Patients who fail a weaning trial develop hypercapnia as a result of alveolar hypoventilation, which, in turn, is caused by an imbalance between the respiratory muscle load and capacity. In some patients, especially those with obstructive lung diseases, respiratory muscle performance is impaired as a result of dynamic hyperinflation and paradoxical motion of the rib cage and abdomen. Worsening of pulmonary mechanics causes further embarrassment of the respiratory muscles and can lead to marked alterations of oxygen use by the peripheral tissues. The development of rapid shallow breathing together with worsening of pulmonary mechanics results in inefficient clearance of COcf152cf1 during a failed weaning attempt.