B. Royston

We evaluated three commercial indirect calorimetry devices which are used during artificial ventilation. Commercial butane, which had an RQ of 0.615, consumes 6.40 ml oxygen, and produces 3.94 ml CO2/1 ml, was burned in a gas-tight combustion chamber in conjunction with the ventilation of a lung model. During combustion, the flow rate of butane was measured with a soapfilm flowmeter for the calculation of reference values of oxygen consumption (VO2) and CO2 production (VCO2). To investigate the effect of oxygen concentration on the accuracy of these instruments, measurements were carried out at FIO2 values of 0.3, 0.4, 0.5, and 0.6 with a fixed ventilation mode (tidal volume 500 ml; respiratory rate 16 breath/min, intermittent positive-pressure ventilation). For the Datex Deltratrac Metabolic Monitor, the mean relative errors of measured VO2, VCO2, and RQ were all within 4.0%, 2.9%, and 4.0%, respectively. For the Engstrom Metabolic Computer, the corresponding values were 1.4%, 5.7%, and 6.0%, and for the SensorMedics MMC Horizon, 5.7%, 2.9%, and 5.9%.

OBJECTIVE: To compare measurement of oxygen consumption (VO2) by spirometry and the reversed Fick method. DESIGN: Within-patient comparison using simultaneous measurements by the two methods, one previously calibrated on a metabolic simulator. PATIENTS: Twenty sets of observations on eight patients (57 to 83 yrs) requiring mechanical ventilation in a critical care unit. INTERVENTIONS: None during or immediately before the measurements. MEASUREMENTS AND MAIN RESULTS: Duplicate pairs of measurements of VO2 were made with a previously validated spirometric technique and the reversed Fick method (Qt[CaO2 - CVO2]), where Qt is cardiac output, CaO2 is arterial oxygen content, and CVO2 is mixed venous oxygen content. The coefficient of variation of the difference between duplicate measurements by the former technique was only 2.53% compared with 10.4% for the latter. The mean VO2 measurement by the spirometric method was 285.7 +/- 40.7 (SD) mL/min standard temperature and pressure, dry (STPD) and for the reversed Fick method, the mean VO2 measurement was 249.3 +/- 38.5 mL/min STPD. The mean difference was 36.4 +/- 28.5 mL/min STPD (p less than .001). CONCLUSIONS: The repeatability of the spirometric method was four times better than the reversed Fick method. The latter gave a significantly lower value that probably, in part, reflects the VO2 of the lung, which is included in the spirometric method but not in the reversed Fick measurement.

Rats were exposed to 100% oxygen for up to 60 h to determine early changes in lung permeability leading to the development of pulmonary edema. The time course of development of increased solute flux was assessed by the clearance of 99mTc-labeled diethylenetriamine pentaacetate (99mTc-DTPA) from the lung and the accumulation of 125I-labeled albumin (125I-albumin) in the lung. These end points were related to the development of pulmonary edema by the measurement of the wet-to-dry weight ratio of the lung and the weight of fluid in the pleural cavity. No significant changes occurred until 48 h of hyperoxia, when sharp increases in both indexes of lung permeability and wet-to-dry weight ratio occurred. By 60 h of exposure, pleural effusions had developed. The volume of this effusion was significantly correlated to both 99mTc-DTPA clearance and 125I-albumin flux.

The rate of inactivation of hepatic methionine synthase by nitrous oxide has been determined in 22 patients undergoing laparotomy during general anesthesia, including 70% nitrous oxide. Mean half-time of inactivation was 46 min. Metabolic consequences of nitrous oxide are, thus, critically dependent on the duration of anesthesia, and are unlikely to be significant during exposures of less than 40 min. Inactivation of methionine synthase is very much more rapid in the rat exposed to 50% nitrous oxide, with a half-time of 5.4 min.