Regarding “Overestimation of a stenosisin the internal carotid artery by duplex sonography caused by an increasein volume flow”

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• Regarding “Overestimation of a stenosis in the internal carotid artery by duplex sonography caused by an increase in volume flow”
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Regarding “Overestimation of a stenosis in the internal carotid artery by duplex sonography caused by an increase in volume flow” 

To the Editors:

I read with interest the article by van Everdingen et al (1998;27:479-85). The purpose of the study was to show that increased volume flow, as shown by means of magnetic resonance angiography, is associated with higher blood flow velocities and overestimation of the severity of a carotid stenosis in cases of significant contralateral carotid obstruction.

This article lacks some essential information about the basic physics of blood-flow analysis with duplex scanning that is important in understanding the reason for overestimation of the degree of stenosis in cases of contralateral severe stenosis or occlusion. The problem of overestimation is already known for a longer time, as the authors also state in the introduction. The amount of flow (Q) is simply the product of the velocity (v) and the luminal cross-sectional area (A) or Q = vA. Because the cross-sectional area in the ipsilateral carotid artery does not change in cases of contralateral obstruction, the only explanation for the overestimation is an increase of the velocity as a result of increased flow. On the basis of these principles, the authors could already have anticipated the outcome of this study.

In the discussion, the authors suggest that the use of velocity ratios appears to be helpful to correct for the increase in volume flow. This is correct and is also on the basis of some simple physics. With ratios, the velocity in the internal carotid artery at the site of the stenosis (V _sten) is compared with flow distally in the internal carotid artery (V_dist). This ratio can also be written as:

In vessels without side branches (eg, the extracranial part of the internal caortid artery), Q at one point of the vessel is the same as at any other point. By canceling Q, it is clear that the ratio is flow independent and directly related to the changes in the cross-sectional area.¹, ²

Furthermore, it would be interesting if those investigators who already performed comparative studies with angiography and duplex scanning in patients with contralateral obstruction would reanalyze their data and calculate likelihood ratios for several cut-off levels of absolute velocities and velocity ratios. Such an analysis would probably help to obtain a better insight as to which patients should be recommended for angiography and which patients can safely be treated solely on the results of duplex scanning.

24/41/96142

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References 

1. Legemate DA, Teeuwen C, Hoeneveld H, Ackerstaff RGA, Eikelboom BC. Spectral analysis criteria in duplex scanning of aortoiliac and femoropopliteal arterial disease. Ultrasound Med Biol. 1991;17:769–776
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2. Burns PN. Hemodynamics. In:  Taylor KJW,  Burns PN,  Wells PNT editor. Clinical applications of Doppler ultrasound. New York: Raven Press; 1988;p. 56–57
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3. In: 2nd ed.  Sackett DL,  Haynes RB,  Guyatt GH,  Tugwell P editor. Clinical epidemiology. A basic science for clinical medicine. Boston: Little, Brown and Company; 1991;p. 69–152
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