terça-feira, 31 de janeiro de 2012

PHT e análise de Prótese Mitral - by Fábio Soares

É comum vermos nos laudos dos ecocardiogramas, a estimativa da área efetiva de uma prótese mitral utilizando o "Pressure Half Time". Mas será que isto é o ideal? Vejamos o que diz a literatura




Pressure Half-Time.
The rate of blood flow across the mitral valve is dominated by the mitral orifice area in the presence of moderate or severe stenosis. However, when the mitral stenosis is only mild or there is a normally functioning valve, the rate of flow also depends on atrial and ventricular compliance, ventricular relaxation, and the pressure difference at the start of diastole. Thus, a large rise in pressure half-time on serial studies or a markedly prolonged single measurement (>200 ms) may be a clue to the presence of prosthetic valve obstruction, because the pressure half-time seldom exceeds 130ms across a normally functioning mitral valve prosthesis. However, minor changes in pressure half-time occur as a result of nonprosthetic factors, including loading conditions, drugs, or aortic insufficiency. Pressure half-time should not be obtained in tachycardic rhythms or first-degree atrioventricular block when E and A velocities are merged or the diastolic filling period is short.


Calculation of EOA from pressure half-time, as traditionally applied in native mitral stenosis, is not valid in prosthetic valves, because of its dependence on LV and LA compliance and initial LA pressure. Therefore, EOA calculation by the continuity equation is preferable to that measured by pressure half-time in mitral prostheses. In bileaflet valves, the smaller central orifice has a higher velocity thant he larger outside orifices, which may lead to underestimation of EOA by the continuity equation. Thus, the accuracy of EOA by the continuity equation may be better for bioprosthetic valves and single tilting disc mechanical valves. EOA is derived as stroke volume through the prosthesis divided by the VTI of the mitral jet velocity:

Fernandes V, Olmos L, Nagueh SF, Quinones MA, Zoghbi WA. Peak early diastolic velocity rather than pressure half-time is the best index of mechanical prosthetic mitral valve function. Am J Cardiol 2002;89:704-10.

Malouf JF, Ballo M, Connolly HM, et al. Doppler echocardiography of 119 normal-functioning St Jude Medical mitral valve prostheses: a comprehensive assessment including time-velocity integral ratio and prosthesis performance index. J Am Soc Echocardiogr 2005;18: 252-6.

Dumesnil JG, Honos GN, Lemieux M, Beauchemin J. Validation and applications of mitral prosthetic valvular areas calculated by Doppler echocardiography. Am J Cardiol 1990;65:1443-8

Bitar JN, Lechin ME, Salazar G, ZoghbiWA. Doppler echocardiographic assessment with the continuity equation of St. Jude Medical mechanical prostheses in the mitral valve position. Am J Cardiol 1995;76:287-93

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