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Proximal Isovelocity Surface Area: The Three-Dimensional “Correction”

Skubas, Nikolaos J. MD, FASE, DSc; Lang, Roberto M. MD

doi: 10.1213/ANE.0000000000000617
Editorials: Editorial

From the *Department of Anesthesiology, Weill Cornell Medical College, New York, New York; and Department of Medicine, University of Chicago Medical Center, Chicago, Illinois.

Accepted for publication November 20, 2014.

Funding: No funds.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Nikolaos J. Skubas, MD, FASE, DSc, Department of Anesthesiology, Weill Cornell Medical College, M302C, 525 East 68th St., New York, NY 10605. Address e-mail to njs2002@med.cornell.edu.

“If I am walking with two other men, I will pick out the good points of the one and imitate them, and the bad points of the other and correct them in myself.”

Confucius, Analects

The decision to intervene on an incompetent mitral valve (MV) is based on the severity of mitral regurgitation (MR), which is graded by qualitative and quantitative analysis of the MR jet. The most frequently used measurement is the effective regurgitant orifice area (EROA), which represents a surrogate of the incompetent MV orifice, because the calculation is based on flow measurements.1

The principle of EROA calculation is based on the continuity principle: blood flow is considered to be constant along the MR jet and flow rate (= area × velocity) through the heart does not change. As the blood cells move from the wider left ventricular area toward the much smaller regurgitant MV orifice, they accelerate and converge into concentric hemispheric shells or proximal isovelocity surface areas (PISAs). The closer to the incompetent anatomic MV orifice, the smaller the PISA is and the faster the velocity of the blood is. This blood movement is imaged with color flow Doppler (CFD), when the CFD sector is placed over the mitral leaflets, including both their left ventricular and left atrial sides. When the blood velocity exceeds the CFD velocity limit (usually set at ±60–70 cm/s), it will be color coded with the opposite color, that is, red will change to blue, as the MR jet is facing the transducer in a midesophageal transesophageal echocardiography (TEE) plane. The distance (r) from the hemispheric PISA at which this color change occurs to the center of the regurgitant orifice can be measured with digital calipers and the area (A) of the hemispheric PISA can be calculated as A PISA = 2π × r 2. The flow rate of this PISA will be PISAflow = A PISA × aliasing velocity. The MR flow rate is considered constant based on the theory of the continuity principle; therefore, PISAflow will be equal to the flow rate at the regurgitant orifice. There, the MR jet velocity can be measured with continuous-wave Doppler. Solving for this effective regurgitant orifice will calculate the EROA as EROA = 2π × r 2 × aliasing velocity/MR jet velocity.2

The limitations of the PISA approach mainly result from misalignment between the Doppler beam and the MR jet, a noncircular EROA and eccentric or multiple MR jets.3 Consequently, the PISA approach for the evaluation of MR has a rather poor interobserver variability (0.37, with 95% confidence interval, 0.16–0.58).4 It is important to emphasize that the EROA calculation assumes that the PISA has a hemispheric shape. However, 3D echocardiographic examination of patients with MR shows that the PISA cells are in fact hemielliptical in 98% of cases (49 of 50 patients). Using Doppler-based calculation of EROA, the hemispheric PISA resulted in a statistically significantly smaller EROA (0.34 ± 0.14 cm2) compared with a hemielliptical PISA calculation (0.48 ± 0.25 cm2) with 45% of patients having the MR down-graded from moderate-severe to mild-moderate.5 To further complicate matters, the MR jet differs between structural and functional MR. In functional MR, the pathology is subvalvular, and tethering of structurally normal mitral leaflets typically results in an incompetent MV orifice along the coaptation line, which is more crescent shaped than the hemispheric or hemielliptical incompetent MV orifice encountered in structural MR.6

In this issue of Anesthesia & Analgesia, Ashikhmina et al.7 retrospectively compared the 2D and 3D TEE evaluation of PISAs and resulting EROAs in 24 patients undergoing cardiac surgery with at least mild functional preoperative MR from a database. All these patients had 3D TEE examinations, comprised of full-volume CFD datasets, which were acquired over 14 heart beats, thus resulting in volume frame rates of 30 to 40 Hz. With off-line analysis in a proprietary system and using standardized gray and color scales, the full-volume CFD datasets were dissected to image the MV and MR jet in 2 orthogonal midesophageal (5-chamber and mitral commissural) and 1 short-axis (across the base of the MR jet) views. First, the authors calculated PISA using the standard 2D technique: PISA was considered a hemisphere (HS), the radius of the first aliasing velocity was measured in the midesophageal 5-chamber view, and subsequently the HS-PISA and resulting HS-EROA were calculated. Similarly, PISA was considered a hemiellipse (HE), wherein the PISA radius was measured in both midesophageal views and HE-PISA and HE-EROA were calculated. Subsequently, the authors calculated PISA with 3D techniques. The PISA shell was traced in a semiautomated manner, using proprietary software for analysis of the MV anatomy (the PISA was analyzed as if its surface was the mold of the left atrial side of the mitral leaflets) and 3D-PISA and 3D-EROA were calculated. The 3D-PISA measurements were statistically significantly larger than 2D measurements (in descending order: 3D-PISA > HE-PISA > HS-PISA) with similar findings for their respective EROA measurements. The 2D- and 3D-EROAs were then compared against the vena contracta area (VCA), which was measured by manual planimetry from the short-axis 3D full-volume CFD dataset used for 3D-PISA calculation.8 The VCA, which is essentially the “footprint” of the base of the MR jet as it traverses the incompetent MV orifice, correlated closely but differed less from 3D-EROA than either HS-EROA or HE-EROA.

This work by Ashikhmina et al. proves that the geometric assumptions of 2D PISA do not allow for an accurate estimation of the severity of functional MR because the PISA calculations are based on an hemispheric or hemielliptical shape of the base of the MR jet. On the contrary, with 3D echocardiography, the precise measurement of the PISA (and EROA if the MR jet velocity can be accurately measured) as well as the VCA by planimetry is feasible, resulting in a more accurate assessment of MR severity. One of the flaws of this study is the fact that VCA was used as the reference method, instead of measuring the MR volume using 3D volumes, 2D method of discs, or Doppler as previously performed in other studies. The authors have also pointed out that their results do not apply to patients with multiple MR jets or structural MR. Also, a single measurement may not accurately describe the dynamic nature of functional MR. The technique used to measure the 3D area of the PISA is novel and requires significant expertise with 3D echocardiography and has not been reproduced by others yet. Last but not least, no clinical question was addressed in this study and future prospective similar studies will determine whether using this novel methodology will impact treatment and outcomes. However, one should consider the strengths of this study, namely the standardization of the 3D acquisition of full-volume datasets, the meticulous control of salient but important imaging details, such as imaging settings, and an excellent interobserver reproducibility.

There are important take home messages in the study by Ashikhmina et al.7 The traditional 2D PISA should not be considered a reliable technique to evaluate functional MR. The technique assumes a circular or an elliptical base of the MR jet and as shown, this is not the case. At no time should a 2D PISA be the only criterion of severity of functional MR. Another important point to make is that the measurement of a 3D-PISA is, currently, a daunting task, as it requires time to perform, an operator with significant expertise and proprietary software. The imaging and measuring of the VCA from a short-axis view is, however, relatively easy. As presented in their Figure 5, it is clinically advisable to try to acquire the VCA from a full-volume CFD dataset in all cases of functional MR. The practicing echocardiographer-anesthesiologist, currently equipped with 3D TEE, should make acquisition and measurement of the VCA part of the routine examination, and perhaps in the future also incorporate 3D-PISA and derived 3D-EROA. We should all definitely avoid the very simple “eye-balling” estimation of MR as the only method used in our studies.9

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DISCLOSURES

Name: Nikolaos J. Skubas, MD, FASE, DSc.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Nikolaos J. Skubas approved the final manuscript.

Name: Roberto M. Lang, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Roberto M. Lang approved the final manuscript.

This manuscript was handled by: Martin J. London, MD.

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