An oxygen consuming lung model was used for evaluation and validation of a technique for metabolic gas exchange measurements during controlled ventilation. The technique comprised a Servo 900 C ventilator (Siemens) and separate oxygen and carbon dioxide analysers (Beckman). Measurements of oxygen consumption and carbon dioxide production were made either by measuring inspired and expired ventilation and gas fractions in these volumes or by measuring gas fractions and calculating expired ventilation from inspired by transformation (Haldane). Irrespective of the FIO2, measured values correlated well with lung model settings: measured values were within +/- 2% of simulated. When Haldane transformation was used with an FIO2 of 0.5 there was a significant underestimation of oxygen consumption. Carbon dioxide production values correlated well irrespective of the FIO2 used or method of measurement of ventilation volume. Metabolic gas exchange measurements by measuring both inspired and expired ventilation volumes may be used when inert gases are not in equilibrium, for example during nitrous oxide anaesthesia.

A system for metabolic gas exchange has been used during nitrous oxide-opioid anaesthesia incorporating a Servo Ventilator 900 C and external analysers for oxygen and carbon dioxide. Oxygen consumption and carbon dioxide excretion were calculated as differences in content between inspired and expired minute ventilation. Nitrous oxide uptake was calculated similarly, assuming it was the only other gas present in addition to oxygen and carbon dioxide. The mean value for oxygen consumption was 3.25 ml kg-1 min-1, declining by 8% during the 2 h of anaesthesia. The formula for the best fit curve of nitrous oxide uptake was 18.3 . t-0.48 ml kg-1 min-1 when FIN2O was 0.7. To simplify measurement procedures and avoid measurements of expiratory volume, we also calculated metabolic gas exchange when expiratory minute ventilation was expressed as a function of inspiratory minute volume and nitrous oxide uptake. The latter value was obtained from the overall best fit curve for nitrous oxide uptake.

The overall performance of the device is satisfactory and well comparable with any equipment tested. It allows near-continuous non-invasive monitoring of EE, VO2, VCO2, VA, VD/VT in ventilated, critically ill patients, providing a rationale for ventilator settings and nutritional support.

Oxygen uptake and carbon dioxide excretion during aorto-coronary bypass surgery were studied in seven patients by indirect calorimetry and compared to blood-gas based measurements. Medium-high dose fentanyl, droperidol and midazolam were used for maintaining anaesthesia. During the period of extracorporeal circulation no external oxygenator was used. Circulation was maintained by two pumps by-passing the left and right heart respectively and the patient's lungs were ventilated with O2/N2 using a Servo 900C ventilator. For indirect calorimetric measurements gas concentrations were analysed by Beckman instruments and gas volumes were measured by the Servo 900C ventilator. Oxygen uptake and carbon dioxide excretion decreased by 31% and 39%, respectively. For invasive measurements during extracorporeal circulation, arterial and venous blood gases and pump flow were used. Using pump flow instead of cardiac output when calculating oxygen uptake circumvented errors in thermodilution measurements. There was a good correlation (r = 0.88) between the invasive and the indirect calorimetric measurements. Further, there was a good correlation between naso-pharyngeal temperature and indirect calorimetric measurements of oxygen uptake (r = 0.87).