The assessment of mitral regurgitation (MR) remains one of the most common and important uses of intraoperative transesophageal echocardiography (TEE). Accurate characterization of the cause and severity of MR along with the determination of valve repairability represent critical goals of the TEE examination. Broadly, MR may be characterized as degenerative or functional. Degenerative MR includes conditions in which primary structural abnormalities of the mitral valve such as prolapsing or flail leaflets lead to incompetence. Functional MR, however, describes a condition in which left ventricular remodeling and annular dilation decrease the coaptation of relatively normal mitral leaflets. In either condition, a careful preoperative assessment of MR severity is critical in determining the timing and nature of any possible surgical intervention. Certainly, any assessment of MR requires an appreciation of the differences in both the pathoanatomic changes in valvular or ventricular anatomy and the pathophysiologic aspects of valve leakage between patients with MR because of functional and degenerative mechanisms. To that end, the currently accepted systematic approach to assess MR by TEE focuses on a careful determination of its severity and the identification of the mechanism of MR and the precise location of mitral pathology.
In this issue of Anesthesia & Analgesia, Cobey et al.1 used sophisticated 3-dimensional (3D) TEE imaging with offline analysis to precisely define not only the size and shape of the regurgitant orifice (RO), but also the timing of maximal RO in patients with MR. Important temporal differences were noted between the regurgitant jets of those with functional versus those with degenerative MR.
Published guidelines recommend the use of multiple echocardiographic modalities in the assessment of MR.2 Echocardiographers often evaluate MR qualitatively based on characteristics of the color Doppler jet as well as 2-dimensional (2D) assessment of valve architecture and chamber size. Quantification of MR volume and effective regurgitant orifice area (ROA) are possible using either the continuity equation or the proximal isovelocity surface area (PISA) method.3
Measurement of the vena contracta (VC) is another semiquantitative means of grading MR severity. The VC width, the narrowest width of the regurgitant jet located at or just downstream from the RO, is identified using color Doppler imaging, and its width is recorded with electronic calipers. Although it is a linear dimension, the VC width is assumed by many to accurately grade MR severity irrespective of jet orientation and mechanism of regurgitation.4 Assuming a perfectly circular shape, the ROA can be calculated using the VC width as the diameter4 such that:
In clinical practice, RO geometry is frequently noncircular. Previous studies using 3D echocardiography5,6 have shown that the RO shape is often asymmetric, particularly in patients with functional MR.7 This concept confounds the notion of ROA calculation using only the VC width as the diameter of a circle, and it calls into question the validity of any 2D measurement of VC width. To limit potential error in the determination of VC width, some authors recommend obtaining this measurement in a long-axis view.4
As an alternative to VC width, echocardiographers can measure the VC area (VCA). The VCA is a short-axis slice through the narrowest portion of the MR jet in a plane that is roughly orthogonal to the direction of MR propagation. Once this narrowest cross-section of the MR jet is located, its area is traced in a manner analogous to planimetry of a stenotic mitral or aortic valve orifice. Earlier investigations have shown VCA to more accurately reflect MR severity compared with VC width7 because it overcomes many of the geometric limitations of VC. Although it is theoretically possible to trace the ROA in a systolic frame using 2D imaging, 3D echocardiography affords a more reproducible approach to the determination of VCA. In particular, the use of a multibeat, full-volume acquisition followed by multiplanar image review allows precise determination of the cross-sectional area of the MR jet.4,7
In their report, Cobey et al.1 used offline 3D multiplanar review to determine the VCA in 2 groups of patients, those with functional MR and those with degenerative MR. However, the goal of the study was not simply to apply an established, albeit sophisticated, technique to the measurement of ROA. These investigators obtained 3D data sets with a high level of temporal resolution and methodically traced the VCA frame by frame throughout systole (an average of 12 systolic frames per patient). This detailed and laborious analysis revealed that the VCA changes throughout systole in patients with both functional and degenerative MR. Notably, the temporal patterns in VCA variation differed importantly between the 2 patient groups. Patients with functional MR showed a biphasic peak in VCA with maximal values reached both in early and in late systole. In contrast, those with degenerative MR displayed a single peak in VCA in mid-to-late systole.
The notion that the ROA of MR changes throughout systole is not new. The dynamic nature of the ROA was described 20 years ago,8 a fact well known to the current authors. More recent studies have further elucidated temporal patterns in the RO of functional9 and degenerative10 MR.
Although neither the techniques used nor the results obtained by Cobey et al.1 are novel, their work should be of significant interest to many readers of this journal. First, although 3D echocardiography and multiplanar review have been used to determine the VCA in patients with MR, it has not, to our knowledge, been used to contrast temporal ROA changes in patients with various mechanisms of regurgitation. The authors have developed a new application for a new technology.
In addition, this study provides further evidence of the incremental value of 3D imaging in general and postprocessing multiplanar review in particular. Other investigators have described the incremental value of 3D over 2D TEE in the accurate sizing of the aortic annulus in transcatheter aortic valve replacement,11 but work such as that by Cobey et al.1 provides further impetus for training in and routine implementation of these advanced imaging techniques.
Perhaps the most important implications of the current study relate to how and when we measure MR by any technique. For example, PISA analysis remains a commonly used quantitative measure of MR in the operating room. Derivation of the effective ROA by the PISA method requires input of the peak MR velocity. Common teaching in echocardiography suggests that the PISA radius should be obtained at the same point in the cardiac cycle as the peak MR velocity. However, as the authors of the current study note, the peak VCA, the peak MR velocity, and the largest PISA radius may not coincide temporally, especially in patients with functional MR. When performing PISA analysis, there may be a tendency to scroll through systole until the largest or most distinct hemispheric velocity transition becomes apparent, regardless of the mechanism of MR or the timing of the peak MR velocity. What then is the validity of a calculation that uses measurements obtained at different times in the cardiac cycle? The many shortcomings of PISA-derived calculations are well known12 and include technical limitations (Doppler gain, Nyquist limit), geometric limitations (shape of the RO and shape of the PISA shell), and temporal limitations (timing of various measurements). This information only further undermines a technique already fraught with many limitations.
Ultimately, if existing imaging criteria cannot be applied equally to all types of MR, Cobey et al.1 challenge us to ponder whether future guidelines for quantification of MR should be different depending on the mechanism of MR.
Although the growth of intraoperative 3D imaging seems inevitable and the applications of multiplanar analysis will undoubtedly expand, a few important facts relative to the work of Cobey et al.1 should be considered. First, this study was performed by a group of experienced investigators who obtained data sets with a temporal resolution (30–40 Hz) that is very high for color 3D imaging. This was accomplished by saving gated, 14-beat, full-volume clips, a laudable accomplishment for intraoperative imaging and one that may be difficult to replicate in many clinical situations. Second, the study was performed in a large surgical practice over a 3-year period. Only 42 patients were enrolled. It is possible that the included patients represent a carefully selected subset of those in whom high-resolution 3D imaging was feasible. The ability to measure VCA with 3D imaging in “standard” surgical populations is unclear and will be determined by future studies and clinical practice. Should intraoperative echocardiographers routinely seek to measure the VCA in patients with MR at this time? In most cases, probably not. The use of color 3D imaging and multiplanar review requires training and practice and, for this specific application, further validation.
It is difficult to imagine this type of study being conducted anywhere other than the cardiac operating room. The need for high temporal resolution afforded by 14-beat, full-volume, gated acquisitions almost certainly renders this protocol unsuitable to the outpatient or hospital echocardiography laboratory where prolonged patient immobility and breath-holding would be difficult to achieve. The current investigators have nicely demonstrated that the opportunity to perform TEE examinations in large groups of mechanically ventilated patients creates a unique environment in which cardiac anesthesiologists can become pioneers in innovative imaging techniques.
Overall, Cobey et al.1 remind us of several important concepts: (1) the size of the RO changes during systole and the temporal pattern differs based on the underlying mechanism of MR, (2) the RO or VCA is often asymmetric, especially in patients with functional MR, and (3) current MR assessment techniques such as PISA analysis likely have important temporal limitations as well, because of the changing size of the RO. The insights provided by the work of Cobey et al.1 may further erode clinical confidence in the quantification of MR by the PISA method. For now, an integrated approach that combines the results of multiple echocardiographic techniques should remain the cornerstone of MR assessment.
Name: Kent H. Rehfeldt, MD, FASE.
Contribution: This author helped write the manuscript.
Attestation: Kent H. Rehfeldt approved the final manuscript.
Name: A. Stephane Lambert, MD, FRCPC.
Contribution: This author helped write the manuscript.
Attestation: A. Stephane Lambert approved the final manuscript.
This manuscript was handled by: Martin J. London, MD.
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