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The effect of 'non-critical' (<50%) stenosis on vein graft longitudinal resistance and impedance.

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 (Pprox) 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 (DeltaP=Pprox-Pdist) and Q (%stenosis=1-{DeltaPbaselineQstenosis/Delta PstenosisQbaseline}1/4). Vein graft longitudinal resistance (RL) was calculated as DeltaP/Q. After Fourier transformation, vein graft longitudinal impedance (ZL) was calculated as DeltaP/Q at each harmonic, with ZL determined by integration over 0-4 Hz. Results are reported as mean+/-S.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.