Significance of pathologic oxygen supply dependency in critically ill patients: Comparison between measured and calculated methods

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Abstract
Oxygen supply dependency at normal or high oxygen delivery rate has been increasingly proposed as a hallmark and a risk factor in critical illnesses. We hypothesized that as fas as an adequate oxygen delivery is provided, oxygen consumption, when determined by indirect calorimetry, is not dependent on oxygen delivery in critically ill patients whereas calculated oxygen consumption is associated with artefactual correlation of oxygen consumption and delivery. Oxygen delivery, oxygen consumption and their relationship were analyzed prospectively. Metabolic data gained from both measured and calculated methods were obtained simultaneously before and after volume loading. The study was completed in the intensive care unit as part of the management protocol of the patients. 32 consecutive patients entered the study and were divided into 3 groups according to a clinical condition known to favour oxygen supply dependency: sepsis syndrome, adult respiratory distress syndrome and acute primary liver failure. The rise in oxygen delivery was obtained by colloid infusion (oxygen flux test) performed in hemodynamically and metabolically stable patients. All were mechanically ventilated. No change in therapy was allowed during the test. Oxygen consumption was simultaneously evaluated by calculation (Fick Principle) and direct measurement using indirect calorimetry. Oxygen delivery was derived from the cardiac output (thermodilution) and arterial content of oxygen. Oxygen supply dependency was considered while observing an increase in oxygen delivery greater than 45 ml/min·m2. Irrespective of patient's clinical diagnosis and outcome, measured oxygen uptake remained unaltered by volume infusion whereas both oxygen delivery and calculated oxygen consumption increased significantly. Arterial lactate level>2 mmol/l and measured oxygen extraction ratio>25% failed to identify oxygen supply dependency when measured data were considered. Analysis of oxygen uptake, when measured by indirect calorimetry, failed to substantiate oxygen supply dependency in the vast majority of the critically ill patients irrespective of diagnosis and outcome. Mathematical coupling of shared variables accounted for the correlation between oxygen delivery and calculated oxygen consumption.

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