Non-invasive measurement of blood flow-rate in expanded polytetrafluoroethylene grafts

Four 6 mm e-PTFE grafts were placed subcutaneously into four dogs between the carotid artery and jugular vein to model shunt flow through an access graft. Flow-rates were varied from 120 to 2000 ml/min (N = 42) by use of a cardiac stimulant (Aramine) and external constrictor. Measures of graft flow were made simultaneously by electromagnetic flowmeter (EMF) and Doppler ultrasound based on a known graft diameter, use of a flat-head probe providing a fixed beam-to-vessel angle and derivation of the mean Doppler-shift frequency from first moment computation of the frequency spectra. Correlation coefficients between the Doppler ultrasound and EMF recordings were 0.54, 0.96 and 0.92 at measurement sites just beyond the arterial/graft anastomosis, at the graft loop and just proximal to the graft/venous anastomosis, respectively. Availability of a simple and accurate non-invasive flowmeter for superficially placed e-PTFE grafts may be valuable for evaluating the systemic effects of vascular shunts, for monitoring the patency of access grafts and for assessing the function of bypass grafts for limb ischaemia.