VO2 and DO2 can be calculated from cardiac output determined with the help of a pulmonary artery flotation catheter and from arterial and mixed venous blood gases (reverse Fick method). The development of direct measurements of VO2 and VCO2 as a difference in gas concentrations between inspired and expired gases has allowed a non-invasive assessment of whole body metabolism. This has revealed a difference11-13 between the values derived from thermodilution techniques and those obtained from indirect calorimetry. Indirect calorimetry overestimates the values obtained by the reverse Fick method, which is explained by pulmonary oxygen consumption not assessed by the reverse Fick method of calculation.
Measurement of VO2 is performed by mass spectroscopy or by paramagnetic oxygen analyzers; measurement of VCO2 is usually infrared-based. The accuracy of commercially available metabolic monitors is guaranteed up to a normobaric FiO2 of 0.6.
The hyperbaric environment poses some methodological problems. Arterial blood gases measured under normobaric conditions from samples obtained under hyperbaric conditions need to be interpreted with regard to misleading oxygen tension values, as it is impossible to calibrate blood gas machines accurately for gas tensions higher than ambient barometric pressure. The only way to obtain exact results is to measure blood gases inside the hyperbaric chamber. However, Weaver14 has shown that normobaric blood gas measurements may provide reliable results in healthy individuals at rest exposed to HBO if a correction formula is used.
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