The microdialysis technique was introduced experimentally in animal studies in the 1970s to determine the concentrations of low-molecular-weight compounds in the interstitial fluid of the brain.[1, 2] The technique was used in humans in the 1980s to assess the status of tissues into which they had been inserted before changes in blood or plasma levels could be noted.
The technique is based on the passive diffusion of substances according to their concentration gradients from the extracellular fluid to the dialysate. The catheter has a tubular, semipermeable dialysis membrane and has a function very similar to the passive function of a capillary blood membrane. The catheter is perfused with an electrolyte solution such as normal saline. The perfusate is chosen to allow the diffusion of selected substances so that equilibration takes place with surrounding tissues. In this way, microdialysis provides continuous monitoring of metabolic changes in tissues. Larger pore catheters may require the addition of albumin or dextran to the solution to prevent fluid loss into tissues. The size of the pores in the catheter determines how large the molecules to be measured can be. The smaller pores (20 kDa) can measure glucose, glycerol, pyruvate, and lactate, and the larger pores (100 kDa) can measure inflammatory cytokines and some drugs. This is opening the door to studying drug levels directly at the level of the drug target and to not relying on plasma concentrations to determine the pharmacodynamic and pharmacokinetic profiles of a therapeutic agent. This may enhance the management of immunosuppressant drug therapy in the future through the measurement of the drug concentration at the effector site. This unique technology can also be used to measure target site concentrations of antibiotics or anticancer drugs in different tissues and organs and lead to more effective dosing or delivery systems. Experimental catheters have been made with up to a 3000-kDa cutoff for measuring even larger molecules.
The limitations of microdialysis catheters include the effect of local tissue trauma occurring during placement, but this has been shown to clear after 30 minutes of perfusion. The probes are usually perfused with aqueous solutions such as Ringer's solution, so they are conceptually limited to the study of hydrophilic molecules.
Continuous microdialysis measurements of metabolic changes in the interstitial space offer the opportunity for the earlier detection of adverse changes in an organ before reflections in the peripheral blood or any clinical symptoms develop. Cerebral microdialysis is being used clinically in patients with traumatic brain injury as a unique source of pathological changes at a molecular level in the brain. Cerebral ischemia is being detected early through the measurement of lactate/pyruvate ratios in the dialysate. The viability of microvascular flaps, especially when direct visualization is difficult, can be determined by continuous microdialysis monitoring.
In liver transplantation, major complications that present early include preservation injury, acute rejection, and ischemia. The differential diagnosis rests on clinical suspicion, nonspecific clinical symptoms, and laboratory abnormalities such as elevations of serum aminotransferase, alkaline phosphatase, and bilirubin levels, but these values are nonspecific and do not correlate with the severity of rejection. Despite close posttransplant follow-up for enzyme elevations and intermittent ultrasound Doppler screening of blood flow, hypoperfusion may not be detected early, and this can result in preventable organ failure. Acute rejection occurs in approximately 20% to 40% of recipients, and the gold standard for confirmation is liver biopsy. This is an invasive procedure that is not without risk and may not be tolerated by the pediatric population.[7, 8]
Tønnessen's team from Oslo has developed an increasingly robust data set on the safety, accuracy, and precision of using microdialysis catheters in humans.[9-11] The catheters were implanted in the right and left liver lobes just before wound closure, and monitoring of the dialysate proceeded continually for several days after transplantation. The catheters were placed with a split-needle technique to set the introducer, through which the catheter with a diameter of 0.6 mm was passed. The dialysis tubing was tunneled through the abdominal wall. The catheters were perfused with a fluid containing dextran and electrolytes at a rate of 1 μL/minute by a microinjection pump (CMA 107, M Dialysis AB, Stockholm, Sweden). Lactate, pyruvate, glucose, and glycerol—the metabolic markers—were analyzed at the bedside every 1 to 2 hours. The analyzer calculated the lactate/pyruvate ratio automatically (Iscus, M Dialysis AB). These data were used to identify and also differentiate ischemia from rejection. Ischemia was recognized by an increase in lactate levels and an increased lactate/pyruvate ratio. However, the specificity for identifying rejection was not robust, so inflammatory mediators were also collected in the dialysate with catheters with a 100-kDa cutoff. The dialysate fluid was collected and frozen twice daily and was analyzed with an immunosorbent assay (Kit II, BD Biosciences, San Jose, CA).
These results have been reported in a series of 3 articles published in Liver Transplantation.[9-11] They have demonstrated that dialysate analysis can distinguish acute rejection from ischemia. Under ischemic conditions, there will be anaerobic metabolism and an increase in lactate levels as well as an increase in the lactate/pyruvate ratio. This was also reported by Isaksson et al. for patients who underwent portal triad clamping and whose intrahepatic metabolism was continually assessed by microdialysis. They demonstrated an increase in the lactate/pyruvate ratio in response to ischemia. This has been confirmed by other authors.[13-15]
In patients who sustained acute rejection, Tønnessen et al. demonstrated simultaneous increases in lactate and pyruvate with a stable lactate/pyruvate ratio, which reflected an increase in aerobic metabolism due to lymphocyte activation. Now using larger pore membranes, they were able to measure inflammatory cytokine levels and identified 2 biomarkers: chemokine (C-X-C motif) ligand 10, which was prominent in rejection, and complement activation C5a, which was elevated exclusively in ischemic grafts. This did not confirm their earlier data, which showed that an increase in inducible protein 10 reflected a normal pathophysiological response to transplantation, whereas interleukin-8 and C5a were increased only in patients with rejection. However, this was a study analyzing only 3 patients. They demonstrated that microdialysis catheters with a 100-kDa cutoff recovered chemokines as well as metabolic molecules and, therefore, could be used to detect both metabolic and inflammatory parameters after liver transplantation A recent study reported an examination of 73 patients who had undergone liver transplantation. However, further validation with multicenter clinical trials is necessary.
The advantages of this technology are that it is minimally invasive, it can be used in the pediatric population, and patients can be transferred from the intensive care unit with the catheters in place. The catheters can be withdrawn by gentle traction. The only type of complication that the authors reported was minor bleeding at the insertion of the catheters. No bleeding or infection was noted upon the withdrawal of the catheters, which were kept in place for up to 21 days.
The data from these studies need further validation from large multicenter clinical trials, but this method shows great promise for identifying in real time ischemia and rejection and so will be a valuable tool in the early detection of impaired graft function for early intervention. This minimally invasive technology is very attractive not only for adult patients but especially for pediatric recipients. It has great promise for reducing early graft loss and also for further determining the significance of a positive crossmatch.