SIGNIFICANCE OF PATHOLOGICAL OXYGEN-SUPPLY DEPENDENCY IN CRITICALLY ILL PATIENTS - COMPARISON BETWEEN MEASURED AND CALCULATED METHODS

Objective: 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 pa tients whereas calculated oxygen consumption is associated with artefa ctual correlation of oxygen consumption and delivery. Design: oxygen d elivery, oxygen consumption and their relationship were analyzed prosp ectively. Metabolic data gained from both measured and calculated meth ods were obtained simultaneously before and after volume loading. Sett ing: the study was completed in the intensive care unit as part of the management protocol of the patients. Patients: 32 consecutive patient s entered the study and were divided into 3 groups according to a clin ical condition known to favour oxygen supply dependency: sepsis syndro me, adult respiratory distress syndrome and acute primary liver failur e. Intervention: the rise in oxygen delivery was obtained by colloid i nfusion (oxygen flux test) performed in hemodynamically and metabolica lly stable patients. All were mechanically ventilated. No change in th erapy was allowed during the test. Measurements and results: oxygen co nsumption was simultaneously evaluated by calculation (Fick Principle) and direct measurement using indirect calorimetry. Oxygen delivery wa s derived from the cardiac output (thermodilution) and arterial conten t of oxygen. Oxygen supply dependency was considered while observing a n increase in oxygen delivery greater than 45 ml/min.m(2). Irrespectiv e 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 lacta te level>2mmol/l and measured oxygen extraction ratio>25% failed to id entify oxygen supply dependency when measured data were considered. Co nclusion: analysis of oxygen uptake, when measured by indirect calorim etry, failed to substantiate oxygen supply dependency in the vast majo rity of the critically ill patients irrespective of diagnosis and outc ome. Mathematical coupling of shared variables accounted for the corre lation between oxygen delivery and calculated oxygen consumption.