The Coanda Effect

Figure 1.: Underestimation of mitral regurgitation (MR) severity by jet area estimation by 2-dimensional (2D) and 3-dimensional (3D) transesophageal echocardiography (TEE). 2D midesophageal long-axis view (A) and corresponding R-wave-gated 3D view (B) interrogating eccentric MR jet by TEE showing the Coanda effect. Sole reliance on visual estimation of jet area can lead to underestimation of eccentric jet severity because of Coanda effect. AV indicates aortic valve; LA, left atrium; LV, left ventricle; MV, mitral valve.A 57-year-old man presented for an urgent coronary artery bypass grafting surgery. On routine intraoperative 2-dimensional transesophageal echocardiography (2D TEE), an eccentric mitral regurgitant (MR) jet hugging the left atrial (LA) wall was appreciated in the 2D midesophageal long-axis view. (Figure 1A; Supplemental Digital Content 1, Video 1, left panel, https://links.lww.com/AA/B459) and 3- dimensional (3D) TEE (Figure 1B; Supplemental Digital Content 1, Video 1, right panel, https://links.lww.com/AA/B459). The MR was graded as moderate to severe, and mitral valve repair was performed using an annuloplasty ring. DISCUSSION Depending on the underlying valvular pathology, regurgitant valvular jets may either be central or eccentric. Certain eccentric regurgitant jets that are directed toward the walls of the receiving chamber appear smaller than a central jet of same regurgitant volume. This is attributed to the Coanda effect. Definition The “Coanda effect” explains the tendency of a high-velocity jet to adhere to and flow along an adjoining surface; it was described by a Romanian aeronautical engineer, Henri M. Coanda, in the early 20th century.1 Although not specific to regurgitant jets, the Coanda effect is commonly used to describe the behavior of eccentric MR jets that hug the left atrial wall. Coanda effect is also known to affect the behavior of eccentric regurgitant jets associated with aortic, tricuspid, and repaired valves (Supplemental Digital Content 2, Video 2, https://links.lww.com/AA/B460; and Supplemental Digital Content 3, Video 3, https://links.lww.com/AA/B461). Early in vivo studies using color flow Doppler (CFD) reported that eccentric, wall-impinging MR jets had a smaller jet area than central jets with comparable regurgitant volume.2 This underestimation of the severity of eccentric MR jets was initially attributed to a reduction in velocity and subsequent loss of “momentum flux” after the jet came into contact with the LA wall.3 However, in vitro experiments showed that the change in shape of the regurgitant jet was also because of the Coanda effect.4,5 Physical Principle of Coanda Effect When an eccentric regurgitant jet is directed toward an adjacent wall, it rapidly entrains the fluid between the jet and the wall6 and results in a regional increase in fluid velocity and a fall in pressure between the jet and the adjacent wall; thus, resulting in inward suction of the jet toward the wall (Coanda effect). Coanda Effect and MR The Coanda effect in eccentric MR results in significant change in 3D jet trajectory and area affecting its eventual appearance on CFD imaging. Although the regurgitant volume is the most important determinant of MR severity,7 color map of the jet area obtained with CFD only offers a cursory assessment of the MR. The CFD MR jet area is dependent on the aggregate of regurgitant volume, driving pressure as well as turbulence generated by the MR jet in the LA. Therefore, it is possible to have different regurgitant velocity maps (jet areas) for the same total regurgitant volume. Other factors, ie, machine settings, receiving chamber compliance, and geometry of the jet, and its relationship to chamber walls can also affect jet area and appearance.7 Because of the asymmetric constraint related to Coanda effect, eccentric jets appear to have varying severity on CFD in different multiplane angles compared with central jets (Figure 2), which might lead to erroneous conclusions. This is the reason why estimating the MR severity by simply “eyeballing” the jet by CFD should not be the sole criterion for grading MR severity.Figure 2.: Biplane imaging of central and eccentric mitral regurgitant (MR) jets by 2-dimensional (2D) transesophageal echocardiography (TEE). A central MR jet (A and B) expands symmetrically, and hence, has similar appearance on orthogonal views. An eccentric jet exhibiting Coanda effect (C and D) adheres to the adjacent walls resulting in significantly different appearance (A and B) of the same jet on 2D biplane TEE imaging. LA indicates left atrium; LV, left ventricle.Table.: TEE Parameters Useful in Grading Severity of Mitral Regurgitation Jets Exhibiting Coanda EffectThe effect of MR jets exhibiting Coanda effect on various 2D-TEE parameters and the recommendations for grading their severity have been summarized in the Table. In brief, vena contracta7 and volumetric measures8,9 of quantification (such as regurgitant volume and regurgitant fraction) are less susceptible to inaccuracies. 3D TEE 3D TEE improves MR quantification by improving estimation of the effective regurgitant orifice area10 with direct 3D echocardiographic vena contracta area11 or proximal isovelocity surface area12 measurements especially in patients with eccentric MR jets exhibiting Coanda effect. In patients with irregular rhythms, biplane imaging (simultaneous display of 2 orthogonal planes) with matrix TEE probes can be helpful in accurate assessment of severity. Teaching Points Coanda effect causes eccentric regurgitant jets to hug the adjacent recipient chamber wall. Interrogation of an MR jet hugging the LA wall leads to underestimation of jet area by CFD because of plastering of jet against the LA wall (Coanda effect). The smaller CFD area obtained while measuring wall-hugging regurgitant jets is a limitation of 2D-imaging planes. Use of simultaneous orthogonal imaging and 3D CFD imaging can possibly improve assessment of CFD area. Vena contracta and volumetric methods of MR assessment are less susceptible to the influence of Coanda effect and, hence, are useful for the assessment of eccentric jets. DISCLOSURES Name: Amit Bardia, MD. Contribution: This author helped write the manuscript. Conflicts of Interest: This author declared no conflicts of interest. Name: Rabya Saraf, MD. Contribution: This author helped write the manuscript. Conflicts of Interest: This author declared no conflicts of interest. Name: Kamal Khabbaz, MD. Contribution: This author helped design the study. Conflicts of Interest: This author declared no conflicts of interest. Name: Andrew Maslow, MD. Contribution: This author helped design the study. Conflicts of Interest: This author declared no conflicts of interest. Name: Feroze Mahmood, MD. Contribution: This author helped design the study and write the manuscript. Conflicts of Interest: This author declared no conflicts of interest. This manuscript was handled by: Martin J. London, MD.