Pitfalls of the glucose pump test for access flow measurements

Nikolai Krivitski

Transonic Systems Inc. Ithaca USA Email: nikolai{at}transonic.com

Sir,

I read with interest the recent publication by Magnasco et al. [1] in which they introduced the glucose pump test (GPT) method for access flow measurement. Their in vivo comparisons showed a statistically significant overestimation of access flow (235±117 ml/min, P<0.001) by the GPT vs the Transonic HD01 Hemodialysis Monitor. The authors attribute this difference to changes in haemodynamic conditions (as measurements were not made simultaneously) and to the influence of counter-current flow due to the blood-line reversal required by Transonic's technology [2].

These explanations do not appear to be convincing. Firstly, haemodynamic changes may occur in either direction and cannot produce a strong statistical difference in one direction of such magnitude. Secondly, the underestimation of the blood flow with the reversed line position by Transonic HD01 is on the order of 50–70 ml/min [2] (it may reach up to 100 ml/min in special circumstances [2]). This cannot account for such large differences that the authors found to be as much as 450 ml/min.

Before attributing this large discrepancy to patient haemodynamics and the reversed line technique, the authors should first extensively analyse the errors of their own GPT method.

There are two sources of access flow overestimation by GPT. The first is related to the participation of unmixed blood in their blood samples. As described in the GPT protocol, the authors ‘empty [the arterial needle tubing] with 2 ml of air’. The reason for this manipulation is that the sample should consist only of blood mixed with glucose, for including in the sample any unmixed blood would affect the calculation. It obvious that there is always some blood left in the tubing, the volume of which could be approximated as, for example, 0.3–0.5 ml. Then if ~5 ml of blood is withdrawn, the overestimation resulting from this factor may be 6–10%.

The second and principal source of overestimation is related to the manner by which the second blood sample is taken after glucose infusion: ‘withdraw the second sample vigorously from the venous needle with a rapid and strong aspiration (~5 ml of blood in 2 s)’ [1].

I conjecture (as there is no clarification in the text) that the need for strong aspiration is related to the fact that the slow infusion (20 ml/min) of glucose may not produce complete mixing of the indicator with blood. The authors' recommendation for a quick withdrawal may be an attempt to produce turbulence that will enhance mixing at the tip of the withdraw needle. This attempt to achieve mixing is not without cost. A simple ratio of volume withdrawn to time of aspiration shows that blood flow in the access will be immediately and significantly changed by this procedure (Table 1Go).


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Table 1.  Blood flow increase between access needles during blood sample withdrawal

 
The influence of these flow changes on glucose concentration in the blood sample is complicated and depends on the distribution of haemodynamic resistances in the vascular access, the actual transit time for glucose between two needles, and the rate of blood sample withdrawal.

If, for example, all the haemodynamic resistance is on the inflow side, then the flow between needles will not change and glucose dilution between the needles will not be disturbed. This may be the reason why the authors did not observe any obvious overestimation with their in vitro validation. The pump (main resistance) is located upstream of the access model (Figure 2 of [1]). In reality, blood flow in the access is driven by a pressure gradient, so the model used in Figure 7 of [2] would be more appropriate.

If all the resistance is on the outflow side, then all the withdrawal will add to the initial blood flow, and the maximum overestimation by the GPT method would be observed (Table 1Go).

There is a principal difference between the manners of access flow disturbance by the above method and by blood-line reversal. When lines are reversed, blood flow is simultaneously delivered and withdrawn from the access. In the situation when blood is quickly withdrawn, the initial access flow may change to full withdrawal rate to maintain mass balance.

The amount of residual blood in the tubing needle, the distribution of haemodynamic resistances in the access, and the rate of blood sample withdrawal will define the overall overestimation of access blood flow by GPT. I hope this analysis will assist the developers of the method in improving the accuracy of access flow measurements through more detailed modelling and analysis of their data.

References

  1. Magnasco A, Alloatti S, Martinoli C, Solari P. Glucose pump test: a new method for blood flow measurements. Nephrol Dial Transplant 2002; 17:2244–2248[Abstract/Free Full Text]
  2. Krivitski NM, Depner TA. Development of a method for measuring hemodialysis access flow: from idea to robust technology. Semin Dial 1998; 11:124–130[ISI]