EVALUATION OF OXYGEN-UPTAKE AND DELIVERY IN CRITICALLY ILL PATIENTS -A STATISTICAL REAPPRAISAL

Objective: The evaluation of oxygen consumption (VO2) and oxygen deliv ery (DO2) has gained increasing importance in the monitoring of critic ally ill patients. They can be obtained from either direct measurement s or by indirect calculations based on the Fick principle. However the choice between these two approaches remains controversial. The aim of the study was to investigate whether these 2 methods provide similar results, and if not, to define the best one in terms of reproducibilit y. Design: Oxygen delivery and oxygen consumption were prospectively a nalyzed in 171 consecutive critically ill patients. Metabolic data wer e obtained simultaneously. Setting: The study was completed in the int ensive care unit as part of the management of the patients studied. Pa tients: A first ''group'' of 279 evaluations was carried out in 73 con secutive critically ill patients. The results were subsequently valida ted by 423 observations performed in the 98 following patients. Interv entions: Before and during each evaluation, the patients were kept in stable hemodynamic and metabolic conditions. All were mechanically ven tilated. Measurements and results: VO2 was evaluated by calculation (F ick principle) and direct measurement using indirect calorimetry Cardi ac output was both measured by the thermodilution technique and calcul ated (Fick principle) and the data were used for the evaluation of the directly measured and indirectly calculated DO2. For both VO2 DO2 the agreement between direct and indirect evaluations was not satisfactor y. Differences as great as 55 ml/min.m(2) and 267 ml/min.m(2) between simultaneously measured and calculated VO2 and DO2 respectively may be expected. Finally, the indirect calculated methods were less reproduc ible than the measured ones. These observations resulted mainly from t he cumulative effects of the random errors in the metabolic data enter ing into the calculation of VO2 and DO2. Conclusions: Our data suggest ed that the indirect calculation (Fick equation) and the direct measur ement (indirect calorimetry, thermodilution) of both VO2 and DO2 did n ot provide similar results. Direct measurements are more reproducible methods and must be preferred.