“ Let me show you in a figure how far our nature is enlightened or unenlightened: Behold! Human beings living in an underground cave. ” In Plato’s Allegory of the Cave, prisoners were chained to a wall and forced to look at two-dimensional (2D) flickering shadows that represented their entire reality. Peri-operative echocardiographers relying on 2D imaging for the assessment of mitral regurgitation (MR) face similar dimensional limitations.
The American Society of Echocardiography recommends that the echocardiographic assessment of MR severity involve a multi-modal approach including both qualitative and quantitative techniques. However, the limitations of conventional echocardiographic techniques used to evaluate MR have to be considered. Qualitative assessment of MR severity using color flow Doppler (CFD) is load dependent, and therefore the size and direction of MR jets may vary significantly especially in the dynamic intraoperative hemodynamic environment. CFD techniques are also angle dependent, have poor spatial and temporal resolution, and often do not reflect ideal hydraulics. Quantitative techniques using CFD including vena contracta width (VCW) and proximal isovelocity surface area (PISA) radius are actually one-dimensional, and often cannot capture the complex, three-dimensional (3D) geometry of effective regurgitant orifice area (EROA). The limitations of 2D-CFD are not just theoretical, as significant inter-observer variability using these techniques has been reported to be over 50%. The accuracy of echocardiography to grade MR has also been repeatedly challenged when compared to MRI. While there is substantial clinical outcome data supporting 2D approaches for grading MR severity, further consideration for more reliable techniques including three-dimensional (3D) parameters is warranted.
The vena contracta area (VCA) as a measure of EROA, can be obtained from 3D data sets without geometric assumptions by using multiplanar reformatting to crop the color jet to its narrowest planimetered area. By multiplying the VCA by the velocity time integral of the regurgitant jet, a regurgitant volume can also be calculated. Published MRI data supports this approach, clearly demonstrating 3D-VCA’s superiority over the 2D-VC method for evaluating MR severity. Nonetheless, several limitations to this approach need to be considered. As with all CFD techniques, gain settings should be standardized. Furthermore, this metric may account for the geometric complexity of the EROA, but still remains a static measurement. The VCA can also vary temporally by etiology with substantial dynamic differences throughout systole between patients with primary versus secondary MR. Finally, while we assume that we are indeed measuring the hydraulic VC’s cross-section, this is likely not the case. Two in vitro studies using laser have demonstrated that 3D-VCA can significantly over-estimate the size of the true hydraulic VC. Corroborating this data, the EROA derived from 3D-PISA in vivo is also smaller than the 3D-VCA.
Immediately after the introduction of the PISA technique for evaluating MR severity in the 1990’s, echocardiographers questioned its accuracy. Using a 2D approach, a single, one-dimensional radius is acquired and extrapolated to generate the area of a perfect PISA hemisphere for the calculation of the EROA. However, the true shape of a PISA and its corresponding EROA is often geometrically complex. Thus, 3D dimensional PISA techniques which are less susceptible to geometric assumptions enable more accurate and highly feasible estimates of MR severity. Regurgitant volumes acquired from semi-automated 3D-PISA techniques compare very closely to those obtained with MRI.
Despite its advantages, limitations remain with 3D-PISA. Because mitral regurgitation, as previously mentioned, is dynamic, the EROA calculated from PISA in a static manner is not adequate. In secondary MR, the PISA is biphasic with a peak that does not correspond temporally to the peak gradient that occurs in mid systole. In addition, all flow-based methods that rely on CFD are angle dependent. Consequently, blood flow in horizontal directions creating the sides of the PISA may be incorrectly represented by CFD. Doppler’s angle dependence is a particularly significant limitation for highly eccentric jets, potentially leading to further geometric distortions of hydraulic reality. Thus, a perfect single echocardiographic technique for the quantitative assessment of MR severity has yet to be identified.
Three-dimensional approaches for quantifying MR severity represent a significant advance in perioperative cardiac imaging overcoming geometric limitations of 2D echocardiography. Regurgitant volumes calculated using 3D-VCA and 3D-PISA both compare favorably with MRI measurements when compared to conventional 2D imaging. As these new techniques are enthusiastically promulgated in the echocardiography community, we must be cautious to recognize that CFD in its current form with limited spatial and temporal resolution may not be able to accurately depict the actual hydraulics of flow convergence and flow contraction with perfect accuracy. Novel approaches which use 3D-CFD to more accurately measure differences between left ventricular in-flow at the mitral annulus and out-flow in the left ventricular outflow tract may have unique promise.
Compared to conventional 2D techniques for evaluating MR severity, the recent adoption of 3D techniques are limited by the paucity of longitudinal studies demonstrating their clinical value. As we emerge from the 2D cave and explore exciting new territories in perioperative diagnostic imaging, further robust investigation will be needed to determine the clinical value of incorporating these new techniques in formal guideline recommendations for risk stratification and determining the most appropriate intervention.
Frederick C. Cobey, MD, MPH, FASE is an Assistant Professor of Anesthesiology and Division Chief of Cardiac Anesthesiology at Tufts Medical Center.
Stanton K. Shernan, MD, FAHA, FASE is a Professor of Anaesthesia and Chief of the Division of Cardiac Anesthesia at Brigham and Women’s Hospital, Harvard Medical School. Dr. Shernan is the Immediate Past President of the National Board of Echocardiography and the current Secretary/Treasurer of the Society of Cardiovascular Anesthesiologists.