The idea behind microdialysis is to place a needle probe into the fluid surrounding a tissue, the interstitial tissue fluid, where you can reach the cells and other target structures directly. The probe has a semipermeable membrane at the tip and is guided into place with a cannula. An inflow tube is attached at the other end from the tip and then connected to a microperfusion pump. The pump sends physiological saline into the probe at a rate of about 1 microliter per minute. The flow passes into the interstitial fluid, then back into the needle probe, and finally out through another tube for collection. An appropriate device now analyzes the fluid. Just about every molecule able to dissolve in the fluid can be evaluated if the analytical device in use is capable of doing so. In fact, currently available devices are extremely sensitive, can check for almost everything, and only require a few microliters of fluid. In spite of this, it appears from the article that chemical assays for certain substances will have to be even more sensitive in the future. The devices are sold by several companies as kits costing roughly $220, and are therefore quite affordable. To date tissues involved in studies include adipose tissue, brain, heart, lung, intestines, and solid tumors. In the brain, it has been found most useful for monitoring ischemia following injury or intracranial hemorrhage. This approach has become routine in Sweden. For the heart, microdialysis has been used to monitor troponin T and aspartate transaminase after heart surgery. Ischemia (local anemia) of the intestines as a complication has also been monitored. Another use has been for checking ischemia when myocutaneous (muscle and skin) flaps are used for plastic surgery reconstruction. One of the first chemicals measured was glucose and portable devices now exist for subcutaneous checks. Finally the technique is gradually playing a more important role in antibiotic research, for the study of anticancer agents used in solid tumor treatments such as for breast cancer and melanoma, to evaluate the concentration reaching tissues when topical drugs are applied to the skin, to measure several drug concentrations directly in human brain parenchyma, and to evaluate penetration across the blood-brain barrier (for example in HIV infection).

From a practical standpoint, the procedure appears to be fairly easy for most health professionals to learn and is no more painful for the patient than having an intravenous catheter put in your vein. Ten minutes later, most subjects have no more pain even though the probes may have to remain in place for a few hours to several days. Since they can be flexible it is even possible to do some exercise with a probe in place.

Okay, there you are. Is microdialysis the wave of the future or is it just another "flash-in-the-pan?" Some of the most promising new techniques have gone up in smoke for all kinds of reasons including lack of interest or funds, competition from other techniques, failure of the researching company, etc., etc., etc. Will microdialysis survive and eventually become a standard procedure? Only time will tell. Of course, the patient who has to walk around with a probe inserted in a vein for several days may have something to say about what the future will bring for microdialysis.

What! It goes in my brain? Don't all needles go in a vein? You sure there ain't much pain, You go first. See if you complain. Huh! Keep it in 96 hours? How do I take my showers? Oh sure I can leap tall towers, After all, I've got Superman powers.

Cartoons and Poems following each article are created and copyrighted by Dr. Ackerman and cannot be copied or reproduced without his permission. Copyright © 2006 by Marvin Ackerman, M.D.