My clinic uses the crit-line for ABF and recirculation tests, This question came up by one of the MDs the other day, What value or parameter are we measuring when we do a recirculation test? What is the area under the curve or spike, on the venous or arterial representative of? Is this saline or HCT? what math equation is the device using to measure recirculation thanks for any information that I can pass onto the docs

First off, some background. As you may know recirculation does not occur until access flow drops below pump speed. Therefore if you have any recirculation at all you know that the access is already having problems as its flow is below pump speed (300-500ml/min.) Now to your question. The Crit-Line measures the absolute hematocrit of the blood just before it enters the dialyzer. By injecting saline into the blood, the hematocrit is changed (diluted or lowered). An injection into the arterial line of a known volume becomes what can be considered a "calibration" bolus. The hematocrit change over time will be "bell" shaped and represents 100% recirculation. An identical bolus is injected into the venous line - if recirculation is occuring, a percentage of this saline exiting the venous needle is "recirculated" back to the arterial needle. This saline also causes a hematocrit change - a change that is proportional to the actual amount of recirculation.

To calculate the percentage - the area under the hematocrit dilution curve of the venous injection is divided by the area under the curve of the arterial dilution curve. The result is the percentage recirculation.

Although not as predictive as access flow measurements, recirculation tests provide a quick (2 minute) indicator for proper needle placement and access patency.

In addition, the venous bolus is pushed first during the Crit-Line III test, allowing visual proof of whether recirculation is occuring. If no hct change occured, seen as no "bump" on the screen - then there is no need to do an arterial injection.

Russell, Thanks for the information Do you know the math formula?