Veterinary Echocardiography Newsletter 12: Estimating Pulmonary Pressures

 

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Estimating Pulmonary Pressures

 

Pulmonary hypertension is often overlooked in veterinary echocardiography. Yet, it is a complication of a wide variety of conditions, including:

  • Mitral regurgitation
  • Heartworm
  • Respiratory disorders

Estimating pulmonary artery systolic pressure is a relatively straightforward procedure with a decent Colour Doppler ultrasound machine, but is infrequently performed.

 

What is pulmonary artery systolic pressure?

In the absence of pulmonary stenosis or right ventricular outflow tract obstruction, pulmonary artery systolic pressure (PASP) = right ventricular systolic pressure (RVSP). Ruling out pulmonary stenosis or RVOT obstruction is quick and easy – if you need a reminder on how to do this, just click here.

PASP = RVSP because, during systole, the pulmonary valve opens and the pressure in the right ventricle is equal to the pressure in the pulmonary artery. There is no pressure gradient between them; with the valve leaflets open, it’s as if they are a single chamber.

The tricuspid valve, on the other hand, opens during diastole to allow blood to flow in from the right atrium into the right ventricle. During diastole when the tricuspid valve is open, the right ventricle and right atrium are like a single chamber; but when the tricuspid valve is closed during systole, the pressure in the right ventricle is – with very few exceptions – higher than in the right atrium.

Wherever there is incomplete closure of the tricuspid valve leaflets, some blood will always be forced from the higher pressure right ventricle backwards into the lower pressure right atrium. The greater the pressure difference between the two chambers, the higher the velocity of the regurgitating blood. For this reason, if you know (or can reasonably estimate) the pressure of the right atrium, measuring the velocity of the tricuspid regurgitant jet can tell you the pressure in the right ventricle.

 

How to estimate pulmonary pressures

Tricuspid regurgitation is very common, and even animals without significant cardiac disease are likely to have a small jet. In animals with pulmonary hypertension, dilatation of the right ventricle in response to elevated pressures stretches the tricuspid annulus. This means that the tricuspid valve leaflets do not meet as well upon closure, and often results in regurgitation of mild or greater severity. This is fortuitous for the echocardiographer: it means that in the very patient group that you most need an estimation of pressures, you will invariably be presented with an easy means of obtaining it.

Annular dilatation

Above: Left: Normal sized annulus with good leaflet coaptation. Right: Dilated annulus which pulls the leaflets apart and prevents them from creating a tight seal during ventricular systole.

 

Pulmonary artery systolic pressure can be estimated quickly and easily from the tricuspid regurgitant jet using Continuous Wave (CW) Doppler. All phased array probes should give you CW Doppler capability, but it is usually absent when using a microconvex probe.

An ultrasound machine with a reasonable colour Doppler frame rate and good sensitivity makes obtaining a trace so much easier. Colour Doppler allows you to see the regurgitant jet clearly, and therefore align your Continuous Wave beam with it, maximising your chances to obtaining a clear trace. It is best to take measurements from a number of different views, and even some modified ones (a foreshortened apical view is demonstrated in the video below), in order to find the best trace you can. The demonstration video is taken on a normal patient, but remember that in patients with pulmonary hypertension, there is likely to be significant tricuspid regurgitation, making your job a lot easier!

 

 

In canine echocardiography, right atrial pressure is subjectively estimated based primarily on the size of the right atrium. Kittleson and Kienle (2014) suggest adding 5mmHg if the right atrium appears normal in size, 10mmHg if it appears enlarged but without any other signs of right sided heart failure, and 15mmHg if it’s enlarged and accompanied by signs of right sided heart failure. This is then added to the pressure gradient obtained from the CW Doppler trace through the tricuspid regurgitant jet. However, it may be easier to simply be aware that anything over 3m/s warrants further investigation, with the implications of such findings dependent upon the aetiology and on the patient’s symptoms.

 

A word of caution

It is important to remember that the peak CW Doppler velocity cannot be interpreted in isolation. Occasionally, no regurgitation may be present. In addition, where tricuspid regurgitation is very severe, this method may underestimate pulmonary pressures. In such a situation, right atrial pressure is likely to also be so severely elevated that the pressure gradient between the two chambers will be lower. Always look at the overall picture and interpret in context. Is the right ventricle dilated? Is there septal flattening? Is the pulmonary artery dilated? These are strong indicators of pulmonary hypertension, too. You can view more examples here.

 

References

Borgarelli M, Abbot J, Braz-Ruivo L, et al. Prevalence and prognostic significance of pulmonary hypertension in dogs with myxomatous mitral valve degeneration. J Vet Intern Med 2015;29:569–574.

Kittleson M., Kienle R. (2014). Pulmonary arterial and systemic arterial hypertension. Small Animal Cardiovascular Medicine. 433–449.