Echocardiographic assessment of patient-prosthesis mismatch
Patient-prosthesis mismatch is present when the effective prosthetic valve area is less than that of the normal human valve . The reduction in valve area is usually mild to moderate and may not be off immediate clinical significance. Occasionally, it can be severe and patients may be hemodynamically and symptomatically worse after valve replacement. This typically occurs after aortic valve replacement for aortic stenosis as the aortic annulus is not dilated and permits insertion of only a smaller prosthetic valve compared to that in aortic regurgitation. Similar situation can be there after mitral valve replacement for mitral stenosis.
Patient prosthesis-mismatch is an important cause of increased transvalvar gradient detected by Doppler echocardiography. Effective orifice area of an aortic prosthetic valve may be too small in relation to patient’s body surface area and can result in abnormally high gradients . Indexing of calculated valve area with respect to body surface area is usually done to assess patient prosthesis mismatch. Indexed effective orifice area of an aortic prosthetic valve should be more than 0.85 cm2/m2 to avoid significant gradient at rest and exercise . This corresponds to the concept that in moderate stenosis of native aortic valve, the indexed effective orifice area is <0.90 cm2/m2. An indexed effective orifice area ≤0.60 cm2/m2 will correspond to severe stenosis and requires reoperation.
Effective orifice area is usually calculated using continuity equation and involves two dimensional and Doppler echocardiography. Effective aortic prosthetic orifice area = cross sectional area of left ventricular outflow tract x (velocity time integral of left ventricular outflow/velocity time integral of aortic jet) . The cross sectional area of left ventricular tract can be calculated from the diameter measured by two dimensional echocardiography or the documented diameter of the sewing ring of the prosthetic valve. A simplified method for calculation of effective orifice area is cross sectional area of left ventricular outflow tract x (peak velocity of left ventricular outflow/peak velocity of aortic jet) .
In case of mitral valve replacement, an indexed effective orifice area <1.2 cm2/m2 is considered as patient prosthesis mismatch . Mitral patient prosthesis mismatch has been classified as moderate if indexed effective orifice area is >0.9 and ≤1.2 cm2/m2 and as severe if ≤0.9 cm2/m2.
Speckle tracking echocardiography has been used to assess patients with severe patient prosthesis mismatch . They defined severe patient-prosthesis mismatch after aortic valve replacement as indexed effective orifice area of 0.65 cm2/m2 or less. They found that global longitudinal strain and global circumferential strain were significantly decreased in the 54 patients with severe patient prosthesis mismatch, suggesting subclinical left ventricular dysfunction. These patients had preserved left ventricular ejection fraction.
Three dimensional and transesophageal echocardiography can be used to get better measurements of the left ventricular outflow tract diameter compared to transthoracic echocardiography. Outer margin of the stent or cage of the prosthetic valve should be measured. While assessing transprosthetic velocity by continuous wave Doppler, multiple windows including apical and right parasternal views should be used. For measurement of left ventricular outflow velocity, pulse-wave Doppler sample should be placed immediately below the apical border of the stent with no valve opening or closing clicks visible .