Resistive Index May Not Accurately Reflect Renal Flow Resistance in the Presence of Significant Aortic Insufficiency

In Response: What Yang and Wong1 describe is, of course, correct: the segmental artery depicted will demonstrate flow away from the transducer as blood flows from the medulla to the cortex and will be shown in the spectral display below the baseline. However, in the figure in the editorial reproduced from the previous article by Garwood et al.,2 the resolution of the 2-dimensional image is not sufficient to determine exactly where the sampling volume of the Doppler interrogating line is located. It appears to be approximately in the center of the kidney as displayed so that flow from the medulla to the cortex could be either above or below the baseline. Their point is well taken; however, either better care could have been exercised in choosing a more obvious illustrative image or more attention given to the language used in the editorial.3 If the wording had been “a branch of the renal artery” rather than “a branch artery” (implying a specific artery, either the ventral or dorsal branch artery), this would have been a moot point. With respect to severe aortic insufficiency (AI) and renal blood flow, Yang and Wong do bring up an important point. As with all Doppler volumetric measurements, significant regurgitant flow will cause gross errors in any hemodynamic Doppler calculation (other than the regurgitant flow itself or respective orifice). Hence, hemodynamic calculations are not recommended from Doppler equations when there is significant regurgitation. This is one of the basic rules of Doppler echocardiography. However, the point is very interesting: what does happen to blood flow both within the renal artery and subdivisions thereof when there is severe enough AI to cause retrograde diastolic flow in the aorta? Is there retrograde flow at this level or does the flow approach 0 as stated by Yang and Wong? Salgado et al.,4 reporting on increased intrarenal arterial impedance in a normal-functioning transplanted kidney, state that other workers have similarly reported significantly increased intrarenal indices in the native kidneys of patients with AI but the 2 references given do not investigate this phenomenon (one is a dog model of external pressure on the kidney with 2 clinical examples and the other is an in vitro model using electrical circuitry analogy). Krumme et al.5 noted that indices in transplanted kidneys are directly related to the pulse pressures of the recipient. Clearly, this interesting question should be studied. Susan Garwood, MB, ChB Department of Anesthesiology Yale University School of Medicine New Haven, Connecticut [email protected]