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respiratory system. From these, the McMaster AHCPR group identified 8 parameters that had consistently significant LRs to predict successful discontinuation in several studies. Some of these measurements are made while the patient is still receiving ventilatory support; others require an assessment during a brief period of spontaneous breathing. These parameters, their threshold values, and the range of reported LRs are given in Table 4. It should be noted that despite the staTable 4. Measurements Performed Either While Patient Was Receiving Ventilatory Support or During a Brief Period of Spontaneous Breathing That Have Been Shown to Have Statistically Significant LRs to Predict the Outcome of a Ventilator Discontinuation Effort in More Than One Study Parameter Studies (n) Threshold Values Positive LRs Range Measured on Ventilator V˙ E 20 10–15 L/min 0.81–2.37 NIF 10 20 to 30 cm H2O 0.23–2.45* PImax 16 15 to 30 cm H2O 0.98–3.01 P0.1/PImax 4 0.30 2.14–25.3 CROP score 2 13 1.05–19.74 Measured During a Brief Period of Spontaneous Breathing RR 24 30–38 breaths/min 1.00–3.89 VT 18 325–408 mL (4–6 mL/kg) 0.71–3.83 f/VT ratio 20 60–105/L 0.84–4.67 LR likelihood ratio V˙ E minute ventilation *1 study reported a likelihood ratio (LR) of 35.79. NIF negative inspiratory force (maximum inspiratory pressure) PImax maximum inspiratory pressure P0.1 mouth occlusion pressure 0.1 s after the onset of inspiratory effort CROP index including compliance, respiratory rate, oxygenation, and pressure RR respiratory rate VT tidal volume f/VT respiratory rate/tidal volume ratio Table 5. Frequency of Tolerating an SBT in Selected Patients and Rate of Permanent Ventilator Discontinuation Following a Successful SBT Study Patients Receiving SBT Patients Tolerating SBT n (%) Patients Discontinuing Ventilation Patients Having Ventilation Reinstituted n (%) Esteban et al105 546 416 (76) 372 58 (16) Ely et al108 113 88 (78) 65 5 (4) Dojat et al110 38 22 (58) 22 5 (23) Esteban et al101 246 192 (78) 192 36 (19) Esteban et al102 270* 237 (89) 237 32 (14) Esteban et al102 256† 216 (84) 216 29 (13) SBT spontaneous breathing trial *30 min SBT †120 min SBT EVIDENCE-BASED GUIDELINES FOR WEANING AND DISCONTINUING VENTILATORY SUPPORT RESPIRATORY CARE • JANUARY 2002 VOL 47 NO 1 75 tistical significance of these parameters, the generally low LRs indicate that the clinical applicability of these parameters alone to individual patients is low. Although assessments that are performed while a patient is receiving substantial ventilatory support or during a brief period of spontaneous breathing (Tables 3 and 4) can yield important information about discontinuation potential, assessments that are performed during a formal, carefully monitored SBT appear to provide the most useful information to guide clinical decision-making regarding discontinuation. Indeed, because of the efficacy and safety of a properly monitored SBT (see below), the assessments in Table 4 that are performed to predict SBT outcome are generally unnecessary. In concept, the SBT should be expected to perform well, as it is the most direct way to assess a patient’s performance without ventilatory support. Indeed, the evidence for this concept is quite strong. As can be seen in Table 5, multiple studies have found that patients tolerant of SBTs that are 30 to 120 min in length were found to have successful discontinuations at least 77% of the time. Because the 12 to 42% of patients in the Table 5 studies failing the SBT were not systematically removed from ventilatory support, the ability of a failed SBT to predict the need for ventilator dependence (ie, negative predictive value) cannot be formally assessed. Indeed, it is conceivable that iatrogenic factors such as endotracheal tube discomfort or continuous positive airway pressure (CPAP) demand-valve insensitivity/unresponsiveness, rather than true ventilator dependence, caused the failure of the SBT in at least some of these patients.125 Thus, it is thus unclear how many patients who are unable to tolerate an SBT would still be able to tolerate long-term ventilator discontinuation. Although the

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