Oxygen supply dependency in the critically ill--a continuing conundrum

There was little dispute that endotoxin treatment of experimental animals could recreate the O2 extraction defect that had been observed in critically ill patients. The remaining question was whether or not this necessarily signified pervasive tissue hypoxia. Some limitation to O2 diffusion in the tissues had been postulated because of known effects of endotoxin that ultimately result in damage to endothelium. We were unable to alter the critical DO2 or 0(2)ER in endotoxic dogs by manipulating the arterial PO2. This tended to rule against there being a diffusion limitation created by the endotoxin as a result of endothelial disruption or microvascular dysfunction. The results of the DCA and dopexamine experiments served to remind us that arterial lactate measurements may or may not indicate widespread tissue hypoxia. Sepsis, as emulated by endotoxin infusions, is also a metabolic disease that can cause inactivation of PDH and thus cause lactacidosis without tissue hypoxia. Regional measurements of lactate flux indicated that gut was hypoxic in spite of DO2 above critical because of maldistribution of blood flow between muscularis and mucosa. The questions persist of how much tissue hypoxia is caused by sepsis or endotoxin when DO2 is supported at supposedly adequate levels and whether there are marked regional differences. Such questions still await answers. Newer technological advances that permit assessment of tissue oxygenation by noninvasive methods, such as near infrared spectrophotometry or nuclear magnetic resonance measurement of tissue energy potential, may soon be feasible in critically ill patients. This kind of information will be of vast importance in designing the most effective therapeutic regimen.