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Objective:vein graft stenoses <50% cause minimal flow impairment, velocity elevation, or symptomatology and are therefore usually assumed to be “non-critical”. The purpose of this study was to assess the effect of <50% vein graft stenosis on vein graft longitudinal impedance, as elevated impedance has been found to correlate with clinical graft failure. Methods:eight segments of non-reversed cryopreserved vein (mean length 23±1 cm; mean outer diameter 4.7±0.2 mm) were saline-perfused in vitro utilising a variable pulsatile perfusion pump, Windkessel, and clamp resistor simulating the haemodynamic conditions of arterial bypass. Proximal (Pprox) and distal (Pdist) pressure were continuously measured by fluid-filled catheter transduction, and flow (Q) by ultrasonic transit-time flowmetry. Waveforms were digitally recorded at 200 Hz at pulse rates ranging from 60–180 b.p.m. with mean flow (Q;) of 154 ml/min and mean proximal pressure (P;prox) of 100 mmHg (max/min 120/90). Graded mid-graft stenoses of <50% were created using an inflatable vascular occluder and measured by the corresponding changes in mean pressure gradient (ΔP=Pprox−P;dist) and Q; (%stenosis=1−{ΔP;baselineQ;stenosis/ΔP;stenosisQ;baseline}1/4). Vein graft longitudinal resistance (RL) was calculated as ΔP;/Q. After Fourier transformation, vein graft longitudinal impedance (ZL) was calculated as ΔP/Q at each harmonic, with ZL determined by integration over 0–4 Hz. Results are reported as mean±.e.m. Results:the desired levels of pressure and flow were established in all vein segments. Graded inflation of the occluder resulted in vein graft stenosis of 23±3% and 39±3%. This was accompanied by a mild reduction in Q (12% and 30%) and considerable increases in both RL (180% and 710%) and ZL (140% and 430%). Conclusions:“non-critical” vein graft stenosis (<50%) causes minimal change in mean flow, but substantial elevations in longitudinal resistance and impedance. The contribution of “non-critical” stenosis to vein graft failure may be under-appreciated.

Purpose: Clinical studies have revealed that the most important predictor of successful bypass grafting is the origin and quality of the bypass conduit. Attempts at intraoperative evaluation of the hemodynamic properties of the conduit, including assessment of blood flow (Q), pressure gradients (@DP), and resistance (R), have not been useful. This is because each of these parameters measures the characteristics of the graft plus the outflow bed. To date, no specific measurement of the resistive properties of the conduit only is available. The purpose of this investigation was to evaluate longitudinal impedance (Z"L) as a measure of conduit-specific resistance and to evaluate its potential in predicting the outcome of infrainguinal vascular reconstructions. Methods: Z"L was measured during surgery in 73 infrainguinal autologous vein reconstructions performed in 68 patients in two separate institutions over a 21-month period. Vein graft ultrasonic transit time Q and @DP (from proximal to distal anastomosis) were measured at baseline and after maximal peripheral vasodilatation with an intraarterial injection of papaverine 30 mg. Waveforms were recorded for 10 seconds at 200 Hz using a digital acquisition system. R was calculated as proximal mean pressure divided by mean blood flow (Q@?). After Fourier transformation, Z"L was calculated as @DP/Q at each harmonic and total Z"L (@!Z"L) was defined as the integral of moduli from 0 to 4 Hz. Results: All hemodynamic variables were significantly affected by papaverine vasodilatation (@DP@?, 3.9 +/- 0.5 vs 6.3 +/- 0.8 mm Hg; Q, 78.2 +/- 7.0 vs 126 +/- 11 ml/min; R, 134 +/- 17 vs 72.7 +/- 6.2 x 10^3 dyne . sec . cm^-^5; p < 0.0001), except @!Z"L, which remained constant (31.1 +/- 2.8 vs 30.8 +/- 2.8 x 10^3 dyne . cm^-^5; p = NS). After follow-up of 1 week to 17 months (median, 5 months), the 1-year primary, primary-assisted, and secondary patency rates were 72% +/- 7%, 77% +/- 6%, and 81% +/- 6%, respectively. Using Cox analysis, primary patency was significantly associated with decreased @!Z"L (p = 0.0001), but not with baseline or papaverine-stimulated @DP@?, Q@?, @DP@?/Q@?, or R. @!Z"L > 47 x 10^3 dyne . cm^-^5 predicted primary failure with 90% positive and negative predictive value. Conclusions: Intraoperative measurement of @!Z"L in infrainguinal vein grafts is independent of outflow conditions (that is, does not change with papaverine), and hence describes the resistive properties of the conduit only. In addition, these preliminary data suggest that @!Z"L is predictive of short-term primary patency. @!Z"L is the first available hemodynamic measurement that is conduit-specific and may therefore be a better predictor of graft patency than currently available methods.