Prosthetic valves often cause some degree of obstruction to the blood flow in comparison with native heart valves. How can we evaluate the valve obstruction with echo? What’s the role of the Bernoulli equation? And what’s the cutoff value for the blood flow peak velocity? Find out in this lesson presented by Samir Sulemane, a Senior Clinical Scientist and echocardiography expert at Royal Brompton and Harefield NHS Trust, UK.
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Echocardiography of prosthetic heart valves is much more demanding, both to perform and to interpret, compared with the assessment of native heart valves. Learn how to use tools such as 2D, 3D, and transesophageal echocardiography in order to become a skilled decision-maker who can confidently talk to any cardiologist or cardiac surgeon about your patient’s prosthetic valve problem.
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The hemodynamic performance of most prosthetic heart valves is inferior to that of native valves. This is because prosthetic heart valves often cause some degree of obstruction to blood flow, which varies depending on the prosthesis and the size of the patient. The parameters used to evaluate prosthetic heart valve function include the peak velocity, pressure gradient, effective orifice area, and Doppler velocity index. In this Medmastery lesson, I will focus on the first two and you will learn how to use these parameters to determine if your patient's valve is obstructed.
These measurements are based on the simplified Bernoulli equation, which describes the relationship between velocity and pressure. Typically, the instantaneous pressure gradients across the valve, indicated by P, is related to the instantaneous blood flow velocity through the prosthetic valve indicated by v. The peak velocity is obtained using continuous wave Doppler across the prosthetic valve. To do this, the ultrasound beam should be positioned parallel to the blood flow with no significant angle.
This is to ensure that the blood flow velocity is not over or underestimated. Different valve types have different ranges of velocities and gradients. Compared to the 27 millimeter bio-prosthetic valve, a 21 millimeter mechanical valve in the aortic position will have higher velocities and gradients. So it is very important to have all the information related to the model, size, and type of valve before performing the echocardiogram. The hemodynamic profiles and reference values for the different prosthetic valves can be found in the handbook. Let's go through some examples of how to calculate transprosthetic velocities and gradients.
The apical 4-chamber view is the best view to measure the peak velocity across the prosthetic mitral valve, because the ultrasound beam is parallel, or at very small angle to the blood flow. Next, trace the Doppler profile to determine the peak velocity and mean pressure gradient. Here, the peak velocity is 2.3 meters per second, and the mean pressure gradient is 7.62 millimeters of mercury. Generally, a peak velocity less or equal than three meters per second is considered normal.
To check whether the pressure gradient is within the normal range, the next step is to determine the model and the size of the prosthesis and check the reference values for this specific valve. This is a 25 millimeter stented bio-prosthesis, and the reference mean gradient for this specific valve is 3 millimeters of mercury. Therefore, a mean gradient of 7.62 suggests increased flow and a degree of obstruction. In this case, you should report these findings to the cardiologist that follows the patient, and the patient should be monitored more frequently.
For a prosthetic valve in the aortic position, the apical 5-chamber view is the most widely used as the ultrasound beam is parallel to the blood flow across the valve. So again, we trace the Doppler profile to assess the flow and pressure gradients across the valve. For this patient, the peak velocity is 4.13 meters per second. With a mean pressure gradient of 39.4 millimeters of mercury. The peak velocity is more than 3 meters per second, and based on the reference mean pressure gradient, these numbers suggest severely raised gradients across the valve. In this case, other valve parameters should be evaluated to confirm the significant stenosis, which will be covered in the next lesson.