1 Nephrology and Dialysis Unit S. Andrea Hospital La Spezia 2 Aosta Hospital Aosta Italy Email: alberto.magnasco{at}ausl5.la-spezia.it
Sir,
The careful analysis of Krivitski has allowed us to look further into some aspects of the GPT, our new method for blood flow measurements based on a constant glucose infusion [1]. Krivitski criticizes the GPT performance because of a mean overestimation (235 ml/min) vs Transonic HD01, but Krivitski himself states that Transonic HD01 underestimates up to 100 ml/min the real Qa, and other authors confirmed an underestimation of 9% [2,3]. Therefore it is clear that both methods have similar but opposite errors. In fact, if Qa is the real blood flow of a patient, the Transonic HD01 measures
Qa-100 ml/min; the GPT
HD01+235 ml/min; that is GPT
Qa+135 ml/min. Neither method produces exact values because of systematic errors that have negligible clinical effects in serial Qa measurements (surveillance protocol).
The first specific criticism by Krivitski concerns the washout procedure of the needle tube before C2 sampling. It is easy to verify that a correct and simple emptying procedure leaves less than 0.2 ml residual basal blood in the needle tube, with a possible overestimation of C2 of 2%, which is clinically trivial and technically undetectable because of the glucometer coefficient of variation (2%). In addition, the overestimations calculated by Krivitski (610%) are wrong because he has considered the residual blood with zero glucose, while obviously it has the basal glucose of the patient, higher than zero (mean C1 of our patients=132 mg/dl, mean C2=297 mg/dl; correctly recalculated Krivitski's overestimations 35%). Nevertheless it is possible to insert a three way stopcock with two syringes into the venous needle in order to avoid the emptying procedure with air. In this case you can wash out the needle with a low flow withdrawal during the glucose infusion and then, at 11 s, turn the stopcock and take sample C2 with a second syringe.
The second criticism concerns the C2 sampling procedure and its theoretical effects on Qa. The strong (but rapid) aspiration during C2 withdrawal is used to improve the blood sampling through the whole vascular access size, avoiding taking only the peripheral blood stream, especially when the venous needle tip is placed near the wall of the graft. In theory this withdrawal flow (about 150 ml/min) could produce haemodynamic consequences in the vascular access flow, but these aberrations are mostly less than those reported in Table 1 by Krivitski for the following reasons: (i) the aspiration time is technically impossible under 2 s; (ii) the procedure is so fast that it is difficult to reach a new steady state in so short a lag time (also considering that only the early first half of C2 is used for the test, while the second half (2 ml) remains in the dead space of the needle tube, further reducing the real lag time); (iii) at least a part of the withdrawal volume derives from the vascular access compliance. Summarizing, there are plenty of haemodynamic variables influencing glucose (or saline) mixing, and the real effect of C2 on Qa, such as (i) intravascular turbulence (not laminar flow), (ii) varying density of blood that is not an ideal fluid (peripheral rouleaux formation of red cells with preferential flows), (iii) haemodynamic resistance, (iv) vascular access compliance.
In such a complex haemodynamic situation, our glucose mass balance equation is a simplification and its accuracy must be shown with in vitro tests and, mainly, in vivo comparisons with other methods. For example, up to now we have found: (i) mean difference of GPT vs in vitro gravimetric measurements=9.8 ml/min; n=18; r=0.94 [1], (ii) mean difference of GPT vs Transonic=235 ml/min; n=23; r=0.95 [1], (iii) mean difference of GPT vs US Doppler=0.4 ml/min; n=23; r=0.88 [1], (iv) mean difference of GPT vs reversed urea test=31 ml/min; n=20; r=0.88 [4], and (v) mean difference of contemporary GPT vs Transonic)=110 ml/min; n=33; r=0.91 [5].
This published data confirms that GPT gives good clinical results. In particular, Barz's contemporary comparison between GPT and Transonic shows a smaller difference and a good concordance between the two tests. This data reinforces the hypothesis that the difference in time between GPT (pre-dialysis) and Transonic (during dialysis) is an important source of the discrepancy of our first results.
In conclusion, GPT has proved to be a very simple and economic method of measuring Qa. The aim of our first paper [1] is to present the new glucose method, allowing other researchers to verify its clinical performance in the vascular access surveillance protocol.
References