Page 29 - Journal of Structural Heart Disease Volume 5, Issue 3

Original Scientific Article
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the RVOT, it is still difficult to predict the exact rela- tionship of final stent diameter and ultimate RVOT size. This study attempted to account for this as much as possible by measuring the RVOT during peak sys- tole on cSSFP imaging. Presumably during this time the RVOT would be expanded to its widest point. This most likely explains why cSSFP images showed the best correlation and agreement with BWD. MRA is not gated to the cardiac cycle and the measurements became an average of systole and diastole which would limit the utility in using this protocol. 3D-SSFP is gated to the cardiac cycle, but is taken at a single phase when the heart is most quiescent which may not be peak systole, thus it is not possible to know if these measurements were at the time when the RVOT was largest.
Currently, there is a significant amount of research being done into reliably predicting the distensibility within an artery [3, 8] A number of different compli- ant materials at different thicknesses have been test- ed to make in-vitro models of arteries in an attempt to be able to predict how distensible a vessel is non-in- vasively. These models are created using advanced images, mostly MRI to obtain the in-vitro data [8]. While this method is extremely expensive and time consuming, the theory behind these models is that MRI can be used to predict the characteristics of the vessel. Thus there is data to suggest that cardiac MRI can be used to non-invasively determine true RVOT size, especially when using cSSFP imaging during peak systole, as our study did [1, 8].
In addition to the size of the outflow tract, the shape and character of the outflow tract can play an import- ant role in the ability to place a percutaneous valve. Schievano et al. defined five distinct types of RVOT morphology in patients whom had undergone sur- gical repair of congenital heart disease affecting the RVOT. Type 1 morphology was the only morphology not amenable to PPVR. However, this type account- ed for 44.6% of the patients in the Schievano study. This suggests nearly half of patients are ineligible for current PPVR based on shape of the RVOT alone [1]. The frequent large size and this pyramidal morphol- ogy of many native RVOTs after surgery have led to the use of a new self-expanding stent PPVR. This tech- nology has shown promise in early feasibility studies and looks encouraging as an approach that will allow
many more patients to receive a percutaneous valve [4, 9-11]. While this technology is still being devel- oped and tested there have been new techniques for using the current PPVR including implanting valves in the branch pulmonary arteries [12]. Cardiac MRI will continue to be useful for both of these techniques and future studies should determine if cardiac MRI can predict an accurate cross sectional area for the safe deployment of self-expanding stents and current PPVR in the branch pulmonary arteries.
ECG gated CTA may better assess RVOT size and shape, but it is generally not utilized to follow right ventricular function and volumes over time, plus se- rial CT exposes patients to unnecessary radiation. However, cardiac MRI is an ideal test to attempt to predict the candidacy for PPVR because most of these patients will undergo cardiac MRI as part of their standard clinical evaluation prior to consideration for valve replacement. This is due to the fact that many of the guidelines for pulmonary valve replacement are based upon severity of pulmonary insufficiency and right ventricular volume on cardiac MRI. Addition- ally, cardiac MRI can be used to screen for another common cause of PPVR placement failure, coronary compression [3, 5]. Malone, et al recently showed that pre-procedural screening with cardiac MRI or CTA can reliably predict which candidates are at risk for coro- nary compression [5].
This study shows promising results that cardiac MRI has benefit in patient screening for PPVR, but there were a number of limitations to this study. The retrospective design does not allow for determining eligibility by measurements on the cardiac MRI at the time of catheterization. Additionally, the patient population was biased by the fact that only those who were deemed to be good candidates were rec- ommended for PPVR attempt, this decision may have included the pre-procedural cardiac MRI measure- ments. This study could have been improved if cardi- ac MRI measurements were categorized prior to cath- eterization as suitable for PPVR or not. Going forward a larger, multi-center study, should be undertaken to determine if these results are true amongst a larger population. A larger population could improve the uncertainty caused by wide lines of agreement and be used to create a regression formula to translate a cardiac MRI RVOT measurement to BWD measure-
Journal of Structural Heart Disease, June 2019
Volume 5, Issue 3:62-69