Krivitski NM
Theory and validation of access flow measurement by dilution technique during hemodialysis

Kidney Int (Jul) 48:244-250 1995

A sensor which measures the ultrasonic transit time across a dialyzer bloodline is normally used to measure blood flow rate. In this application, the injection of saline upstream to the sensor is used to estimate access recirculation and, with reversal of the lines, access blood flow rate. Saline dilution alters the ultrasonic transit time across a column of flowing blood. If saline is injected upstream to the sensor, the sensor will register an indicator dilution curve. If the sensor is placed on the arterial bloodline, injection of blood into the venous line should not affect the arterial bloodline sensor reading for the initial 10-15 sec after injection (until the saline traverses the heart and some of it comes back via the arterial circulation). If, however, there is access recirculation, then injection of a 5 ml saline bolus in the venous line will show up at the arterial bloodline sensor, and the amount of recirculation can be quantified as a percentage of the dilution curve obtained by injecting the same amount of saline in the ARTERIAL line just upstream to the sensor. Access blood flow can be measured by deliberately reversing the bloodlines for a few minutes before making the injections. With the lines reversed, the dialyzer is fed from the downstream needle. The venous line empties into the upstream needle. Saline injected into the venous line will now go via the upstream needle through the access segment and downstream needle into the arterial line and will be detected by the sensor on the arterial line. The percent of injected saline that will be detected depends on the ratio of blood pump flow to blood pump flow + access flow. In this way, access blood flow rate can also be quantified.

The present paper presents details of the mathematics and in vitro validation using various degrees of separation of access needles, and also needle orientations. Access recirculation was accurately quantified with only minor degradation of accuracy at a 3 cm separation of the needles. Access blood flow (in vitro) could be accurately measured at line flows as low as 100-150 ml/min, and with blood pump flows down to the same range. When the needles were placed in the same direction into the "access", the accuracy of access blood flow measurements was reduced slightly due to imperfect mixing.

This is very exciting new technology. It was validated in patients for both recirculation and access blood flow at the ASAIO meetings in Chicago in May of 1995 by Tom Depner. We have used this device to examine recirculation as well, and find that most patients (about 90-95%) have zero recirculation. We also picked up several patients with inadvertently reversed needles. The advantage of measuring access flow is that low flows may predict access failure better than any other measure. (Daugirdas)

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Basic hemodialysis : Vascular Access: graft/fistula