Valvular heart disease: MV and AV diseases for the trainee Part 2

Valvular heart disease: MV and AV diseases for the trainee Part 2


As already talked in part 1, severity of the stenotic lesion is evaluated by the ‘orifice area’ and ‘pressure gradient’ Orifice area can be evaluated by 2D or Doppler echocardiograhy, or by cath using Gorlin equation. Pressure gradient can be evaluated by Doppler echocardiography, or by cath. Let’s take a look at the evaluations in the mitral and aortic valves. Mitral valve orifice area in MS, can be obtained by planimetry in 2D echocardiography, as the rate of flow deceleration in early diastolic flow of the mitral inflow depends on the orifice area, that is, smaller the area, slower the rate of deceleration, Rate of deceleration can be represented by the parameter called ‘pressure half time (PHT)’ Pressure half time implies the time elapsed
from the time of peak pressure gradient to the time when the gradient is half. That is, when the velocity becomes the value of peak velocity divided by the square root of 2. If we divide 220 by this pressure half time, we can get mitral orifice area. Gorlin equation is based on the fact that area can be obtained when the flow is divided by the velocity. ‘Flow’ can be obtained from the CO corrected by the diastolic filling period and heart rate, as the ‘velocity’ cannot be obtained by cath, it can be derived from the relationship that ‘velocity’ correlates with the square root of pressure gradient, and the correlation coefficient. For the estimation of pressure gradient, nearly exclusively, Doppler echocardiography has been used. According to the Bernoulli equation, as the fluid energy is conserved when the fluid moves from one to the other places, summation of pressure and kinetic energy at place 1 is same as the summation of pressure and kinetic energy at place 2. Pressure difference between two places, as the half of the blood density corrected by the factor to get the ‘mmHg’ unit, is approximately ‘4’, pressure difference can be obtained by multiplying 4 to the difference in the squares of the velocities at two places. And if the V1 is significantly lower than V2, therefore can be ignored, equation can be simplified as square of V2 multiplied by 4. To obtain transmitral gradient, pressure gradient at a number of time points during diastole must be calculated from the velocities using Bernoulli equation, and the mean value of the pressure gradient must be obtained. In the real world, what we have to do is just tracing the envelope of the velocity profile. In cath, pressure gradient can be directly estimated from the difference between LV diastolic and LA pressures. As we cannot directly measure LA pressure, LA pressure is substituted by the pulmonary capillary wedge pressure, and measure difference between the two pressures during diastole. In AS, severity of the stenosis can be evaluated in a very similar fashion. We can estimate aortic valve area on 2D. Image quality is quite frequently inadequate for the measurement due to heavy calcification and other factors, therefore, in the real world, 2D echo method is used very rarely. Method of measuring aortic valve area using Doppler technique is based on the ‘mass conservation’ law that amount of flow passing through the subvalvular area must pass through the aortic valve area. Therefore, amount of flow passing subvalvular area, A1x V1, must be same as the amount of flow passing through the aortic valve area, A2x V2 Aortic valve orifice area equals to A1x V1 divided by V2. A1 can be obtained by the LVOT diameter assuming the LVOT as a circle. V1 can be obtained by the pulsed-wave Doppler by placing the sample volume just beneath the AV. V2 can be obtained by the continuous-wave Doppler, that is, highest velocity passing along the line of aortic ejection. Gorlin equation to calculate aortic valve area was already mentioned. What is different from the calculation of mitral valve area are using systolic ejection period, instead of diastolic filling period, and by using different coefficient. Pressure gradient can be estimated by Doppler echocardiography. Peak instantaneous gradient can be calculated from the peak velocity obtained by Doppler, using simplified Bernoulli equation. There is one aspect that should be kept in mind. In the past, cath is the only method of estimating pressure gradient. and pressure gradient measured in cath is the peak to peak gradient. As the peak of LV pressure and that of aortic pressure are not occurring at the same timing, peak to peak gradient is always lower than the peak instantaneous gradient. As most of our accumulated data are based on the peak to peak gradient, there had been an effort to approximate the peak to peak gradient with the pressure gradient measured by Doppler. If we estimate the mean gradient with Doppler, this value is close to the peak to peak gradient measured by cath. In cath, peak to peak gradient can be obtained from the difference between the peak pressures of LV and aorta. Once valvular orifice area is obtained, we should determine the severity of stenosis based on the orifice area and pressure gradient. Normal MV orifice area is 4-6cmsq, 4cmsq might be the representative value, normal AV orifice area is 3-4cmsq, a little smaller than the MV orifice area, 3cmsq might be the representative value. In the traditional point of view, MV orifice area less than 1cmsq or pressure gradient greater than 10mmHg has been regarded as severe stenosis, in case of AV, AV orifice area less than 0.75cmsq or pressure gradient greater than 50mmHg has been regarded as severe stenosis. Definition of severe stenosis in not based on the research data. In the MV, if we assume the minimum necessary CO of 5L/min, in the resting state, in patient with MV orifice area of 1cmsq, this amount of CO can be maintained with normal LA pressure. If the MV orifice area is less than 1.0cmsq, higher than normal LV pressure is needed to maintain this CO. If the heart rate is increased to 88/min, upper normal LA pressure is needed to maintain this CO. When the heart rate reaches 100/min, higher than normal LA pressure is needed. Indicating that this condition might cause pulmonary congestion and patient would complain dyspnea. This explanation is based on the theoretically calculated values, under the assumption of minimum necessary CO of 5 L/min. AV does not have research data or theoretical basis in the determination of severe stenosis, either. Given that a quarter of the normal area in the mitral valve can be regarded as ‘severe’ stenosis, as the AV has smaller orifice area than the MV, orifice area of 0.75cmsq, a quarter of the normal area, is a reasonable value as a cut-off value of ‘severe’ stenosis. However, in recent guidelines, as the critera for ‘severe’ stenosis, in the AV, orifice area less than ‘1’cmsq or pressure gradient greater than ’40’mmHg – traditionally this value was 50mmHg – was defined as severe stenosis. Even in the MV, recently, orifice area less than 1.5cmsq was proposed as a definition of severe stenosis. There are several practical problems with this guideline. In the MS, if we define severe stenosis as the valve orifice area55mm. Recent recommendations by ACC/AHA and ESC, although there is a small difference in the recommendations between the two organizations, are not much different from the traditional recommendation. Surgery is recommended in patient with EF50mm, rather than 55mm in the past, and criteria of diastolic dimension>70mm was addded to the traditional indications. In ACC/AHA guideline, surgery is recommended at the diastolic diameter of 65mm instead of 70mm is recommended. Thank you for your attention.