[Inverse fick's principle in comparison to measurements of oxygen consumption in respiratory gases. Does intrapulmonary oxygen uptake account for differences shown by different system methods?].
Automated measurements of respiratory gas exchange recently became available for the determination of oxygen uptake (VO2) in critically ill patients. Whereas these metabolic gas monitoring systems (MBM) are assumed to measure total body VO2, the reversed Fick method in principle excludes intrapulmonary VO2. Previous clinical reports comparing VO2 measured 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 estimated to account for 15% of total body VO2 in dogs with experimental pneumonia. The objective of this study was to compare VO2Fick with VO2MBM in patients with and without pneumonia and to assess the reproducibility of both methods in critically ill patients. METHOD. With institutional approval nine critically ill patients with acute pneumonia were studied under controlled mechanical ventilation. The diagnosis of pneumonia was based on respective changes of chest X-rays, body temperature > 38 degrees C, and WBC counts > 12,000/mm3. Inspiratory oxygen fractions (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 shivering or involuntary movements. Arterial and pulmonary-arterial blood samples were drawn simultaneously after aspiration of the sevenfold catheter dead space. Measurements of haemoglobin concentration (Hb), fractional oxygen saturation (SO2), and O2 partial pressure (PO2) were performed by use of a calibrated haemoximeter and blood gas analyser, respectively; 2 x 5 thermodilution measurements of cardiac output (CO) were spread randomly over the respiratory cycle for each determination of VO2Fick. To minimise systematic errors of CO measurements, the CO computer was calibrated in an extracorporeal flow model using an electromagnetic flowmeter. Calculations of VO2Fick were based on an oxygen binding capacity of 1.39 ml/g Hb. Simultaneous measurements of VO2MBM were obtained by use of a Datex Deltatrac MBM that had been validated in vitro with a gas dilution model of respiratory gas exchange. Calibration of the MBM was performed prior to each measurement. Gas supply of the respirator was provided by an external high-precision mixing device to reduce errors in VO2 measurements that may arise from short-term oscillations in FIO2. All patients with pneumonia were studied on three consecutive days; thus, measurements from 27 days could be analysed. 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 twice, 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 measurements were performed in ten patients undergoing major neurosurgical procedures. None of these patients exhibited signs of pulmonary infection. Except for the number of repeated measures, all VO2 measurements were obtained in the same way as in the study group. Descriptive statistical analysis was performed according to Bland and Altman; comparisons between methods were done by multivariate analysis of variance for repeated measures. RESULTS. Neither in the study group nor in the control group could a significant difference between methods be demonstrated. In patients with pneumonia the mean difference between methods (VO2Fick-VO2MBM) was 15.2 ml/min (4.2%); the double standard deviation of differences (2 SD) was 59.2 ml/min (19.2%).
Weyland
A
A
Zentrum Anaesthesiologie-, Rettungs- und Intensivmedizin, Georg-August-Universität Göttingen.
Weyland
W
W
Sydow
M
M
Weyland
C
C
Kettler
D
D
D001784:Blood Gas Analysis / Q000379:methods*; D006452:Hemoglobinometry; D006801:Humans; D008168:Lung / Q000378:metabolism*; D010101:Oxygen Consumption / Q000502:physiology*; D011014:Pneumonia / Q000503:physiopathology; D011659:Pulmonary Gas Exchange / Q000502:physiology*
Inverses Ficksches Prinzip im Vergleich zu Messungen der Sauerstoffaufnahme aus respiratorischen Gasen. Sind systematische Methodendifferenzen Ausdruck eines erhöhten intrapulmonalen Sauerstoffverbrauchs?