■ The Bernoulli equation should also be used with care in the setting of ventricular systolic dysfunction.. Mean Pressure Gradient The pressure gradient derived from the Bernoulli equati
Trang 1■ 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
Trang 2higher 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 peak-to-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.
Trang 3Application 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 four-chamber 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