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when there is critical aortic or pulmonary stenosis ■ The Bernoulli equation should also be used with care in the setting of ventricular systolic dysfunction In this context, the ventricle may no longer be capable of generating sufficient output and hence produces low proximal velocities The reduction of flow across the valve will be associated with a reduction in gradient In the case of ventricular dysfunction, Doppler gradients may be abnormally low and should be interpreted carefully In contrast to the adult population, this is more unusual in children ■ A final problem can be the phenomenon of recovery of pressure As already emphasized, when flow occurs across a narrowing, potential energy is converted into kinetic energy However, distal to the narrowing, the vessel widens again, and kinetic energy is converted into potential energy This phenomenon is called pressure recovery and is more important in smaller vessels This leads to overestimation of gradients and should be taken into consideration when assessing aortic stenosis severity in children Mean Pressure Gradient The pressure gradient derived from the Bernoulli equation is the peak instantaneous pressure gradient and not the peak-to-peak pressure gradient as measured during cardiac catheterization (Fig 19.50) The peak-instantaneous gradient derived from Doppler measurements across a stenosis will always be higher than the peak-to-peak gradient This means that Doppler interrogation systematically overestimates pressure gradients measured invasively To overcome this problem, the mean pressure gradient can be calculated by integrating the velocity curve during ejection and thus calculating the mean gradient This is the average of all the instantaneous pressure gradients throughout ejection This is calculated electronically by tracing the Doppler curve Mean pressure gradients have been shown to correlate better with peakto-peak gradients obtained during cardiac catheterization and are generally used to assess the severity of a stenosis FIG 19.50 Peak and mean gradients of flow across the aortic valve in a patient with aortic stenosis Application of the Bernoulli Equation in Clinical Practice For the application of the Bernoulli equation in the context of valvar stenosis, reference should be made to the individual chapters devoted to valvar abnormalities in pediatric cardiac disease This chapter focuses on the assessment of right ventricular systolic pressure and pulmonary arterial pressures using Doppler echocardiography because this is an important aspect in the evaluation of patients with congenitally malformed hearts, where elevated right heart pressures, as well as pulmonary hypertension, can be an important topic Estimation of Pressures in the Right Ventricle and Pulmonary Arteries For the assessment of right ventricular pressures and pulmonary artery pressures, different Doppler measurements can be used If tricuspid regurgitation is present, at least mild, a continuous wave Doppler signal can be obtained The fourchamber view is used most commonly, although other views can provide good alignment with the regurgitant jet The right ventricular systolic pressure can be calculated from the peak velocity of tricuspid regurgitation using the modified Bernoulli equation and adding the right atrial pressure, as the regurgitant jet is driven by the pressure difference between the right ventricle and the right atrium (Fig 19.51) If no direct or indirect measurement is present, right atrial pressure is assumed between 5 and 10 mm Hg In the absence of right ventricular outflow tract obstruction, it can be assumed that right ventricular and pulmonary arterial systolic pressures are equal However, even in the presence of pulmonary stenosis, knowledge of the right ventricular pressure and the gradient across the right ventricular outflow tract will provide an estimate of pulmonary arterial systolic pressure ... For the application of the Bernoulli equation in the context of valvar stenosis, reference should be made to the individual chapters devoted to valvar abnormalities in pediatric cardiac disease This chapter focuses on the assessment of right ventricular systolic pressure and pulmonary arterial pressures
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