Comparison of the Resistive Properties of Reversed and Nonreversed Saphenous Veins at Arterial Pressure and Flow: Implications for Optimal Graft Configuration

Abstract

Clinicians continue to debate the hemodynamic advantages of reversed vs nonreversed vein grafts in infrainguinal arterial reconstructions. Vein grafts placed in the reversed configuration do not require valve lysis but have the theoretical drawback of being smaller in caliber at the inflow end than the outflow end. The purpose of this study was to objectively determine the effect of vein valve lysis and flow direction on vein graft hemodynamics by using physiologic levels of pulsatile flow (Q), pressure gradient (AP), outflow resistance (Ro), and longitudinal impedance (ZL).

Nine cryopreserved human greater saphenous veins (length=23 ±1 cm) were perfused via an in vitro circuit utilizing a variable pulsatile perfusion pump, Windkessel, and clamp resistor. Levels of Q and AP were chosen to simulate the known physiologic conditions of infrainguinal bypass grafting while holding Reynolds numbers <2,400. Veins were studied in the reversed configuration prior to valve lysis, after valve lysis by use of a catheter-directed valvulotome with 3 mm cutting head, and in the nonreversed configuration. Ultrasonic transit-time flow and proximal and distal intraluminal pressure were continuously recorded while Ro and pump rate were varied. Waveforms were digitally stored at 200 Hz at pump rates of 60, 100, 140, and 180 beats per minute at a Q of 154 ± 1 and 253 ± 1 mL/min while Pprox was maintained at 100 mmHg. Veins were perfusion fixed at 100 mmHg, sectioned at 2 cm intervals, and analyzed morphometrically. Inner diameter (id) and outer diameter (od) were determined by light microscopy after correction for shrinkage artifact by comparing to outer diameter measured by digital calipers at 2 cm intervals at the time of perfusion. Percent vein taper was calculated as (idmax7idmin)/idmax and wall thickness (t) as (od-id)/2. After Fourier transformation, ZL was calculated as AP/Q at each harmonic and the curves compared by use of Wilcoxon signed-ranks test.

There were one to four valves per vein (idmean=³.⁶ ±0.3 mm; range 2.3-6.2) with an average taper of -8 + 12% (range: -46-36%) and mean wall thickness of 0.51 ±0.03 mm (range 0.42-0.68 mm). ZL curves were smooth and reproducible over the measured frequency range. Neither valve lysis nor flow direction had an effect on ZL at any level of Q or Ro, even in veins with a taper >25%. Mean wall thickness correlated with ZL (r²=0.43; p=0.05).

In this in vitro system, saphenous vein graft impedance (ZL) was independent of valve lysis, flow direction, and the degree of vein taper but was weakly dependent on wall thickness. In veins of adequate size, hemodynamic considerations should not influence the decision to use the reversed vs nonreversed configuration.