Microdialysis: Use in the Assessment of a Buried Bone-Only Fibular Free Flap

Monitoring of free tissue flaps is difficult, especially when the flap is inaccessible or buried. It has been suggested that flap monitoring should be simple and harmless to both patient and flap, and be rapid, repeatable, reliable, recordable, accurate, and ideally undertaken by any member of the microvascular team.1 Clinical measures such as skin color, skin turgor, surface temperature, and capillary refill time remain the favored method in nonburied flaps.2 Other measures described in buried free flap monitoring include use of an externalized skin paddle, Doppler probes, spectrophotometry, laser Doppler flow, oxygen tension probes, indocyanine green, plethysmography, hydrogen clearance, and power Doppler with microbubble contrast perfusion. With the exception of power Doppler microbubble contrast perfusion and the use of an externalized skin paddle, one can comment on inflow and outflow, but there is no measure of the microcirculation and tissue perfusion dynamics. The failure to recognize early a compromised buried flap is clearly demonstrated in studies that have found no flap salvage in these cases.3 Metabolic activity in free flaps has been monitored by microdialysis previously,4–7 although to our knowledge there are no studies of flaps used for head and neck reconstruction. Microdialysis is a sampling technique that studies the biochemistry of organs or tissues. It has been used in free tissue transfer (mainly for rectus flaps in breast reconstruction), transplant, and neurosurgery. A double-lumen microdialysis catheter or probe similar in size to an 18-gauge venous cannula is placed (using an open needle) under direct vision into the subcutaneous tissues, muscles, or viscera (depending on type of flap). It is then connected to a small micropump, which perfuses physiologic fluid across a dialysis membrane in the catheter at a rate of 0.3 l/ minute. This fluid equilibrates with the interstitial fluid surrounding the catheter, and therefore care is taken to ensure that it is not inserted into a blood vessel. To date, we have not experienced any problems with inserting the catheters. The catheter itself can remain in the tissue for as long as is clinically required although, as with any indwelling catheter, the risk of infection rises after a few days in situ. A microvial port forms part of the double-lumen catheter, and microvials are easily inserted and removed from this. The minimal time taken to fill a microvial sufficiently for analysis is 20 minutes. This is therefore the shortest interval between readings, and this frequency can be used if required in the early postoperative period. Once the microvial has been filled, it is easily inserted into an analyzer/monitor (CMA Iscus Microdialysis Monitor; CMA Microdialysis, Stockholm, Sweden) for analysis of glucose, lactate, pyruvate, and glycerol metabolite concentrations. The machine does this automatically and an onscreen graphical display shows values for these metabolites and their respective ratios (if required), reflecting viability of the tissue being monitored. The time delay from inserting the vial into the analyzer and subsequent readings is approximately 10 minutes. The results can be copied to a removable disk for subsequent downloading. A falling glucose and rising lactate-to-pyruvate ratio indicates anaerobic metabolism and thus indicates poFrom the Department of Oral and Maxillofacial Surgery, Queen Alexandra Hospital. Received for publication November 16, 2005; accepted January 12, 2006. Copyright ©2007 by the American Society of Plastic Surgeons