Intracerebral microdialysis in neurointensive care: the use of urea as an endogenous reference compound

When evaluating the results of intracerebral microdialysis, the in vivo performance of the microdialysis probe must be considered, because this determines the fraction of the interstitial concentration obtained in the microdialysis samples. The in vivo performance is dependent on several factors, for example, the interstitial compartment's diffusion characteristics, which may vary during the course of the acute brain injury process. In the present study the authors investigated the method of controlling the in vivo performance by using urea, which is evenly distributed in all body fluid compartments, as an endogenous reference compound and by comparing the urea levels in three compartments: the brain (CNS), abdominal subcutaneous tissue (SC), and blood serum (BS).

Sixty-nine patients with traumatic brain injury or cerebrovascular disease were included in the study. In 63 of these patients a CNS probe was used, an SC probe was used in 40, and both were used in 34. Urea was measured by enzymatic methods, at bedside for the microdialysis samples and in routine clinical laboratory studies for the BS samples, with the probe calibrated to give identical results. The correlation coefficient for CNS/SC urea was 0.88 (2414 samples), for CNS/BS urea it was 0.89 (180 samples), and for SC/BS urea it was 0.98 (112 samples).

Urea levels in the CNS, SC, and BS were highly correlated, which supports the assumption that urea is evenly distributed. The CNS/SC urea ratio can therefore be used for monitoring the CNS probe's in vivo performance. Fluctuations in other substances measured with microdialysis are probably caused by biological changes in the brain, as long as the CNS/SC urea ratio remains constant.