PLASMA-CATECHOLAMINES AND OXYGEN-CONSUMPTION DURING WEANING FROM MECHANICAL VENTILATION

Previous studies on oxygen consumption (V(o)2) during weaning from mechanical ventilation assumed that an increase in V(O)2 (DELTA-V(O)2) reflected oxygen consumption by respiratory muscles (V(O)2RESP), and proposed DELTA-V(O)2 as a weaning predictor. We measured V(O)2 CO2 production (V(CO)2) and plasma catecholamines in 20 short-term ventilated patients during weaning by SIMV and CPAP. DELTA-V(O)2 as a percentage of V(O)2 during spontaneous ventilation (DELTA-V(O)2 %) ranged from 4.8% to 41.5%. V(CO)2 also increased and correlated with V(O)2. 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-V(O)2 represents V(O)2RESP is incorrect. Although mean DELTA-V(O)2% 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-V(O)2% as an accurate weaning predictor.