Chapter 44

• Patients are candidates for liberation from mechanical ventilation when gas exchange or circulatory disturbances that precipitated respiratory failure have been reversed.
• More than half of all critically ill patients can be liberated successfully from mechanical ventilation after a brief trial of spontaneous breathing on the first day that reversal of precipitating factors is recognized. Gradual reduction of mechanical support, termed weaning, frequently is unnecessary and can prolong the duration of mechanical ventilation.
• Once a patient has been liberated from the ventilator, extubation should follow if mechanisms of airway maintenance (e.g., cough, gag, and swallow) are sufficient to protect the airway from secretions. Whether to extubate is a decision that follows successful liberation from the ventilator.
• In patients who fail their first trial of spontaneous breathing, attention should turn to defining and treating the pathophysiologic processes underlying failure.
• Weaning regimens that use ventilator modes with the goal of improving respiratory muscle endurance have not been proved to expedite liberation, but data from animal models suggest that “exercise" may be beneficial. Approaches attempting to exercise the respiratory muscles should not substitute for daily interrogation of readiness for spontaneous breathing.
• One weaning regimen, the gradual reduction of intermittent mandatory breaths, prolongs patients' time on mechanical ventilation.
• Liberation from mechanical ventilation is achieved most expeditiously if patients with a stable circulation (not on pressors or with evolving myocardial infarction) and adequate oxygenation are given a trial of spontaneous breathing (T piece or pressure support ≤7 cm H2O) each day. Patients remain on ventilators unnecessarily when clinicians do not put this simple plan in place.
• Patients who have had most correctable factors addressed and remain marginal with regard to ventilatory capacity in most circumstances should undergo a trial extubation rather than remain intubated for protracted periods. Noninvasive positive-pressure ventilation is extremely useful in these patients to transition them to fully spontaneous breathing following extubation.

Respiratory failure occurs when the lungs and respiratory pump fail to exchange oxygen and carbon dioxide adequately (see Chap. 31). Hypoxemic (type 1) respiratory failure usually results from flooding or collapse of the distal airspaces leading to intrapulmonary shunt and inadequate arterial oxygenation despite generous concentrations of inspired oxygen (see Chap. 38). Hypercapnic (type 2) respiratory failure results from inability to sustain sufficient alveolar ventilation to eliminate the CO2 produced from aerobic metabolism. Perioperative respiratory failure, a special case of types 1 and 2, results when postoperative pain and recumbency result in atelectasis and hypoxemia or when medications to alleviate pain reduce respiratory drive, leading to hypercapnia. Shock-related respiratory failure is another special case in which the underperfused respiratory muscles are unable to compensate for the acidosis resulting from inadequate global tissue perfusion. Mechanical ventilation substitutes for the respiratory pump until these disturbances have been reversed adequately to allow resumption of spontaneous breathing and gas exchange.

Institution of positive-pressure ventilation can be lifesaving for the disorders just described but is also associated with many complications (Table ...

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