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anesthesia or neuromuscular blockade, the term “ventilator-dependent” is usually reserved for patients with a need for mechanical ventilation beyond 24 hours or by the fact that they have failed to respond during discontinuation attempts. Under these circumstances, the clinical focus should be not only on ventilator management but also should include a search for all of the possible reasons (especially potentially reversible ones) that may explain the ventilator dependency. Recommendation 1. In patients requiring mechanical ventilation for 24 hours, a search for all the causes that may be contributing to ventilator dependence should be undertaken. This is particularly true in the patient who has failed attempts at withdrawing the mechanical ventilator. Reversing all possible ventilatory and nonventilatory issues should be an integral part of the ventilator discontinuation process. Table 1. Grades of Evidence Grade Description A Scientific evidence provided by well-designed, well-conducted, controlled trials (randomized and nonrandomized) with statistically significant results that consistently support the guideline recommendation B Scientific evidence provided by observational studies or by controlled trials with less consistent results to support the guideline recommendation C Expert opinion supported the guideline recommendation, but scientific evidence either provided inconsistent results or was lacking EVIDENCE-BASED GUIDELINES FOR WEANING AND DISCONTINUING VENTILATORY SUPPORT 70 RESPIRATORY CARE • JANUARY 2002 VOL 47 NO 1 Evidence (Grade B) There are a number of specific reasons why patients may be ventilator-dependent (Table 2). Determining which factor or factors may be involved in a given patient requires both clinical awareness of these factors as well as focused clinical assessments. The search for the underlying causes for ventilator dependence may be especially important if previously unrecognized, but reversible, conditions are discovered. Neurologic Issues: The ventilatory pump controller in the brainstem is a rhythm and pattern generator, which receives feedback from cortical, chemoreceptive and mechanoreceptive sensors. The failure of this controller can come from several factors.5–12 These factors can be either structural (eg, brainstem strokes or central apneas) or metabolic (eg, electrolyte disturbances or sedation/narcotic usage13,14). The failure of the peripheral nerves also can be the result of either structural factors15 or metabolic/drug factors.16,17A unique neurologic dysfunction that also could cause ventilator dependence is obstructive sleep apnea, in which an artificial airway may be necessary to maintain airway patency.10,11 Respiratory System Muscle/Load Interactions: Often, patients who exhibit ventilator dependence do so because there appears to be a mismatch between the performance capacity of ventilatory pump and the load placed on it (ie, the capacity/load imbalance hypothesis).18–23 There is ample evidence that ventilatory pump performance may be impaired in ventilator-dependent patients because ventilatory muscles are weak. This may be a consequence of atrophy and remodeling from inactivity.2,24 It may also be a consequence of injury from overuse and of insults associated with critical illness neuropathy and myopathy.25–29 A number of drugs (eg, neuromuscular blockers, aminoglycosides, and corticosteroids) also can contribute to myopathy,17,30–32 as can various metabolic derangements (see below). Finally, dynamic hyperinflation can put ventilatory muscles in a mechanically disadvantageous position.33 In a number of studies, patients who failed to respond to a withdrawal from mechanical ventilation tended to be weaker (ie, they had a lower performance capacity) than those who succeeded,34–49 but, in general, the within-group variability in respiratory muscle strength was too large to justify general conclusions. Ventilatory muscle fatigue also could contribute to poor muscle performance. However, the role of fatigue in ventilator dependence is not well understood, and the studies performed to date21,26,50–54 have failed to delineate the sensitivity and specificity of specific fatigue tests in ventilator-dependent

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