Case Studies In Small Animal

Cardiovascular Medicine

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Case 12

Continuous Wave Doppler Tracings

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This is a continuous wave Doppler tracing of the tricuspid regurgitation jet. Normally the peak systolic pressure in the right ventricle is 20 to 25 mm Hg and the right atrial pressure is less than 5 mm Hg. To make it easy, let's assume the peak systolic right ventricular pressure in a normal animal is 20 mm Hg and right atrial pressure is 4 mm Hg. This means the pressure gradient (i.e., difference) is 16 mm Hg across the tricuspid valve. Of course in a normal animal, there is no tricuspid regurgitation of note although in many there is a trace amount. Using the modified Bernoulli equation, we can calculate the velocity of a tricuspid regurgitation jet by dividing the pressure gradient by 4 and then taking the square root of that value. In this situation, 16 divided by 4 is 4 and the square root of 4 is 2. This means that if a normal animal has some tricuspid regurgitation, the velocity of blood flow back across the tricuspid valve should be 2 meters/second (normal is up to around 3.2 meters/second). Working the other way in this dog, if the velocity is 4.5 meters/second the pressure gradient must be 81 mm Hg. We don't actually know what the right atrial pressure is but since the dog appears to be in right heart failure, it must be in the 10 to 15 mm Hg range.

 

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Similarly, we can measure the velocity of the pulmonic regurgitation jet and estimate the diastolic pressure in the pulmonary arteries. In the picture above we have done this and measured the velocity of the jet in early diastole to be about 3.9 meters/second. This translates into a pressure gradient across the pulmonic valve of 60 mm Hg. In early diastole, the right ventricular pressure is usually zero so this means the pulmonary artery early diastolic pressure must be the same as the gradient or 60 mm Hg. In late diastole you can see that the velocity decreases. It does this as the pulmonary artery pressure in diastole decreases and as the right ventricular diastolic pressure increases. The velocity at the end of diastole on the middle tracing comes down to about 3 meters/second. This translates into a pressure gradient of 36 mm Hg. If we assume that right ventricular end-diastolic pressure is 14 mm Hg, this means the diastolic pressure in the pulmonary arteries is approximately 50 mm Hg. We have now approximated the pulmonary artery pressure to be 100 mm Hg in systole and 50 mm Hg in diastole. This is assuming that we were able to line up our continuous wave Doppler beam parallel with both jets. If we didn't, the velocity would be less than normal. Remember that it is always easy to underestimate the velocity of blood flow using Doppler ultrasound. It can never be overestimated.

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Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.