The benefits of separating patients from mechanical ventilation include decreasing the risk of ventilator-associated pneumonia and ventilator-induced lung injury, increasing patient comfort, and decreasing cost. These benefits, however, must be weighed against the complications of removing mechanical ventilation too soon, which include the potential for difficulty in re-establishing the airway and in complications related to impaired gas exchange.

Decisions to discontinue mechanical ventilation cannot be based on the clinical picture alone. “Foot of the bed” assessment is notoriously unreliable in predicting patient suitability for weaning and extubation. Studies have documented sensitivities as low as 35%, implying that often patients who may successfully wean are missed. Overall, clinical assessment is better at predicting those who will fail to wean but still has only a specificity of less than 80% at best. To this end, guidelines for ventilator weaning have been adopted by several professional organizations including the American College of Chest Physicians, the Society of Critical Care Medicine, and the American Association for Respiratory Care.

The guidelines are based on the best currently available evidence and recommend criteria to determine whom should be considered for discontinuation of ventilation, principles to guide that assessment, how to treat patients who fail their weaning assessment, as well as considerations for long-term ventilator dependence and the design and use of weaning protocols by nonphysicians. Criteria suggested prior to a formal assessment of discontinuation include evidence of some reversal of the underlying reason for the patient’s respiratory failure; adequate oxygenation (PaO₂/FIO₂ ≥200 mm Hg, PEEP ≤5–8 cm H₂O, FIO₂ ≤0.4–0.5, and pH ≥7.25;FIO₂, fraction of inspired oxygen; PEEP, positive end-expiratory pressure); hemodynamic stability; and patient ability to generate an inspiratory effort. Spontaneous breathing trials (SBTs) may be attempted when these criteria are met. Trials may be carried out on either T-piece or with so-called “minimal” ventilator settings of 5 cm H₂O continuous positive airway pressure (CPAP) and 5 cm H₂O pressure support. The decision to use one method or the other does not seem to affect outcome. The former has the advantage of removing all supportive positive pressure from the airway but has
the disadvantage of removing the patient from any apnea alarms on the ventilator. At these settings, conceptually CPAP will maintain recruitment of alveoli and pressure support will overcome the resistance to the ventilator circuit. The patient should be observed for respiratory pattern, quality of gas exchange, hemodynamic changes, and subjective comfort. Patients who pass an SBT of 30 to 120 minutes duration should be considered for separation from the ventilator. Patients successfully completing a 2-hour trial have a 90% chance of staying off mechanical ventilation at 48 hours. Patients who fail should be placed on a mode of ventilation that will provide rest and be reassessed for weaning in 24 hours. Other considerations should include the amount and quality of secretions combined with the patient’s ability to protect the airway and the potential for airway edema.