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

To the Editor: To measure renal blood flow, Garwood1 suggested using the segmental or other intrarenal vessels resistive index (RI) and pulsatility index using transesophageal echocardiography (TEE) to avoid the error caused by the cosine of the angle between blood flow and the Doppler beam. In support of his suggestion, Garwood2 cited a figure (Fig. 1, reproduced from J Cardiothorac Vasc Anesth 2001;15: 65–71) to demonstrate measurement of the RI and pulsatility index from a branch renal artery. We believe that there is an error in this cited figure.Figure 1.: Two-dimensional view of the renal arteries obtained with transesophageal echocardiography (TEE) showing that the main renal artery supplies ventral and dorsal branches (A). The branch divided into segmental renal arteries; the low-segmental renal arterial blood flow backward the TEE probe transducer (B). The pulsed-wave Doppler cursor placed in the low-segmental artery and the corresponding spectral waveforms showing that the orientation was away from the baseline (C).Using TEE, the main renal artery supplies the ventral and dorsal branches, which subsequently branch into segmental renal arteries (Fig. 1, A and B). The segmental arteries supply interlobar arteries that divide at the corticomedullary junction into arcuate arteries. These, in turn, supply interlobular arteries within the cortex.3 The orientation of low-segmental or interlobar renal arterial blood flow should be away from the TEE probe transducer (from hilum to cortex), and, accordingly, the corresponding spectral Doppler waveform should be below the baseline (Fig. 1, B and C). In the figure cited by Garwood,1 the Doppler cursor was placed in the low-interlobar renal artery. However, the corresponding spectral Doppler waveform was above the baseline, meaning that blood flow was toward the transducer. Second, RI might not accurately reflect renal resistance and renal blood flow when the patient has aortic valvular insufficiency. In the presence of severe aortic valve insufficiency, the renal artery diastolic velocity will be nearly 0. Thus, the RI (Vmax − Vmin/Vmax) will be approximately 1, suggesting that renal blood flow has a very high resistance, leading possibly to an erroneous conclusion that the cause is a constricted or severely stenosed renal vessel and reduced renal blood flow and possibly renal dysfunction. Thus, in the presence of significant aortic insufficiency, RI might not accurately reflect renal flow resistance, resulting in an inaccurate impression of reduced renal blood flow from increased renal vascular resistance. Ping-Liang Yang, MM Department of Anesthesiology West China Second University Hospital, Sichuan University Chengdu, Sichuan People’s Republic of China [email protected] David T. Wong, MD Department of Anesthesia Toronto Western Hospital University of Toronto, Toronto Ontario, Canada

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