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the McMaster AHCPR report were much smaller than the Esteban et al107 and Brochard et al,109 and both suggested that pressure support was easier to reduce than IMV alone. Because none of these studies offer evidence that gradual support strategies are superior to stable support strategies between SBTs, the clinical focus for the 24 hours after a failed SBT should be on maintaining adequate muscle unloading, optimizing comfort (and thus sedation needs), and avoiding complications, rather than aggressive ventilatory support reduction. Ventilator modes and settings can affect these goals.143 Assisted modes of ventilation (as opposed to machinecontrolled modes) are generally preferable in this setting Table 7. Modes of Partial Ventilator Support Mode Patient Work Adjusted By SIMV Number of machine breaths supplied (ie, the fewer the number of machine breaths, the more spontaneous breaths are required) PSV Level of inspiratory pressure assistance with spontaneous efforts SIMV PSV Combining the adjustments of SIMV and PSV VS PSV with a “guaranteed” minimum VT (PSV level adjusts automatically according to clinician VT setting) VAPS(PA) PSV with “guaranteed” minimum VT (additional flow is supplied at end inspiration if necessary to provide clinician VT setting) MMV SIMV with a “guaranteed” V˙ E (machine breath rate automatically adjusts according to clinician V˙ E setting) APRV Pressure difference between inflation and release (ie, the less the pressure difference, the more spontaneous breaths are required) SIMV synchronized intermittent mandatory ventilation PSV pressure support ventilation VS volume support VAPS(PA) volume-assured pressure support (pressure augmentation) VT tidal volume MMV mandatory minute ventilation V˙ E minute ventilation APRV airway pressure-release ventilation EVIDENCE-BASED GUIDELINES FOR WEANING AND DISCONTINUING VENTILATORY SUPPORT 78 RESPIRATORY CARE • JANUARY 2002 VOL 47 NO 1 because they allow patient muscle activity and some patient control over the ventilatory pattern. Although good clinical supporting data are lacking, these features may help to avoid muscle disuse atrophy24 and may reduce sedation needs in these types of patients.143 With assisted modes, sensitive/ responsive ventilator triggering systems,144–147 applied PEEP in the presence of a triggering threshold load from autoPEEP,132,133 flow patterns matched to patient demand,148–152 and appropriate ventilator cycling to avoid air trapping153,154 are all important to consider in achieving patient comfort and minimizing imposed loads. In recent years, several ventilator support modes (volume support,155 adaptive support ventilation (ASV),155,156 minimum minute ventilation (MMV),155 and a knowledgebased system for adjusting pressure support110,157) have been developed in an attempt to “automatically” wean patients by feedback from one or more ventilator-measured parameters. Minimum minute ventilation set at either 75% of measured minute ventilation158 or to a carbon dioxide target,120 and a knowledge-based system for adjusting pressure support110,157 all have been shown to be capable of automatically reducing support safely in selected populations. However, none of these has been compared to the daily SBT approach described above. Moreover, the premises underlying some of these feedback features (eg, that an ideal volume can be set for volume support or that an ideal ventilatory pattern based on respiratory system mechanics can be set for ASV) may be flawed, especially in sick patients. Indeed, potentially flawed feedback logic may, in fact, delay support reduction. Further work is clearly needed to establish the role (if any) of these automated approaches. There has been increasing interest in the use of noninvasive positive-pressure ventilation (NPPV) in recent years. Although NPPV has been used primarily as a method to avoid intubation, it has also been used as a technique to facilitate the discontinuation of invasive ventilatory support. Data from the pooling of results of 2 prospective, randomized controlled trials159,160 in patients with chronic respiratory disease suggest the need for