Microdialysis: an innovative new tool to detect endotoxin tolerance

Metabolic changes in different organs in the critical state are often subtle and remain unrecognized with conventional hemodynamic and metabolic parameters. Since the development of the microdialysis technique, which mimics the passive function of a capillary blood vessel, it is now possible to examine the interstitial space of intact tissue.

Preconditioning an animal with low doses of endotoxin has been shown to induce endotoxin hyperresponsiveness, the so-called 'endotoxin tolerance'. The exact pathophysiological mechanism involved has still not been fully elucidated. It appears to represent an essential mechanism to prevent unreasonable expansion of inflammatory or indiscriminate immune and metabolic responses before specific antibodies are developed. Up to this study, no data were available about the effect of endotoxaemia with and without endotoxin preconditioning regarding regional metabolic monitoring by the microdialysis technique.

Materials and methods

Ten pigs (25 ± 8 kg) were randomly assigned to a control group (n = 5) and a pretreated group (n = 5) before endotoxaemia was induced. Pretreatment consisted of injecting incremental doses of endotoxin (SAE) on days 5–2 before the experiment ranging from 5 ng/kg on day 5, 10 ng/kg on day 4, 30 ng/kg on day 3 to 50 ng/kg on day 2. The control group received saline injections at the same time intervals. The day prior to the experiment, neither group received any injection.

Discussion

The main findings of this study describe various metabolic consequences of endotoxin preconditioning prior to lethal endotoxin shock, but also the effects of endotoxaemia per se on local metabolism in three tissue compartments. This was only possible by using sequential microdialysis technique.

Molecules in the tissue (extracellular fluid) surrounding the microdialysis capillary diffuse through the semipermeable part of the capillary to the dialysate, which is pumped through the capillary. Changes in the concentrations of a substrate in the surrounding milieu are reflected by the concentration in the dialysate. Thereby, sampling of the dialysate at different time intervals, and subseqeuent analysis of the concentration, gives an idea of a substrate's concentration in the extracellular space over the observation time.

The role of cellular metabolism during sepsis is a highly controversial issue. While some studies suggest that tissue blood flow is reduced during endotoxaemia (leading to cellular oxygen debt and, consequently, to anaerobic metabolism), other studies suggest the septic insult to be a cellular dysoxia.

Our data clearly confirm that endotoxaemia is accompanied by an increase in metabolism in various tissues and in systemic oxygen consumption.