Effective hemodynamic diameter: An intrinsic property of vein grafts with predictive value for patency

Introduction: Conduit size and quality are major determinants of the long-term success of infrainguinal autologous vein grafting. However, accurate measurement of the internal diameter of vein grafts is difficult given their variable wall thickness and taper. The purpose of this study was to define the 'effective' internal diameter of a vein graft according to its hemodynamic properties and to determine its significance for graft patency. Methods: Sixty infrainguinal bypass grafts performed on 57 patients were evaluated intraoperatively. Proximal and distal graft pressure and blood flow (Q(meas)) were measured with fluid-filled catheter transduction and ultrasonic transit-time flowimetry, respectively, after unclamping. Waveforms were recorded digitally at 200 Hz under baseline conditions and after stimulation with 60 mg of papaverine. According to Fourier transformation of the measured pressure gradient (ΔP), the Womersley solution for fluid flow in a straight rigid tube was used to calculate theoretical flow waveforms (Q(calc)) for a range of graft diameters. The theoretical waveforms were then compared with the measured flow waveforms and the best-fit diameter chosen as the 'effective hemodynamic diameter' (EHD). Only grafts in which the correlation coefficient of Q(calc) versus Q(meas) was more than 0.90 were accepted (n = 47) to assure validity of the hemodynamic model. After a mean follow-up of 12.5 months (range, 0.1-43.9 months), patency was determined by the life table method. Hemodynamic and clinical variables were tabulated, and their effect on patency determined the use of univariate and multivariate Cox regression. Results: Mean EHD was 4.1 ± 0.1 mm with a range of 2.5 to 5.7 mm. Administration of papaverine caused profound changes in ΔP (+78% ± 17%) and Q(meas) (+71% ± 12%) as expected, but had no effect on EHD (+0.05% ± 0.1%). Univariate regression identified five variables associated with decreased secondary patency (P < .10): low END, conduit source other than the greater saphenous vein, high baseline ΔP(mean), female sex, and redo operation. Of these, only low EHD was significant after multivariate analysis (P = .03). Patency of small diameter grafts (END < 3.6 mm; n = 11) was compared with patency of larger grafts (EHD > 3.6 mm; n = 36) to test a frequently espoused clinical guideline. Grafts with an EHD less than 3.6 mm exhibited significantly lower secondary patency compared with larger grafts (P = .0001). The positive and negative predictive values for an EHD less than 3.6 mm for secondary graft failure for grafts with at least 1 year follow-up were 86% and 88%, respectively. Conclusion: An EHD is a unique parameter that quantifies conduit size and has a significant impact on vein graft patency. An EHD less than 3.6 mm portends graft failure.