Renal Sympathetic Denervation Does Not Consistently Affect Renal Input Impedance

Abstract

The renal sympathetic nerves are important regulators of renal blood flow, and the investigation of renal blood flow markers of sympathetic innervation is important to the field of therapeutic renal denervation. Input impedance analysis is a classic method used to analyze the pressure-flow relationship of a vascular bed at the cardiac frequency and its harmonics, and renal sympathetic nerve activity shows robust cardiac cycle-related synchronization. Thus, we hypothesized that renal sympathetic denervation in two different animal models would result in consistent changes in renal input impedance, making it a potentially useful clinical marker of renal sympathetic innervation. Traditional ensemble-averaged impedance analysis (Figure 1) was performed on renal arterial pressure-flow data from ten conscious rabbits that underwent chronic, surgical renal denervation and nine anesthetized pigs that underwent acute, functional renal denervation via thoracic intrathecal administration of ropivacaine. A novel method in which input impedance was calculated for each beat of the recording, allowing for assessment of beat-to-beat impedance temporal variability, was also used. While acute functional renal denervation increased the impedance modulus in the swine model, no difference was observed in the highly controlled unilaterally denervated rabbit model (Figure 2). Renal denervation decreased beat-to-beat variability of the impedance modulus in both models of renal denervation. Renal denervation increased impedance phase shift variability in rabbits but not in swine. Renal sympathetic denervation does not consistently affect classic measures of input impedance. Beat-to-beat impedance modulus variability was greater in the innervated kidneys of both rabbits and pigs, highlighting the time-varying nature of renal sympathetic outflow which is not captured by traditional impedance analysis. Other pressure-flow analysis methods may be better suited for the identification of renal sympathetic innervation for clinical applications.