The Coanda effect refers to the phenomenon by which a narrow jet of liquid (or air) passing through an orifice directly in sequence with a solid (especially convex) surface will deviate from its path and adhere to this curved surface, following its shape in parallel.
The mechanism is thought to be related to entrainment of fluid from the downstream chamber into the jet, creating an area of low pressure around the jet margins; if a surface is sufficiently close to one of these low-pressure margins, the jet deviates toward the surface 1.
Radiographic features
Ultrasound
This phenomenon underlies the issue in estimating mitral regurgitation severity on echocardiography when using color flow Doppler;
- primarily affects eccentric, as opposed to centrally directed, mitral regurgitation jets, which have a resultant propensity to adhere to left atrial walls
- jet area, one parameter used as to grade mitral regurgitation severity may be underestimated by more than 40% with these eccentric jets adhering to atrial walls
- no relationship between jet area and regurgitant volume in wall adherent jets, as compared to a modest relationship in central jets 1
- postulated mechanisms include transfer of momentum to atrial walls and deformation of the jet into a thinner perpendicularly directed vector, increasing the angle of insonation and therefore decreasing the measured doppler shift
- jet area, one parameter used as to grade mitral regurgitation severity may be underestimated by more than 40% with these eccentric jets adhering to atrial walls
- Assessment of the morphology of a regurgitant jet, then, is paramount prior to quantitation of severity, especially when using color flow Doppler indices
Other clinically relevant situations in which the Coanda effect is thought to be notable include 2:
- supravalvular aortic stenosis 3
- patent ductus arteriosus (PDA) flow 4
- the typical systolic flow orientation from a PDA hugs the left wall of the main pulmonary artery (MPA), as it originates adjacent to the left pulmonary artery
- with an absent forward stroke volume from the right ventricle (RV) in e.g. pulmonary atresia, the Coanda effects may result in a flow loop, with a jet adhering to the right wall of the main pulmonary artery, reversing direction and traveling toward the MPA bifurcation
- this may mimic physiologic, systolic flow from the RV through the pulmonic valve into the MPA
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