Doppler Ultrasonography in Posttransplant Renal Artery Stenosis

THE VALUE OF DOPPLER ULTRASONOGRAPHIC PARAMETERS FOR THE ESTIMATION OF HEMODYNAMICS IN RENAL ALLOGRAFTS Transplant renal artery stenosis is an important hemodynamic aspect for deterioration of graft function. This vascular problem is attributable to scarring or narrowing of the graft artery, intimal hyperplasia because of immune injury at the site of anastomosis, size mismatch between donor and recipient, inadequate suturing technique, reaction to suture material, or atherosclerosis in elderly recipients. The frequency of graft renal artery stenosis ranges between 2% and 15%, with a higher incidence in pediatric transplant patients (1). As a consequence of hypoperfusioninduced angiotensin II formation and water and sodium retention, arterial hypertension occurs. The latter represents a serious clinical problem in transplant patients because it is not only a risk factor for long-term graft survival but also accelerates cardiovascular disease in the recipient, with a negative impact on patient survival. Timely treatment of transplant renal artery stenosis is therefore essential. To reach this goal, rapid and reliable diagnosis is necessary. In this context, Doppler ultrasonography represents a powerful diagnostic tool, because the transplanted kidney in the iliac fossa and its vasculature can be readily explored by ultrasound, significantly more so than orthotopic kidneys. Especially the correction for the angle of insonation alongside the axis of the renal artery, which determines exact flow velocity estimation, is easier to perform in transplanted kidneys. Doppler ultrasonographic parameters indicating an artery stenosis are increased peak systolic velocity (PSV) at the stenotic site in the main vessel and decreased resistive index (RI) in poststenotic, intrarenal interlobar arteries. A PSV of greater than 200 cm/ sec can be considered an indicator for a hemodynamically relevant (>60%) stenosis. A normal RI in a healthy kidney should be approximately 0.6. There are numerous studies that show increasing skepticism regarding the value of an increased RI for the diagnosis of rejection in transplanted kidneys because it seems to be a rather nonspecific phenomenon (2). However, a decreased RI (tardusparvus waveform phenomenon) should at least initiate an intensive Doppler ultrasonographic reevaluation of the anastomotic region and the mainstem renal artery (2). In transplanted kidneys, the renal-aortic ratio of PSV cannot be used as a classic parameter for renal artery stenosis detection (3). The study of Bruno et al. presents for the first time comparative data regarding pre- and postinterventional measurements of Doppler ultrasonographic and renal function parameters during the treatment of transplant renal artery stenosis. Renal function parameters were measured by means of classic clearance methods, and there is no doubt about the reliability of these data with regard to glomerular filtration rate (GFR), renal plasma flow (RPF), and renal vascular resistance (RVR). As systolic arterial pressure decreases after angioplasty and stenting, GFR remained unchanged, whereas RPF increased and RVR decreased. The decrease in RVR is most probably because of the decrease of angiotensin II formation after successful restoration of intrarenal blood flow with consequent vasodilatation at the level of the efferent arteriole. Interestingly, the Doppler ultrasonographic parameter RI did not at all show a linear relationship with RVR, as is always stated in former ultrasonographic studies (2). Even in vitro experiments show a direct dependence of RI and RVR, although it seems clear now that other factors such as vascular compliance significantly influence this relationship (2). In the literature, there is no further information on the finding of an inverse relationship between RI and RVR after successful restoration of renal blood flow in a transplanted kidney. For native kidneys, these comparative measurements are not feasible because differential clearances in the presence of two kidneys are usually difficult to obtain. From the pathophysiologic point of view, the reninangiotensin system seems to play a key role in the setting of a transplanted kidney with arterial stenosis. This one-kidney, one-clip situation is known to excessively stimulate renin secretion followed by increased angiotensin II formation. Angiotensin II constricts the efferent arteriole to maintain glomerular perfusion pressure under reduced RPF. This explains increased RVR. As a consequence, peritubular perfusion and medullar perfusion are reduced. In combination with reduced RPF, this might be an explanation for decreased RI. As RPF is restored, angiotensin II is reduced, RVR drops, and RI will increase with increased filling of the poststenotic vasculature and restoration of intrarenal blood flow. Besides that, the postinterventional decrease of vascular wall shear stress and renal blood velocity gives no problems for interpretation. The purpose of the study was to show a correlation of Doppler ultrasonographic parameters and clearance parameters during treatment of transplant renal artery stenosis. It can be stated that restoration of RPF and reduction of RVR seem to be noninvasively detectable in this setting by means of Doppler ultrasonography. However, this is only true if comparative measurements before and after intervention are performed to determine changes in the same individual. The absolute RI value gives no significant information if measured at a single time point, because it might be influenced by various other parameters in the transplanted kidney (2). Furthermore, it should be clearly pointed out that Doppler ultrasonography is extremely investigator dependent. In the cited study, only one investigator performed all the measurements, which excludes a serious source of variation. The Materials and Methods section of the article explicitly shows that many methodologic ultrasonographic parameters need to be carefully considered to obtain reliable and reproducible results. Therefore, this noninvasive technique must not be underestimated as an easy technique for general practice. There is one further concern with this study. All measurements were performed during a cyclosporine A-free time period. From the methodologic point of view, this was absolutely necessary to exclude intrarenal vasoconstrictor effects of calcineurin inhibitors, which affect renal hemodynamic parameters. However, discontinuation of any immunosuppressant carries a risk for graft rejection, and it is questionable whether this procedure should be recommended for Doppler ultrasonographic evaluations in general. The RI values might be influenced by effects of calcineurin inhibitors on the intrarenal vasculature, and it remains unclear whether the described correlation between RI and RVR remains reproducible at the same level of significance under continuous immunosuppressant treatment.