Synopsis

We quantified the kinetic energy (KE) of intraventricular RV flow (KE_RV,flow), PA forward flow (KE_PA) and PA retrograde flow (KE_PA,regur). The purpose is to establish 4D flow-derived KE-related indices for better understanding of the RV-PA interaction. We observed the interaction between RV and PA by assessing the correlation between diastolic KE_PA,regur and late diastolic KE_RV,flow as well as between systolic KE_PA and RV mean pressure. In conclusion, we verified that altered hemodynamic conditions of RV interacted with severe PR, which may lead to RV overloading pathology and adverse outcome in rTOF patients.

Introduction

Method

This study recruited 15 rTOF patients (22.9±3.4 y/o, male/female: 12/3) and 15 age-matched normal subjects (22.1±1.2 y/o male/female: 8/7) without any known cardiovascular diseases. Images were acquired at a 3.0-T MR scanner (Tim Trio or Skyra, Siemens, Erlangen, Germany). All subjects underwent 4D flow MRI (TR/TE=10.8/2.9 ms, flip angle=7°, VENC=1.5 m/s, voxel size=1.2×1.2×3.5 mm³). Prospective ECG-triggering and navigator-echo were performed to synchronize with cardiac and respiratory motion, respectively.

In Figure 1, the planes for computing KE_RV,flow and KE_PA or KE_PA,regur were determined manually on a 4-chamber view for RV and on a 3D angiogram for PA. The kinetic energy was calculated as:

$$ KE=\frac{1}{2}\times\rho\times V\times v^{2} $$

where ρ was blood density with a value of 1060 kg/m³, V denoted voxel volume, and v represented the voxel velocity. The late diastolic KE_RV,flow was calculated as the summation of RV intraventricular KE on a 4-chamber view during the period from peak KE_RV,flow to the end of the cardiac cycle. We characterized the histogram of late diastolic KE_RV,flow by several indices, including the total, maximum, second quartile (Q2), and third quartile (Q3) for each subject. Further, the top 5% of the KE_RV,flow in the histogram was selected as the local maximum, and voxels with KE_RV,flow greater than local maximum were labeled as high KE region. The mean KE value and the area of the high KE region were also determined.

Two-tailed Student t test was used to assess the statistically significant difference. A P value<0.05 was considered statistically significant. The Pearson correlation coefficient was calculated when appropriate.

Results

Table 1 summarizes the basic characteristics of cardiac function and pressure-related indices of the study population. rTOF patients were with the presence of RV dilation but preserved RV ejection fraction (P = 0.154). All pressure indices in rTOF group were significantly higher than the normal standards reported by AHA, suggesting that patients presented pressure overload.

As illustrated in Figures 2a and 2b, the rTOF group presented substantial higher KE_RV,flow and KE_PA during late diastolic period (after 180 %ES) and the whole time course, respectively. The increased KE_PA after 160 %ES can be attributed to the regurgitation flow. In Figure 2c, higher summation of KE_PA and KE_PA,regur over the whole time course were observed in rTOF group (both P<0.01).

As shown in Figures 3a and 3b, a normal subject typically demonstrated a narrower distribution of late diastolic KE_RV,flow than an rTOF patient, reflected by the lower maximum (0.09 mJ vs. 0.20 mJ). Figure 3c presents the characterized indices of the histogram distribution of the two groups. The rTOF group demonstrated significantly larger maximum, Q2, and total KE_RV,flow (all with P<0.05). As for high KE area, rTOF group exhibited higher local maximum and sum high KE than normal group (both P<0.05), implying the RV flow in rTOF group maintains at high energy level during the late diastolic period.

In rTOF group, total late diastolic KE_RV,flow correlated with KE_PA,regur (R=0.80, P<0.05), as illustrated in Figure 4a. Further, Figure 4b illustrates a strong positive correlation between KE_PA and RV mean pressure in rTOF group (R=0.86, P<0.001).

Discussion

In this study, the rTOF patients demonstrated significantly higher diastolic KE_RV,flow, increased systolic KE_PA, higher diastolic KE_PA,regur, and abnormal late diastolic KE_RV,flow distribution. We observed the interaction between RV and PA by assessing the correlation between diastolic KE_PA,regur and late diastolic KE_RV,flow as well as between systolic KE_PA and RV mean pressure. Our finding is consistent with the previous study with visualization of flow patterns in right heart.⁶

In conclusion, we verified that altered hemodynamic conditions of RV interacted with severe PR, which may lead to RV overloading pathology and adverse outcome in rTOF patients. To assess the RV-PA interaction by evaluating KE can help to monitor the abnormal hemodynamics and establish 4D flow-derived KE-related indices in rTOF patients.

Acknowledgements

References

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