Longitudinal impedance is independent of outflow resistance

Background. Many investigators have measured outflow resistance (R) following peripheral bypass procedures, but correlations with graft patency have been weak. This is because the primary determinants of graft patency are the size and quality of the conduit, not its outflow bed. Efforts at separating conduit resistance from outflow resistance have been unsuccessful. Recently, the concept of longitudinal impedance (∫Z_L) has been suggested as a measure of conduit resistance independent of outflow resistance. The purpose of this in vitro experiment was to test the hypothesis that ∫Z_L is independent of R within physiologically relevant ranges. Methods. Rigid polyethylene tubing of known internal diameter and length (4.3 mm, 375 cm) was perfused with a glycerin/saline mixture mimicking the viscosity of blood (4.1 cp), utilizing a variable pulsatile pump and Windkessel, with outflow into multiply branched tubes of decreasing diameter simulating the hemodynamic conditions of arterial bypass. Flow and pressure were measured using ultrasonic transit time and catheter transduction, respectively, and waveforms digitized at 200 Hz. Flow was varied while maintaining "systemic" pressure and resistance. After Fourier transformation, ∫Z_L was calculated as ΔP/Q at each harmonic and integrated over 4 Hz. Results. ∫Z_L calculations were remarkably reproducible within the same day with a coefficient of variation (CV) = 4.0% (at 100 dyne · s/cm⁵; n = 4) or over 4 successive days (CV = 4.3%). Furthermore, ∫Z_L was largely independent of R over the physiologic range tested, with ∫Z_L remaining relatively constant as R was increased sixfold. Conclusion. ∫Z_L is a consistent and reproducible measure of conduit resistance independent of R over a wide physiologic range. It may be useful for measuring the adequacy of bypass graft conduits.