So, for all practical purposes nowadays we take the criteria of 180 centimeters per second that is if you find any turbulence on a color Doppler in the main renal arterial segment then you put a spectral Doppler on that particular area and you will get the velocities of 180 centimeters per second or more. Then there is another criteria that is renal to aortic ratio. So why is there a need for renal? artery to aorta ratio.
Normally whenever we are scanning any peripheral arteries or say carotid arteries we usually take the proximal to stenotic area PSV and then the stenotic area PSV. But in renal arteries as we saw most of the times especially in elderly patients the renal artery stenosis is at the origin. So there is no proximal segment what of the renal artery where you can compare that flow velocity.
And that is why we take the aorta to the stenotic segment of the renal artery PSV ratio. Sometimes in elderly patients because of the heart disease the overall cardiac output is low and then you will start seeing that the aorta is itself pumping blood slowly and in the aorta the PSV is something like 35 or 40 centimeters per second. And now in such patients if you get renal artery PSV. at the area of stenosis as 140 centimeters per second which is almost 3 and half times more than the aorta but it is not fitting into your criteria of 180 centimeters per second.
Then will you call it a stenosis or not? You should call it as a stenosis because proximal which is aorta and then at origin of the renal artery if the velocity is more than 3.5 times then you can still call it as a stenosis. a renal artery stenosis. That is why there is a criteria of renal to aortic ratio. So, renal artery PSV is coming 3.5 times more than the aorta then also you can infer that it is a renal artery stenosis.