Detection of unexpected flow patterns in arterio-venous fistulae using ultrasound dilution

G Hossannee, A Cundiff, E Lindley, A Plou, S Sanasy

St James University Hospital, Leeds

Our centre began routine screening of blood flow in arterio-venous (AV) grafts in 1998, using ultrasound dilution (HD01, Transonic Inc). The recirculation and access flow measurements are carried out during dialysis by the nursing staff and reviewed in a regular multidisciplinary team meeting. The screening measurements immediately revealed that over 25% of patients with loop grafts had been routinely dialysed with the arterial needle downstream of the venous needle because the flow direction was incorrectly recorded. This was found to be due to ambiguous surgical notes and ineffective palpation of newly inserted grafts.

The programme is now being extended to AV fistulae and we have been surprised to find antegrade (towards the hand) blood flow in two native fistulae. With this reversed flow, the screening measurements gave a high recirculation (>20%) with the needles and lines in the usual configuration for dialysis, and an error message when attempting to measure access flow with the blood lines reversed. The flow direction was confirmed by repeating the recirculation test with the lines reversed.

The first patient with reversed fistula flow is a 64 year old who had a brachial fistula created in March 2003. Her Kt/V (equilibrated) was lower than would be expected for a 55 kg female dialysing for 4 hours with a prescribed blood flow of 300 ml/min and a 2.4 m² dialyser, but not low enough to initiate a clinical review. A routine access flow measurement in October 2003 showed 32% recirculation with the needles in the usual configuration. After reversing the needles, so the arterial needle was above the elbow and the venous needle below, there was no recirculation and a flow of 590 ml/min. Using these needle positions, the patient’s Kt/V increased by more than 20%.

More recently, we found the same phenomenon in a 41 year old with a brachial fistula that was created in October 1999. His Kt/V measurements were acceptable until June 2004 when they dropped to 0.90. He was referred for a fistuloplasty in July after which his Kt/V increased to 1.16. An access flow measurement was carried out in September 2004 to see if the patient needed further intervention. It showed 21% recirculation in the usual needle configuration with a set blood flow of 400 ml/min. When the needles were reversed there was no recirculation and an access flow of 1120 ml/min. With the new needle positions, the patient’s Kt/V increased to an acceptable level of 1.26.

We don’t have a physiological explanation for the unusual flow patterns in these fistulae yet, but we do know that both patients experienced an improvement in dialysis adequacy when the actual flow direction had been identified and the needle positions reversed. Without the ultrasound dilution measurements, the antegrade flow is unlikely to have been detected as the first patient did not have a low Kt/V and it would have been assumed that the second patient’s poor results were due to uncorrected stenosis.