Estimating pulmonary artery systolic pressure (PASP)
Learn how to use point-of-care echocardiography to calculate an estimate of PASP.
Pulmonary hypertension is associated with increased mortality, but the gold standard method for diagnosing this—right heart catheterization—is invasive and not necessary for all patients. In this video, from our Point-of-Care Ultrasound Masterclass course, we'll show you how to use point-of-care echocardiography to calculate an estimate of PASP.
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[00:00:00] What causes elevated pulmonary artery pressure? Well, there are multiple reasons why the pulmonary artery pressure could be elevated, this include pulmonary arterial hypertension, thromboembolic disease, left-heart disease or lung disease. Pulmonary hypertension is associated with increased mortality regardless of the cause. The gold standard for determining is to perform
[00:00:30] a right-heart catheterization but this isn't practical or needed in all patients. You can make a good estimation of the pulmonary artery pressure using point-of-care echocardiography. This can help with understanding the hemodynamics of the critically ill or in the evaluation of a patient with dyspnea. We can do this by measuring the pulmonary artery systolic pressure and using this to approximate the mean pulmonary artery pressure. To do this, we first need to determine the pressure gradient
[00:01:00] that exists across the tricuspid valve. To determine the pulmonary artery pressure, we'll use the modified Bernoulli equation. We'll need to be able to calculate the velocity across the tricuspid valve and then convert this into the pressure gradient across the tricuspid valve. To be able to calculate the pulmonary artery systolic pressure, we'll then need to add the pressure contribution coming from the right atrium as well and this will give us the pulmonary artery systolic pressure.
[00:01:30] Our first step is to be able to identify tricuspid regurgitation. To do this, we will activate a color Doppler box seen here and place it over the tricuspid valve in an apical four- chamber view. We'll identify a regurgitant jet using color Doppler. You can see the regurgitant jet here, the blue signal coming back into the right atrium. Then we need to measure the peak velocity of this jet. To do this, we need to activate
[00:02:00] the continuous wave Doppler to measure the peak velocity of the regurgitant jet. We place the continuous wave Doppler at the site of the regurgitant jet and we'll obtain a tracing, such as you see here. The peaks that we see here correspond to regurgitant jets coming back across the tricuspid valve. We can then measure the velocity of the peak jet using the controls on the ultrasound machine. In this image, you can see the callipers are designating the peak velocity,
[00:02:30] which is then depicted at the top left of the image. The value, in this case, is 291 cm / s. We can then use this to calculate the pulmonary artery systolic pressure. We'll use the 291 cm / s, obtained in the prior tracing but need to change this value to m / s, so it's now 2.91. With this, we can calculate the pressure gradient across the tricuspid valve, it's 33.9. Now, we have
[00:03:00] the delta pressure. Next, we need to add the pressure contribution from the right atrium, to complete our calculation with the modified Bernoulli equation. We can estimate the right atrial pressure using the inferior vena cava or IVC. There is a standard table that allows us to predict the approximate right atrial pressure based on the IVC size and collapsibility. We'll obtain an image of the patient's IVC. In this case, we can see that it's dilated
[00:03:30] and it's poorly collapsible and then we'll freeze the image and obtain the measurement. It measures 2.18 cm. We can see then that this patient had poor collapse and an IVC of 2.18 cm, which puts the estimated right atrial pressure at 10 to 15 mmHg. Now, we can go ahead and calculate the pulmonary artery systolic pressure. We can either use 10 or 15 as the right
[00:04:00] atrial pressure estimate or an average between the two of 12.5. For simplicity, let's use 10 mmHg. We add this together and get 43.9. A normal pulmonary artery systolic pressure is less than 25 mmHg. Between 40 and 50 mmHg is considered a mild elevation. Anything greater than 50 mmHg is considered moderate elevation, and greater than 60 is considered severe.
[00:04:30] In our example, the pressure was 43.9 mmHg or could be as high as 48.9, if we use the higher estimate for the right atrial pressure. This corresponds to a mild elevation of the pulmonary artery systolic pressure. There are two things to keep in mind. One, this approach may overestimate the pulmonary artery systolic pressure somewhat. And two, that we've just calculated the systolic pressure and not the mean pressure through the cardiac cycle. That's the traditional approach to measuring
[00:05:00] the pulmonary artery systolic pressure, but there's a couple of potential problems. IVC measurements have the potential for error. One potential problem is if you measure through the wall. This can produce a falsely low IVC measurement. On the other hand, if you measure on the tangent to the IVC, you may have a falsely elevated value for your IVC measurement. And as we saw, this approach to estimating right atrial pressure produced a range of possible values. So, are there
[00:05:30] other ways to estimate pulmonary artery systolic pressure? One approach is to estimate it using a measurement of the tricuspid regurgitation itself. We can do this by measuring the maximum velocity jet of the tricuspid regurgitation, using continuous wave Doppler. Then, there are ranges of values that we can use to predict the likelihood of a correspondingly elevated pulmonary artery systolic pressure. A low probability value is anything less than 2.8 m / s.
[00:06:00] Intermediate probability ranges between 2.8 and 3.4 m / s, and high probability is anything greater than 3.4 m / s. Here's the continuous wave Doppler tracing of another patient. The calculated velocity of the jet here is 273.8 cm / s or 2.7 m / s. This places the patient in the low probability range. Now we know two different ways to calculate pulmonary artery
[00:06:30] systolic pressure and we can use this in evaluating patients who have dyspnea, if we suspect it might be related to elevated pulmonary artery pressures or we can use this to better understand hemodynamics in the critically ill. Give both approaches a try and see which one works best for you.