Despite improvement in dialysis technology, vascular access remains the limiting factor in delivering adequate hemodialysis. No technological or therapeutic advance has been able to compensate for inadequate blood flow through the vasculature and the access remains the "Achilles Heel" for the patient.

The volume flow rate of blood (i.e. ml/minute) must be constant throughout the length of a non-bifurcated vascular segment. Since the volume flow rate is the product of the mean velocity and the cross sectional area, any decrease in cross sectional area will be accompanied by an increase in velocity and vice versa.

Q = V x A = Constant OR Q = VI x A1 = V2 x A2 (Q = Volume flow rate; V = Mean velocity; A = Vessel cross-sectional area; 1 and 2 refer to two different locations in the vessel) If velocity is measured through the length of a graft, narrowing of the graft due to thrombus formation or stenosis from initial hyperplasia of the recipient arterialized vein will be detected simply by noting increased velocity.

A Doppler velocity measuring probe with calibration marks along its length is passed through the venous dialysis needle at the distal end of the graft in the direction of flow to measure velocity of blood in the venous outflow tract of the dialysis graft.

The procedure is performed at the dialysis chair, immediately prior to or after the dialysis treatment.

The catheter is removed at a predetermined speed and then used to calculate the cross sectional area of the vessel.

When the Doppler catheter is positioned beyond the region of suspected stenosis and slowly withdrawn, it registers an increase in velocity wherever there is a narrowing of the vessel.