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FAQ for Clinical Applications

Questions related to the Microdialysis technique

What is recovery?

The recovery of a particular substance is defined as the concentration in the dialysate expressed as percent of the concentration in the interstitial fluid.

How does the length of the membrane affect the recovery?

A longer membrane and a lower flow rate will give a higher recovery.

What is the pH of the CNS perfusion fluid?

The CNS perfusion fluid it is intentially not buffered in order to allow it to take on the same pH as the brain interstitial fluid. The pH of a nonbuffered solution is varying between 5-8.

How does the pH of the CNS perfusion fluid impact the tissue?

Since our perfusion fluid is unbuffered it can hardly itself impact the pH in the tissue besides by dilution. This means that the perfusate will end up with the same pH as the surrounding tissue. But the buffered substances in the tissue can have different recovery over the membrane which indirectly can have a small effect on the pH of the tissue.

If the cut-off of the catheter membrane is 20KDa, why do I not have 100% recovery for molecules of 20 KDa?

The ability of molecules to pass the membrane decreases logarithmically with the increase in molecular weight. By experience we know that most substances with a molecular weight up to 5,000 Da can be dialyzed when using a 20,000 Da membrane. This is of course very dependent on the substance and the sensitivity of the analytical method. It is the same with the 100KDa membrane, molecules with a molecular weight of 100K will not pass, more likely molecules with a weight of 30-40K will pass depending on the molecular shape and if it has the ability to stick to other substances.

Do I need to do a recovery experiment to estimate the correct concentrations?

In clinical practice you normally do not do a recovery test because it‘s rather laboratorious and time-consuming. It has been shown that you reach close to 100 % recovery for small molecules using a 30-mm membrane and a flow rate of 0,3 µl/min. In intensive care you look rather at the trend of parameters than at the absolute values.

How can I estimate the recovery in an easy way?

An easy way is to reduce the flow rate stepwise and measure the concentration at each step. When the concentration remains constant although the flow rate is reduced you have reached 100 % recovery. This can easily be done with the 107 pump.

If I need a higher volume for my assay, can‘t I just increase the flow rate?

The increase in flow rate will lead to a decrease in concentration. At very high flow rates there is a risk to change the physiological conditions in the tissue. In the literature the used flow rates for microdialysis vary between 0.1 and 3 µl/min. Often it‘s more reasonable to dilute the sample when later on analysis requires a higher volume.

What is ischemia?

Ischemia is an inadequate supply of oxygen and Glucose due to low capillary blood flow.

What is the LTC-method?

It is necessary to apply a simple and straight forward method when evaluating multimodal data during neurointensive care. The LTC-method represents a systematic way of looking at microdialysis data alone and in comparison with other data displayed by the ICUpilot software.

Before interpreting microdialysis data it is important to know where the catheter is positioned! In “worse” or in “better” brain tissue? Look for the Gold tip on the CTand use this information to support your interpretation.

  1. Level: Are the levels of microdialysis markers within the physiological range?
  2. Trend: Is microdialysis chemistry becoming more or less pathological over time?
    Look at the trend curves in ICUpilot.
  3. Comparison: How does Microdialysis chemistry compare with other recorded variables, for example ICP and CPP? Use ICUpilot to display different data in the same graph.
Analyte Approx. Values
Glucose 2 mM
Lactate 2 mM
Pyruvate 120 µM
Lactate/pyruvate 15-20
Glycerol 20-50 µM
Glutamate 10 µM

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