FAQ - Clinical Applications
Questions on the Microdialysis technique
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Question:
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What is recovery?
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Answer:
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The recovery of a particular substance is defined as the concentration in the dialysate expressed as percent of the concentration in the interstitial fluid.
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Question:
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How does the length of the membrane affect the recovery?
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Answer:
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A longer membrane and a lower flow rate will give a higher recovery.
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Question:
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What is the pH of the CNS perfusion fluid?
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Answer:
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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.
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Question:
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How does the pH of the CNS perfusion fluid impact the tissue?
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Answer:
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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.
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Question:
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If the cut-off of the catheter membrane is 20,000 Da, why do I not have 100% recovery for molecules of 20,000 Da?
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Answer:
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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.
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Question:
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Do I need to do a recovery experiment to estimate the correct concentrations?
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Answer:
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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.
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Question:
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How can I estimate the recovery in an easy way?
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Answer:
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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 CMA107 pump.
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Question:
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If I need a higher volume for my assay, can‘t I just increase the flow rate?
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Answer:
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The increase in flow rate will lead to a decrease in concentration. At very high flow rates there is a risk to chane 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.
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Question:
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What is ischemia?
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Answer:
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Ischemia is an inadequate supply of oxygene due to low capillary blood flow.
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Question:
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What is the LTC-method?
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Answer:
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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.
- Level: Are the levels of microdialysis markers within the physiological range?
- Trend: Is microdialysis chemistry becoming more or less pathological over time?
Look at the trend curves in ICUpilot.
- 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.
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Analyte
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Approx. Values
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Glucose
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2 mM
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Lactate
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2 mM
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Pyruvate
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120 mM
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Lactate/pyruvate
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15-20
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Glycerol
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20-50 µM
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Glutamate
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10 µM
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Question:
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What are the advantages using Microdialysis measuring tissue ischemia in comparison with PO2?
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Answer:
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PO2 measures only how much Oxygen there is in the tissue. Microdialysis reveals how the cell uses the Oxygen in the cell. When ischemia is suspected look at:
Lactate/Pyruvate ratio - increases
Glucose - decreases
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Question:
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What are normal values for Glucose, Lacate, Pyruvate etc in the brain?
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Answer:
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The normal levels of the various chemical markers in anaesthetized brain when using a catheter with 10 mm membrane and 0.3 ml/min flow are:
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Analyte
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Approx. Values
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Glucose
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2 mM
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Lactate
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2 mM
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Pyruvate
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120 mM
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Lactate/pyruvate
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15-20
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Glycerol
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20-50 µM
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Glutamate
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10 µM
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Question:
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Can the samples from my Microdialysis Analyzer be stored for later analysis e.g. of additional parameters? How?
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Answer:
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The samples should be stored in the microvial racks provided by CMA. They avoid evaporation of the sample by closing the holes in the microvial. The rack can be stored on ice in a small thermo box standing next to the patient. When the rack is filled with samples it should be brought to the freezer. For most substances a storage temperature of -20°C is sufficient. When re-analysing frozen samples please note the instruction in the CMA 600 manual. Also see application note ( Download PDF )
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Question:
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Why do I get blood in my dialysate samples?
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Answer:
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There is probably a small hole in the membrane through which blood cells can enter to the perfusate. If the hole is very small the catheter can be functioning for some hours. The doctor has to decide if the catheter should be replaced or not.
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Question:
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Can I use Microdialysis for drug monitoring?
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Answer:
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During the last 2 decades, microdialysis has become increasingly popular for preclinical and clinical pharmacokinetic studies. The advantage of in vivo microdialysis over traditional methods relates to its ability to continuously sample the unbound drug fraction in the interstitial space fluid (ISF). This is of particular importance because the ISF may be regarded as the actual target compartment for many drugs. In contrast, plasma concentrations are increasingly recognised as inadequately predicting tissue drug concentrations and therapeutic success in many patient populations.
Thus, the minimally invasive microdialysis technique has evolved into an important tool for the direct assessment of drug concentrations at the site of drug delivery in virtually all tissues. In particular, concentrations of transdermally applied drugs, neurotransmitters, antibacterials, cytotoxic agents, hormones, large molecules such as cytokines and proteins, and many other compounds were described by means of microdialysis. (M Müller 2005)
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Questions on the Microdialysis Products
Catheters
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Question:
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What is the difference between the CMA Microdialysis catheters (CMA 60, 61, 62, 70, 71)?
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Answer:
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CMA 60 Microdialysis Catheter is designed for Microdialysis in adipose tissue and resting
skeletal muscle. It is delivered with a slit cannula introducer that makes it easy to implant the catheter into the tissue. It has a shaft length of 20 mm with polyamide 30 mm membrane.
CMA 61 Hepatic Microdialysis Catheter´s dialyzing membrane is designed to achieve excellent diffusion characteristics, allowing a high recovery of substances from the extra cellular fluid. It has a shaft length of 40 mm with a 30 mm membrane.
CMA 62 Gastrointestinal Microdialysis Catheter allows continuous monitoring and detection of local changes in metabolism following gastrointestinal surgery. It has a shaft length of 180 mm with a 30 mm membrane. The catheter is introduced into the intraperitoneal cavity during open surgery.
CMA 70 Brain Microdialysis Catheter
The CMA 70 Microdialysis Catheter is a catheter designed for implantation in brain tissue. A selection of CMA 70 Microdialysis Catheters is available with different membrane and shaft lengths suitable for stereotaxical as well as manual implantation
CMA 70 Bolt Microdialysis Catheter is designed for implantation in brain tissue through a three way bolt fixed to the skull bone. Its Luer connection fits to the Integra 3-channel Bolt, (IM3). The shaft is 130 mm the dialyzing membrane is 10 mm.
CMA 71 High Cut-Off Brain Microdialysis Catheter allows diffusion of large molecules such as Cytokines. The membrane has a cut-off of around 100,000 Daltons. The catheter is available in different membrane lengths 10 mm, 20 mm or 30 mm suitable for different target areas in the brain.
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Question:
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Is the CMA 70 Brain catheter implanted intra-parenchymal or intraventricularly?
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Answer:
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It should be implanted intra-parenchymally
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Question:
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Is the CMA 70 catheter visible on CT scan?
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Answer:
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The CMA 70 catheters are provided with a gold tip that makes it possible to locate the membrane on CT scan.
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Question:
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Can I leave the CMA 70 catheter implanted when the patient is taken to MRI?
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Answer:
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Yes, the needle in the vial holder is from non-magnetic material. But you have to take away the pump during the MRI investigation. It can easily be reconnected afterwards. In case you use the bolt catheter please check with the manufacturer of the bolt weather it can be used in MRI.
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Question:
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Why do I need a special forceps when using CMA 70 brain catheter? Can‘t I use my ordinary forceps?
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Answer:
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The forceps for insertion of the CMA 70 has a special shape at the top that makes it possible to grip thin tubings without squeezing them.
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Question:
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Can I use any of the catheters for applications not mentioned in the intended use?
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Answer:
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In clinical use the CMA guarantee is not valid for anything but for the written intended use. For research you can in a chosen application apply for a special ethical approval. You will then use the catheter at your own risk.
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Question:
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Is there a high cut-off catheter for large molecules e.g. cytokines available?
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Answer:
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Yes CMA can provide you with a high cut-off Brain Microdialysis Catheter for inplantation in the brain tissue for monitoring large molecules such as Cytokines. The membrane has a cut-off of around 100,000 Daltons. The catheter is available in different membrane lengths
10 mm, 20 mm or 30 mm suitable for different target areas in the brain.
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Question:
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What is the charge properties of the microdialysis membranes?
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Answer:
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None of the membranes that we use (Cuprophane, PC, PA, PES eller PAES) have any permanant charge.
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Question:
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What do I do if a catheter membrane is left behind in the tissue?
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Answer:
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It has to be reported ASAP to CMA. Inform the investigator that he should NOT try to get out the membrane by surgical intervention since it‘s a very little piece and probably not possible to find.
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Question:
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Where do I place my Peritoneal Catheter?
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Answer:
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The specially designed CMA 62 Gastrointestinal Catheter is placed in the peritoneal cavity by inserting the catheter into the cavity through the abdominal wall by using a tunnelating needle. It has been shown that changes of chemistry are easily detected in the intraperitoneal fluid (K.Jansson, J.Ungerstedt). Note that the catheter shall not be placed in the interstitial wall or lumen.
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Question:
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How long can the catheter be implanted in the tissue?
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Answer:
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CMA guarantees five days function. When the catheter is peripheral implanted for longer time you have to observe the skin around the catheter for signs of reaction. There are patients who have had catheters implanted up to 30 days without any problems.
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Question:
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Why have I got empty microvials?
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Answer:
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There could be several varying reasons.
Make sure that the vials are inserted into the vial holder properly, e.g. that the needle penetrates the vial membrane.
Check if the microvial holder needle has fallen off.
Check that the pump is functioning.
Check the syringe. If it is empty it has to be replaced with a filled syringe.
There can also be air bubbles in the syringe. Make sure that you always fill the syringe free of air bubbles.
Make sure that the tubings of the catheter are not compressed.
If the needle, pump and syringe are OK probably the catheter membrane is broken and the catheter has to be replaced.
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Question:
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Why does the volume in the vials vary?
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Answer:
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The changes can be caused by air bubbles that are transported from the syringe to the vial during the microdialysis. They can also be caused by differences in the osmotic pressure in the tissue since the membrane is free permeable for water. It has been shown by Hans Rosdahl et al that there is no significant change of the concentration in spite of the loss of water. If necessary the loss can be compensated by adding dextrane to the perfusion fluid.
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Question:
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Why do I have to pre-wet the CMA 60 Microdialysis Catheter before introducing it into the tissue?
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Answer:
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The equilibration with the extracellular fluid will be faster.
We recommend two ways that this can be done:
- Dip the membrane into sterile perfusion fluid for a few seconds.
- Use the CMA106 or CMA 107 Microdialysis Pump to flush the catheter with sterile perfusion fluid. Note! Do not remove the protection tube surrounding the catheter and introducer while flushing.
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Pumps
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Question:
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What is the difference between CMA 106 and CMA 107?
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Answer:
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CMA 106 Microdialysis Pump is a pump designed for clinical practice of Microdialysis. It has a fixed flow rate of 0.3µL/min
CMA 107 Microdialysis Pump is a flexible pump designed for research. The flow rate is adjustable to 8 different settings: 0, 0.1, 0.2, 0.3, 0.5, 1, 2 and 5 µL/min.
You choose a low flow rate for high recovery of low molecular weight substances
You choose a high flow rate for blood flow measuremants and more frequent sample intervals
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Question:
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How long does a new filled syringe in CMA 106 last?
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Answer:
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5 days at a flow rate of 0.3 µl/min
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Question:
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How long does the pump battery last?
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Answer:
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The life time of the battery is 12 days at flow of 3 µL/min.
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Question:
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You recommend your CMA perfusion fluid. Why can I not use ordinary Ringer solution?
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Answer:
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The Perfusion Fluid T1 and CNS are sterile isotonic perfiusion fluids conveniently packed in glass ampulles of 5 mL each, ready to use. Ordinary Ringer solution bought in a Pharmacy is usually unconveniently packed in 1000 mL packages and you need just 2.5 mL for the CMA Pump.
Note that Ringer Lactate is not recommended to use if you intend to monitor Lactate in ischemia.
Further more it is not advisable to use a buffered solution as it might disturb the extracellular fluid.
The CNS Perfusion Fluid is especially designed by CMA mimicing the cerebrospinal fluid.
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Analyzers
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Question:
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What is special about the CMA Analyzers compared to other analytical systems?
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Answer:
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The CMA Analyzers are the first analyzers for bedside analysis of microdialysis samples. It requires only 0.2 to 1 µl sample volume per analysis – about 10 times less then comparable methods. This makes it possible to analyze samples frequently and show trend curves for the patient on bedside.
The analysis takes only 5 min and the data are displayed directly. To achieve the same results you would need three different HPLC systems in the laboratory and receive the data days later which is only of academic interest. With the CMA Analyzersmit is possible to use microdialysis data in clinical praxis.
The analyzer is easy to use and can be run by nurses in clinical routine. The handling of the CMA Analyzers requires only a minimum of time.
The CMA Analyzers can be integrated into the existing clinical monitoring using the ICU Pilot software. This gives you fast and easy access to all monitoring data. It makes it possible to overview a vast amount of information on bedside. The interpretation of the data can be done immediately.
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Question:
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What is the difference between the CMA 600 Analyzer and the ISCUS?
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Answer:
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CMA 600
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ISCUS
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Intended use
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Research
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Routine
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Prize
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1
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½ incl. 3x CMA106
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Patients
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up to 3
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1
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Catheters
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1-3/pat
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1-3/pat
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Reagents available
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6
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5
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Number of reagents analysed at the same time
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4
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5
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Batch analysis
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Yes
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No
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Integrated computer
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No
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Yes
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Integrated printer
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No
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Yes
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Detector
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Laser diode
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Laser diode
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Software
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600 + ICUpilot
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ISCUS
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Operation
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Keyboard/mouse
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Touch screen
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Connect. to ext. instruments
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Yes0
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Yes
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Size/weight
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Big/heavy
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Small/12 Kg
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Service interval
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6 months
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12 months
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Question:
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Why is the MD analyzer system not on line?
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Answer:
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There is an advantage not having the system on-line for ambulant patients and when nursing unconscious patients.
(The analyzer is mostly placed bedside and can analyze samples within minutes.)
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Question:
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When pressing the green button on the CMA 600 the sample tray does not open directly. Why?
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Answer:
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Probably the analyser is occupied with an analysis. This is shown by the button twinkling. The tray will open within 30 seconds.
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Question:
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CMA 600. The results are close to zero for some (all) parameters. What has gone wrong?
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Answer:
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If you observe one sudden drop-down in the curve there has probably been a small air bubble in the vial. The air bubble can be removed after the analysis and the vial can be re-analysed. If all parameters show zero the vial has probably been empty. If not, it may also be that the cannula of the CMA600 is blocked inside. This will also lead to faulty calibrations.
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Question:
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What pH is suitable for the tubings in the CMA 600?
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Answer:
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The tubing (made of PEEK and PVC) in the 600 can be used in pH 1-14 at room temperature for a couple of days. Note that the flowcell of the detector is made of quartz and should not be exposed to high pH for longer periods of time.
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Question:
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Why is the calibration not working on my CMA 600?
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Answer:
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The CMA 600 requires one-hour warm-up time to reach working temperature and lamp stabilization. When the reagents are dissolved with the buffer it is important that the reagent equilibrates in room temperature for at least 30 minutes. (see user manual CMA 600)
If the calibration is started within warm-up time, or if the reagents are not correctly prepared there may be problems.
The sample needle can be broken due to that the stopper of the calibration bottle is not removed. The system can be out of rinsing fluid.
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Question:
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I have difficulty interpreting and comparing my Microdialysis data. Is there a way to copy my results and compare them in e.g. Excel?
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Answer:
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The solution is using a special software developed by CMA called LABpilot.
Advantages:
Works like your brain thinks - in pictures not numbers.
Graphs appear instantaneously as you type or paste in your data.
Visualize, compare and compute data by simple drag and drop. Formulate a hypothesis and test it in a matter of minutes.
MUCH faster than any spreadsheet e.g. Excel, or graphics programs.
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