HDCN Article Review/Hyperlink

Lindsay RM, Burbank J, Brugger J, Bradfield E, Kram R, Malek P, Blake PG

A device and a method for rapid and accurate measurement of access recirculation during hemodialysis

Kidney Int (Apr) 49:1152-1160 1996

This paper describes the hemodynamic recirculation monitor (HDM), a practical, accurate and relatively non-invasive method for measurement of hemodialysis access recirculation. Data from over 1000 measurements were used to validate this method.

The HDM measures the relative conductivities of blood entering and leaving the fistula via the venous and arterial lines. The conductivity sensors require the blood lines to be modified by the insertion of paired toroids into the arterial and venous lines. Differential conductivity is measured by comparing the electromagnetic inductance of the arterial and venous toroids.

For recirculation measurements, one ml of hypertonic (4000 mM) saline is injected rapidly upstream of the venous toroid. The saline 'tracer' is detected at the venous toroid as a transient increase in conductivity. If recirculation is present, some of the saline is immediately drawn into the arterial line and detected at the arterial toroid a short time later. By comparing the size of the arterial and venous signals, the recirculation may be quantified.

The data presented in this paper suggest that the HDM has a precision of 2% absolute. The CV at 10% recirculation was 20%. Recirculation measured by the HDM agreed with the traditional three-sample BUN method (r=0.92), however, Bland-Altman analysis was not done. The paper confirmed previous observations that the three-sample BUN method is imprecise and will 'detect' recirculation at around 5% even when no recirculation exists. The paper also uses the large number of recirculation measurements to illustrate some important clinical points regarding access recirculation;- True access recirculation is rare (2/27 patients with fistulae, 0/22 patients with PTFE grafts). Massive recirculation may occur even in a fistula which appears normal. Timely intervention by angioplasty may restore access function and prevent loss of the access by thrombosis. Access recirculation is inevitable in central venous catheters with flow reversed. Access recirculation, if present, is critically dependent on blood flow rate.

Comment: In principle, the HDM may be re-programmed to measure cardio-pulmonary recirculation and, therefore, cardiac output. This possibility was discussed in the paper. The principle of measuring hemodialysis recirculation by tracer dilution is not new. At least three non-invasive recirculation monitors are already commercially available. All three methods are less invasive and arguably more practical than the HDM; The Gambro Fistula Assessment Monitor (FAM) has been marketed in Europe since 1985. Injected isotonic saline is used as the tracer and is detected optically. The equipment was designed as a module for the AK10 dialysis machine which is now obsolete. Transonics Inc. market a recirculation monitor which detects isotonic saline tracer using ultrasound. This equipment has not been subjected to as rigorous validation as described in Lindsay's paper but is probably at least as precise. Fresenius dialysis machines now contain an optional integral recirculation monitor which uses a bolus of cool blood as the tracer. This method is the simplest to operate as all functions are carried out automatically by the dialysis machine. No injection is required as the cool bolus is achieved by allowing the dialysis fluid to cool. The Fresenius method is probably not as precise as the HDM and cannot distinguish between access and cardio-pulmonary recirculation. None of these three methods need modification of the blood lines or hypertonic saline injection as does the HDM.

Lindsay does discuss the potential drawbacks of the HDM method, namely the potential for vascular endothelial damage caused by the hypertonic saline and the need for blood line modification. I am also uneasy about the additional turbulence and potential for clotting induced by the toroids.

In conclusion, this paper uses a large number of observations to validate the saline-tracer method of recirculation measurements. Many valuable lessons may be learned from Lindsay's observations. This paper will increase awareness and interest in recirculation measurement using one of the commercially available monitors. (James Tattersall, M.D.)