The first fundamental problem in weaning is deciding when to initiate the process. With widespread use of patient-assisted ventilatory modes, it is difficult to stipulate when mechanical ventilation changes from primary support to assistance in weaning. Nevertheless, delayed discontinuation of mechanical ventilation or repeated failed attempts at extubation could be avoided if there were criteria to predict weaning success or failure. The accuracy of these predictors of weaning outcome ( Table 2) are conflicting owing to differences in definitions, study methodologies, and cut-off values to separate success from failure. This is not surprising, given the many determinants of ventilator dependence. One proposed predictor, the ratio of frequency to tidal volume ( Yang„.a.nd..Tob.!0 1991), is simple to measure and is an index of rapid shallow breathing. Nonetheless, there is no consistent evidence to support the usefulness of any set of criteria to hasten the process of weaning ( Slutsky 1994).
The decision to start weaning is still based on subjective bedside assessments, particularly during unsupported or minimally supported breathing. Rapid shallow breaths, use of accessory respiratory muscles, and prominent paradoxical movement of the abdomen and rib cage are signs of probable decompensation.
Table 2 Reported predictors of weaning outcome: cut-off variables suggesting weaning success
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