycle. The LV cavity was manually segmented over diastolic phases from 4D Flow data. For each segmented phase, the kinetic energy within LV was computed as 1/2 mv², with (m) as the mass representing the voxel volume multiplied by the density of blood (1.025 g/ml) and (v) as the 3-directional velocity from 4D Flow MRI. Total kinetic energy (KE) was then computed by integrating the computed kinetic energy over diastole. Using Navier-Stokes energy equations, non-turbulent viscous energy loss (EL) was evaluated in the LV as the integration of viscous energy dissipation over diastolic period as described previously [2] with blood assumed as an incompressible Newtonian fluid. Both KE and EL were normalized by the end-diastolic LV volume (EDV). A dimensionless ratio (EL/KE_avg) was computed with KE_avg as the average kinetic energy over diastole. Measured parameters were compared using unpaired Wilcoxon rank sum test. Association between EL and KE was evaluated using Pearson's correlation.

	Control (N=23)	AVSD-repaired patients (N=23)
Age (yrs)	19 ± 8	20 ± 8
Heart rate (bpm)	87 ± 11	94 ± 15
EL (mJ/mL)	0.21 ± 0.07	0.29 ± 0.12a
KE (mJ/mL)	0.16 ± 0.07	0.24± 0.11b
(EL/KEavg) *	1.31 ± 0.43	1.22 ± 0.5c

^a p<0.05 , ^b p<0.01 and ^c p=0.3 * the dimensionless ratio (EL/KE_avg) represents the ratio of the amount of kinetic energy lost through viscosity (EL) to the amount of kinetic energy(KE) which is still available (retained) during diastole e.g. (EL/KEavg)=1.3 means that the amount of lost kinetic energy due to viscosity is 1.3 times the amount of kinetic energy which is still retained from diastole. KE_avg represents the average KE available over diastole

Detailed results are presented in Table 1. During diastole, viscous energy loss (EL) was significantly higher in patients compared to controls (p<0.05). Additionally, a significant increase in KE was observed in patients compared to controls (p<0.01). However, the ratio between viscous energy loss and average KE over diastole was comparable in patients to controls (p=0.3). Strong correlation (R=0.87) was found between KE and EL (Figure 1).