Previous studies on oxygen consumption (VO2) during weaning from mechanical ventilation assumed that an increase in VO2 (delta VO2) reflected oxygen consumption by respiratory muscles (VO2RESP), and proposed delta VO2 as a weaning predictor. We measured VO2 CO2 production (VCO2) and plasma catecholamines in 20 short-term ventilated patients during weaning by SIMV and CPAP. delta VO2 as a percentage of VO2 during spontaneous ventilation (delta VO2%) ranged from 4.8% to 41.5%. VCO2 also increased and correlated with VO2. Plasma adrenaline and noradrenaline increased significantly to levels known to produce considerable increases in metabolic rate. Mean arterial pressure and heart rate concomitantly increased, but spontaneous minute ventilation decreased. Thus, since the increased plasma catecholamines are calorigenic, the assumption that delta VO2 represents VO2RESP is incorrect. Although mean delta VO2% of successfully weaned patients was significantly less than that of failure-to-wean patients, the wide scatter of individual values in the latter group excludes delta VO2% as an accurate weaning predictor.

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Laryngeal mask airways and tracheal tubes were studied to determine both their resistance to constant gas flows and additional inspiratory work during simulated inspiration. Laryngeal mask airways imposed less resistance and required lower additional inspiratory work compared with the corresponding sized tracheal tubes. If inspiratory loading during anaesthesia is an important consideration, then the laryngeal mask airway may be preferable to a tracheal tube.

-aryTwenty patients having laparoscopic cholecystectomies were studied for changes in thoracic and lung compliance at 5, 10 and l5mmHg of intra-abdominal pressure. Fijteen mmHg of intra-abdominal pressure decreased the thoracic and the lung static compliance by 49 and 39% respectively. This was unaffected by the degree of head elevation. During the procedure the end-tidal carbon dioxide increased by a mean of 1.6 kPa at 1 h and 2.0 kPa at 2 h. This did not appear to be related to either the mean intra-abdominal pressure or the total volume of carbon dioxide insugated. The physiological changes and implications for anaesthetic practice are considered.