Measuring hemodialysis vascular access flow by the temperature gradient method

Wijnen E et al. (2007) Measurement of hemodialysis vascular access flow using extracorporeal temperature gradients. Kidney Int 72: 736–741

A decrease in hemodialysis vascular access flow (Q_a) by 25% or to below 600 ml/min is associated with an increased risk for thrombosis. Current methods of measuring Q_a (based on indicator dilution) are not ideal: saline dilution requires the manual injection of an indicator and thermodilution takes up to 30 minutes. A recent study found that the temperature gradient method (TGM), which is based on the extracorporeal arteriovenous temperature gradients generated when bloodlines are reversed and does not require the injection of an indicator, seems to be a quick and accurate way of measuring Q_a.

The study compared Q_a measurements obtained with TGM with those made using saline dilution (the currently accepted reference technique) for 35 vascular accesses (fistulas and grafts) in 35 patients with end-stage renal disease. Q_a of each vascular access was measured by both techniques during the first hour of dialysis. One week after the first measurements, Q_a of each access was again measured with each technique, providing 70 'measurement pairs' for analysis.

Average Q_a measurements taken using the saline dilution technique and the TGM were similar (960 ± 594 ml/min vs 1,000 ± 588 ml/min, respectively; r² = 0.930). Comparing the first measurements with those taken 1 week later showed that the saline dilution technique and TGM had similar repeatability. The correlation between initial and subsequent measurements was high for both saline dilution and the TGM (r² = 0.88 and r² = 0.85, respectively). On average, it took 5.1 ± 0.7 minutes to measure Q_a with the saline dilution technique and 8.9 ± 3.5 minutes with the TGM.