Input Impedance of Revascularized Skeletal Muscle, Renal, and Mesenteric Vascular Beds

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Abstract
Input impedance describes the relationship between pressure and flow in a vascular system and, hence, characterizes the outflow bed. The purpose of this investigation was to measure input impedance spectra in vascular reconstructions of skeletal muscle, renal, and mesenteric beds. Input impedance was measured in 107 vascular reconstructions in 96 patients. Reconstructions were performed at the aortofemoral/aortoiliac (AF, n = 20), femoropopliteal (FP, n=18), femorodistal (FD, n=41), infrapopliteal-inframalleolar (IM, n = 6), renal (REN, n = 16), or mesenteric (MES, n= 6) level. Grafts were constructed from autologous vein in all cases except AF bypasses in which bifurcated woven Dacron grafts were employed. Input impedance was measured intraoperatively after reperfusion. For impedance calculation, simultaneously acquired intraluminal pressure (transducer-tipped pressure catheter) and blood flow (electromagnetic probe) waveforms of ten-second duration were digitized at 200 Hz and subjected to Fourier transformation in near real-time. AF grafts exhibited the highest blood flow (443 ± 72.8 mL/minute) followed by MES (300 ± 30.4), REN (172 ± 43.9), FP (91.6 ± 20.0), FD (59.3 ± 5.09), and IM grafts (22.4 ± 5.44 mL/minute). A similar (inverse) trend was observed with respect to resistance (Rin), ie, MES≈AF3 dyne•s•cm-5). As expected, AF grafts exhibited the lowest characteristic impedance (Z0 3.5 ± 0.8 x 103 dyne•s•cm-5). However, the Z0 of REN (20.2 ± 3.7 x 103 dyne•s•cm-5) grafts exceeded that of FP and MES grafts (10.5 ± 1.1 and 12.4 ± 4.0 x103 dyne•s•cm-5) and more closely approximated that of FD (22.2 ± 2.2 x 103 dyne•s•cm-5). The highest Z0 was seen in IM grafts (42.1 ± 15.8 x 103 dyne•s•cm-5). Therefore, the highest ratios of Z0/Rin were observed with MES (0.75 ± 0.23) and REN (0.33 ± 0.04) grafts as compared with the other groups (AF 0.23 ± 0.03, FP 0.12 ± 0.13, FD 0.19 ± 0.02, IM 0.16 ± 0.06). As expected, Rin of vascular reconstructions follows the general trend of MES ≈ AF < REN < < FP < FD < < IM. However, examination of the high-frequency components of the impedance spectra reveals that Z0 follows a different pattern, AF < FP MES < FD ≈ REN < < IM and the ratio of ZO/Rin is highest in REN and MES vascular beds compared with skeletal muscle beds. Thus, although the REN and MES beds are “privileged” (ie, maximally dilatated with low arteriolar tone), the intrinsic properties of the graft and larger blood vessels are no different than those of a femorodistal bypass.