MEASUREMENT OF OXYGEN-UPTAKE BY THE REVER SED FICK PRINCIPLE AND RESPIRATORY GAS MONITORING - DOES INTRAPULMONARY OXYGEN-UPTAKE ACCOUNT FORSYSTEMATIC DIFFERENCES BETWEEN METHODS

Automated measurements of respiratory gas exchange recently became ava ilable for the determination of oxygen uptake (VO2) in critically ill patients. Whereas these metabolic gas monitoring systems (MBM) are ass umed to measure total body VO2, the reversed Fick method in principle excludes intrapulmonary VO2. Previous clinical reports comparing VO2 m easured by the reversed Fick principle (VO2Fick) with VO2 measured by MBM (VO2MBM) found that VO2MBM was significantly greater than VO2Fick. It was suggested that these differences between methods represent VO2 of pulmonary and bronchial tissue, as intrapulmonary VO2 had been est imated to account for 15% of total body VO2 in dogs with experimental pneumonia. The objective of this study was to compare VO2Fick with VO2 MBM in patients with and without pneumonia and to assess the reproduci bility of both methods in critically ill patients. Method. With instit utional approval nine critically ill patients with acute pneumonia wer e studied under controlled mechanical ventilation. The diagnosis of pn eumonia was based on respective changes of chest X-rays, body temperat ure > 38-degrees-C, and WBC counts > 12,000/mm3. Inspiratory oxygen fr actions (FIO2) ranged from 0.3 to 0.6; all patients routinely received opioids and hypnotics. Complete muscle relaxation was achieved during the periods of measurement to avoid sudden changes in VO2 due to shiv ering or involuntary movements. Arterial and pulmonary-arterial blood samples were drawn simultaneously after aspiration of the sevenfold ca theter dead space. Measurements of haemoglobin concentration (Hb), fra ctional oxygen saturation (SO2), and O2 partial pressure (PO2) were pe rformed by use of a calibrated haemoximeter and blood gas analyser, re spectively; 2 x 5 thermodilution measurements of cardiac output (CO) w ere spread randomly over the respiratory cycle for each determination of VO2Fick. To minimise systematic errors of CO measurements, the CO c omputer was calibrated in an extracorporeal flow model using an electr omagnetic flowmeter. Calculations of VO2Fick were based on an oxygen b inding capacity of 1.39 ml/g Hb. Simultaneous measurements of VO2MBM w ere obtained by use of a Datex Deltratrac MBM that had been validated in vitro with a gas dilution model of respiratory gas exchange. Calibr ation of the MBM was performed prior to each measurement. Gas supply o f the respirator was provided by an external high-precision mixing dev ice to reduce errors in VO2 measurements that may arise from short-ter m oscillations in FIO2. All patients with pneumonia were studied on th ree consecutive days; thus, measurements from 27 days could be analyse d. On each day two sets of measurements were performed at an interval of 60 min to assess the reproducibility of differences between methods . During each set of measurements duplicate blood samples were drawn t wice, before and after thermodilution measurements of CO, to evaluate the short-term repeatability of VO2Fick. The beginning and the end of each set of measurements were marked in the computer record of the MBM to assess the respective repeatability of VO2MBM. Fifty control measu rements were performed in ten patients undergoing major neurosurgical procedures. None of these patients exhibited signs of pulmonary infect ion. Except for the number of repeated measures, all VO2 measurements were obtained in the same way as in the study group. Descriptive stati stical analysis was performed according to Bland and Altman; compariso ns between methods were done by multivariate analysis of variance for repeated measures. Results. Neither in the study group nor in the cont rol group could a significant difference between methods be demonstrat ed. In patients with pneumonia the mean difference between methods (VO 2Fick-VO2MBM) was 15.2 ml/min (4.2%); the double standard deviation of differences (2 SD) was 59.2 ml/min (19.2%). Control patients exhibite d a mean difference of 7.2 ml/min (3.1%); 2 SD was 41.1 ml/min (20.4%) . Duplicate determinations of VO2Fick and VO2MBM within one set of mea surements showed a repeatability coefficient (2 SD of differences betw een repeated measures) of 43.8 ml/min (13.2%) and 15.3 ml/min (5.1%), respectively. The large variation of duplicate measurements of VO2Fick was caused rather by the variability of arteriovenous O2 content dete rminations than by the variability of CO measurements. Discussion. The se results are in contrast to previous method comparison studies, whic h suggested that in infected lungs VO2 of pulmonary and bronchial tiss ue represents up to 15% of total body VO2. Since the mean differences between VO2Fick and VO2MBM did not differ between the two groups of pa tients, pulmonary infection did not seem to cause a considerable incre ase in intrapulmonary VO2. A minor effect of intrapulmonary VO2 on dif ferences between methods cannot be excluded because of the variability of data. The poor repeatability of VO2Fick measurements, however, see ms to limit the use of method comparison studies for estimation of int rapulmonary VO2.