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* values are expressed as median (+ interquartile range)

"""defined as the need for reintubation within 72 hours after planned extubation Note: Comparison of the outcome between parallel study groups for Epstein et al. [13]. Group 1 consisted of 151 patients who were allowed to progress to a spontaneous breathing trial (SBT) after a liberal daily screen (including a PaO2/FiO2 >150, PEEP <5 cmH2O, hemodynamic stability on minimal vasopressors, arousable to command, and requiring endotracheal tube suctioning no more than every 2 hours) regardless of the result of their f/VT. Group 2 consisted of 153 patients who were not allowed to undergo an SBT unless they passed the daily screening criteria and their recorded f/VT was <105.

* values are expressed as median (+ interquartile range)

"""defined as the need for reintubation within 72 hours after planned extubation Note: Comparison of the outcome between parallel study groups for Epstein et al. [13]. Group 1 consisted of 151 patients who were allowed to progress to a spontaneous breathing trial (SBT) after a liberal daily screen (including a PaO2/FiO2 >150, PEEP <5 cmH2O, hemodynamic stability on minimal vasopressors, arousable to command, and requiring endotracheal tube suctioning no more than every 2 hours) regardless of the result of their f/VT. Group 2 consisted of 153 patients who were not allowed to undergo an SBT unless they passed the daily screening criteria and their recorded f/VT was <105.

Table 3. Criteria to qualify for spontaneous breathing trial (SBT)

Patients receiving mechanical ventilation for respiratory failure should undergo a formal assessment of discontinuation potential if the following criteria are satisfied:

- Evidence for some reversal of the underlying cause of respiratory failure;

- Adequate oxygenation (i.e.; PaO2/FiO2 ratio >150 to 200; requiring positive end-expiratory pressure [PEEP] < 5 to 8 cmH2O; FiO2 0.4 to 0.5); and pH (e.g., > 7.25);

- Hemodynamic stability, as defined by the absence of active myocardial ischemia and the absence of clinically significant hypotension (i.e.; a condition requiring no vasopressor therapy or therapy with only low-dose vasopressors such as dopamine or dobutamine, < 5 ^g/kg/min), and

- The capability to initiate an inspiratory effort.

Note: The decision to use these criteria must be individualized. Some patients not satisfying all of the above criteria (e.g., patients with chronic hypoxemia values below the thresholds cited) may be ready for attempts at discontinuation of mechanical ventilation [2]. On the other hand, there are times in which a patient will meet these criteria and the clinician will appropriately elect to continue without an SBT. These criteria are guidelines only and should not be considered as absolutes.

Those who are advocates ofweaning parameters attempt to identify patients who have a high likelihood of'fatigue' soon after extubation. Laghi et al. [14] recently tried to clarify the role of 'low-frequency fatigue' as it may relate to diaphragm weakness and weaning failure. This entity is also known as long-lasting fatigue, which is a result of muscle injury and may last for days. Twitch transdiaphragmatic pressures were carefully recorded on 11 weaning failure patients and 8 weaning success patients both before and after SBTs. Despite greater mechanical load and diaphragmatic effort experienced in the weaning failure group, no evidence of low-frequency fatigue was found. This small study lays the foundation for advancing our understanding of the diaphragm and its role in the physiology of weaning failure.

In considering the recent body of literature reviewed above, weaning parameters, may best be regarded as 'common-sense' safety criteria that should be applied individually prior to initiating a patient's SBT. Table 3 lists the range of such criteria recommended by the AHCPR [2]. As detailed in this report, any single weaning parameter used in isolation serves poorly and can only be used as an adjunct to clinical experience. Indeed, the likelihood ratios (the expression of the odds that a given test result will be present in a patient with a given condition) for all of these parameters are less than 3, implying only moderate shifts in post-test probability after their use (Table 4).

Therefore, weaning parameters should be inclusive, rather than exclusive, and patients should be allowed to progress to spontaneous breathing at the earliest possible time point. As well, when parameters are used as a screening process prior to a more definitive SBT, they should be performed with a high degree of standardization. This may be possible if one person performs all of the measures [15]. Unfortunately, this degree of accuracy is a major obstacle in current daily practice. In an interesting investigation to determine the actual methods used by registered

Table 4. Predictive values for individual weaning parameters*

Predictor

Likelihood ratio

Sensitivity (%)

Specificity (%)

Minute ventilation

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