Significance of pathologic oxygen supply dependency in critically ill patients: Comparison between measured and calculated methods |
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Authors: | G Hanique T Dugernier P F Laterre A Dougnac J Roeseler M S Reynaert MD |
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Institution: | (1) Intensive Care Department, Saint-Luc University Hospital, Hippocrate Ave, 10, B-1200 Brussels, Belgium;(2) Intensive Care Unit, Saint-Pierre Hospital, Ottignies, Belgium |
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Abstract: | 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 patients whereas calculated oxygen consumption
is associated with artefactual correlation of oxygen consumption and delivery.
Design 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.
Setting The study was completed in the intensive care unit as part of the management protocol of the patients.
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.
Intervention 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.
Measurements and results 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.
Conclusion 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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Keywords: | Adult respiratory distress syndrome Acute hepatic failure Indirect calorimetry Lactate Multiple organ failure Oxygen delivery Oxygen uptake Sepsis syndrome |
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