Preprocedural Evaluation of Ventricular Functional Mitral Regurgitation Upon Transcatheter Edge-to-Edge Repair by Noninvasive Imaging ― An Emerging Clinical Standard ―
Article ID: CJ-24-0150
Details
Article ID: CJ-24-0150
Mitral-valve transcatheter edge-to-edge repair (TEER) is a safe and effective treatment for ventricular functional mitral regurgitation (MR) in patients who are at prohibitive risk for surgery.1 Nevertheless, appropriate patient selection for TEER is essential for procedural success and patient prognoses. One major reason for the poor prognosis of MR is related to left ventricular (LV) volume overload and progressive myocardial fibrosis, resulting in LV dysfunction;2 however, evaluation of LV myocardial fibrosis remains challenging in current preprocedural evaluation strategies for patients with MR. Ideally, noninvasive imaging techniques that are accurate, objective, highly versatile, simple, and practical myocardial tissue evaluation before TEER and mitral-valve surgery should be performed. Standardization of these methods is also required.
Article p ????
In this issue of the Journal, Tsunamoto et al3 report on using native T1 in noncontrast T1 mapping by cardiac magnetic resonance (CMR), which has high clinical practicality, to successfully perform risk stratification of patients undergoing TEER. They found that baseline native T1 Z-scores were independently associated with cardiovascular events after TEER. Surprisingly, fewer cardiovascular events were observed in the patients with a native T1 Z-score <2.41 and who had an effective regurgitant orifice area (EROA) ≥0.30 cm2 than in the patients with a native T1 Z-score ≥2.41 and an EROA <0.30 cm2.
T1 mapping is an imaging method that quantitatively measures myocardial T1 values (T1 relaxation times in milliseconds). This method is expected to be applied for continuous disease-stage evaluation, assessment of severity and outcomes, and evaluation of the therapeutic effects of various cardiac diseases.4 It involves 2 parameters: native T1 and the extracellular volume fraction (ECV). Native T1 reflects the composition of both the intra- and extracellular compartments, whereas the ECV reflects the degree of interstitial expansion. Although the ECV involves the use of gadolinium-based contrast agents, native T1 can be obtained without a contrast agent, making it applicable even in patients with renal failure. Therefore, pre-TEER evaluation using native T1 is simple and practical. However, some precautions are necessary. Unlike the ECV, native T1 is easily affected by the magnetic field strength of magnetic resonance scanners (1.5 or 3.0 T) and imaging sequences, so reference values for each system and imaging sequence must be established.5 Therefore, ECV generally enables stable assessment of myocardial properties.
The authors converted the native T1 values into Z-scores calculated from the population mean and standard deviation obtained from normal subjects at the facility to eliminate differences between scanners and facilities caused by external factors, such as the field strength of CMR systems. This is important for standardizing preprocedural myocardial evaluation among multiple institutions. Familiarity with the standard measurement methods for native T1 is also necessary. To evaluate valvular heart diseases with diffuse myocardial fibrosis, it is recommended that a single region of interest be drawn in the ventricular septum on midcavity short-axis T1 maps to avoid lung, liver, and veins as sources of susceptibility artifacts.5
Echocardiography is the most practical diagnostic method for assessing valvular pathology and is the current preferred imaging modality for this purpose because of its excellent visualization of valve anatomy, availability, and ease of use. Thus, valve morphology and motion and the valvular annulus can be analyzed, valvular regurgitation quantified, ventricle size and function assessed, and MR severity determined. CMR has emerged not only as the gold standard technique for assessing left and right ventricular volume, mass, and function but also as a robust and accurate tool for measuring volume and flow. However, recent guidelines still consider CMR to be “second-line” in the setting of indeterminate echocardiographic findings, because of availability, time, and cost.1 As discussed by the authors, myocardial fibrosis in ventricular functional MR is thought to be caused by preexisting myocardial disease rather than by MR-induced volume overload. However, because ventricular functional MR may be improved by LV reverse remodeling, it is essential to assess the progression of myocardial fibrosis by T1 mapping in patients with ventricular functional MR.
The current study by Tsunamoto et al3 provides important novel insights regarding myocardial damage in patients with ventricular functional MR that support a paradigm shift in which CMR tissue characterization informs the timing and postprocedural prognosis of TEER. A recent prospective multicenter cohort study6 reported that diffuse myocardial fibrosis assessed by CMR native T1 correlated with LV reverse remodeling at 6 months after valvular surgery, and T1 mapping may be a valuable tool to predict LV reverse remodeling in valvular heart disease, including severe ventricular functional MR. Myocardial assessment by T1 mapping in patients with MR should add value to conventional CMR assessments of LV function, geometry, and regurgitant severity. It is essential to raise awareness of the clinical usefulness of T1 and to promote standardization of its analysis.
Although ECV quantification by CMR T1 mapping is an established method, ECV quantification by computed tomography (CT) is an attractive alternative to CMR-derived ECV, similarly using the extracellular contrast agents (gadolinium-based contrast agents for CMR, and iodine-based contrast agents for CT) for ECV calculation.7,8 Compared with CMR, CT has significant practical clinical advantages, such as high accessibility, simple imaging techniques, short examination times, simultaneous evaluation of coronary arteries and extracardiac organs, and suitability for use in patients with mechanical devices. In a very recent study, Malhotra et al9 found that increased CT-derived ECV was associated with worse baseline LV systolic function and increased LV volumes in patients with severe MR. Furthermore, elevated ECV was independently associated with adverse remodeling after TEER. Hence, CT-derived ECV is expected to be widely used in clinical practice as a new quantitative imaging biomarker (Figure).
Preprocedural evaluation of ventricular functional mitral regurgitation by noninvasive imaging. Using cardiac magnetic resonance (CMR), myocardial tissue characterization by late gadolinium enhancement (LGE) imaging and T1 mapping [native T1 and extracellular volume fraction (ECV)] is available in addition to conventional functional and morphological assessment by cine CMR, and volumetric and flow quantification by phase-contrast imaging.
None.
