Many patients admitted to the intensive care unit have respiratory failure and thus require mechanical ventilation. Weaning patients from mechanical ventilation after their primary disease process has been treated can be difficult in approximately 30% of patients. Inadequacies in pulmonary gas exchange and in the performance of the respiratory muscle pump are the most common causes for failure to wean. Assessing whether a patient can be weaned from mechanical ventilation involves two major factors: (1) examining the patient for evidence of an increase in the work of breathing, and (2) measuring spontaneous breathing variables. Although different modalities have been used in weaning patients from mechanical ventilation, none has been shown to be more successful than repeated trials of spontaneous breathing.