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by the American Institute of Ultrasound in Medicine J Ultrasound Med 23:1684-1685 • 0278-4297
Doppler Sonography of Renal Obstruction1 Department of Nuclear Medicine and Ultrasound, Hôpital Bretonneau, Tours, France To the Editor: I read with interest the article entitled "Doppler Sonography of Renal Obstruction: Value of Venous Impedance Index Measurements," by Oktar et al.1 This article highlights the consequences of obstruction on renal hemodynamics. This is of special interest because some articles have described resistance index changes after increased intrarenal pressure. However, the article elicited some questions: The first concerns the concept of impedance applied to veins. To my knowledge, there is no basis to use this concept for veins. Veins and arteries are totally different in terms of vessel walls and properties. Venous flow is almost monotonous without systolic and diastolic components. The final point is that the greatest resistance changes in vessels are related to arteriolar changes and therefore cannot explain venous flow changes, as reported in the article. The changes in flow modulation reported can be explained only by a lower transmission of pulsatility. The second question concerns the figures. The authors claimed an increased venous impedance index value in Figures 2 and 3. Although we could identify a signal rupture on venous flow (less evident in Figure 2), it remains hard to attribute this signal change to increased "resistance." The first hypothesis is the presence of artifacts during recordings. It also could be related to changes in downstream flow, such as in the inferior vena cava or right heart chamber. It would be helpful to visualize the recorded curve from the other kidney. Finally, from my point of view, this article includes some unsolved questions and supports a nonvalidated concept of venous impedance. I would be interested in obtaining some comments from the authors.
Reply2 Department of Radiology, Gazi University School of Medicine, Ankara, Turkey To the Editor: I thank Dr Tranquart for his interest and comments regarding our article. In our study,1 we hypothesized that the reduced compliance in acute renal obstruction results in dampening of renal venous signals when compared with normal or chronic obstruction cases. Considering the controversy in the literature about the use of the arterial resistive index, Bateman and Cuganesan2 previously focused on the venous side of the renal vascular tree for the diagnosis of acute obstruction. Actually, the term "resistive index" is a misnomer, as reported earlier, because the Doppler waveform is altered by the interaction of both vascular resistance and compliance, not by vascular resistance alone. Therefore, it should be called an "impedance index."3 Both vascular resistance and compliance are affected by the same pathologic process causing diagnostic difficulties in using the arterial resistive index, as in the case of renal obstruction. However, on the venous side, the normal resistance is negligible, and the pulsatility directly relates to the compliance.2 Measuring the pulsatility of the hepatic venous signals has been suggested as a method of identifying liver diseases because of the changes in compliance that are produced.4 Similar mechanisms may be in charge in the kidney. In acute urinary system obstruction, pressure within the collecting system increases substantially, causing an increase in interstitial pressure. As a result, a rapid reduction in renal parenchymal compliance and an increase in vascular resistance would be expected to occur.2 Variations in the right-sided atrial pressure due to the cardiac cycle produce a triphasic waveform in the inferior vena cava. The reversal of the flow at end diastole from atrial contraction is propagated into the renal vessels. Increased renal interstitial pressure, such as the case in acute obstruction, reduces compliance of the intrarenal veins; as a result, the end-diastolic flow reduction is reduced.2,3 In a recent experimental study by Sohn et al,5 the interlobar veins in both kidneys of rabbits were checked after unilateral ligation of the ureter. It was observed that the impedance index, measured as (peak flow signal – least flow signal)/peak flow signal, of the intrarenal vein was significantly decreased in the obstructed kidneys compared with nonobstructed kidneys. It was concluded that measurement of the impedance index of the intrarenal vein on Doppler sonography could provide a useful method for diagnosing obstructive uropathy.5
Regarding the chronic obstruction cases, in our study, no statistically significant differences could be found between the venous impedance indices of obstructed and unobstructed sides; even the former seemed to be higher (Figure 1
Dr Tranquart is correct in saying that veins and arteries are totally different in vessel walls and properties. However, the impedance index changes in venous flow detected in our study are thought to occur because of compliance changes in renal parenchyma or, "can be explained only by a lower transmission of pulsatility," as he states, not because of wall properties. This change in parenchymal compliance is also suggested to affect the impedance index at the arterial side, in addition to other complex mechanisms. In conclusion, we think that venous impedance index values may be valuable in differentiating acute obstruction cases from long-standing ones when used in conjunction with arterial impedance index values. We do not claim that this measurement can be used exclusively in the evaluation of renal obstruction; however, it may be helpful in assessing and understanding renal hemodynamics in cases of obstruction.
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