The oxygen cost of breathing (OCB) can be directly estimated during the weaning process from mechanical ventilation: it is a difference between whole-body oxygen uptake (VO 2 ) during spontaneous breathing and VO 2 during controlled ventilation, when respiratory muscles are supposed to be at rest. The recent development of new methods of indirect calorimetry allows an accurate and continuous measurement of VO 2 in intubated and mechanically ventilated patients. The value of the OCB is mainly dependent on the underlying pulmonary disease. Finally, this measurement provides an objective evaluation of the efficay of new modalities of partial ventilatory support, such as pressure support ventilation.Le cout en oxygene de la ventilation (COV) peut etre estime au cours du sevrage de la ventilation artificielle: il s'agit de la difference entre la consommation d'oxygene (VO 2 ) mesuree pendant une periode de ventilation spontanee et celle mesuree pendant une periode de ventilation controlee, entierement passive. Les methodes recentes de calorimetrie indirecte permettent une mesure precise et continue de la VO 2 chez les patients intubes et ventiles. La valeur du COV est tres dependante de la pathologie pulmonaire sousjacente. Enfin, cette mesure permet une evaluation objective de l'efficacite des modes de ventilation partielles, telles que l'aide inspiratoire.