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validity of the RSBI (respiratory rate [f min1 ]/tidal volume [VT; in milliliters]) on T-piece in terms of predicting successful weaning; the actual index number used may vary according to the manner in which it is measured.4 How to Measure the Rapid Shallow Breathing Index A study comparing the RSBI value in patients on pressure support versus continuous positive airway pressure (CPAP) versus a T-piece trial showed, not surprisingly, that the value of RSBI is significantly altered by the mode of ventilatory support that a patient is on when the RSBI is measured. It showed mean values of 46 with pressure support, 63 with CPAP, and 100 during a T-piece trial.4 This reflects the fact that pressure support or CPAP can affect VT and therefore f/VT ratio. Therefore, if the RSBI is to be used, it is advisable to either calculate it under the conditions originally applied by Yang and Tobin, namely with a handheld spirometer attached to the endotracheal tube 1 minute into a T-piece trial after disconnecting from the ventilator, or acknowledge that different cut offs for the index may need to be used in its interpretation.4 WHAT IS THE BEST VENTILATOR MODE TO WEAN ON? Esteban et al compared 4 modes for weaning in patients who experienced respiratory distress during a 2-hour spontaneous breathing trial: synchronized intermittent mandatory ventilation (SIMV; mandatory rate gradually reduced), PSV (pressure support gradually reduced), daily multiple T-piece trials, and a daily single T-piece trial.5 They found the mean duration of ventilation in the SIMV group was 5 days, versus 4 days for the PSV group and 3 days for both T-piece groups. They concluded that, in this study, the T-piece spontaneous breathing trial patients weaned more quickly than the PSV group, who weaned more quickly than the SIMV group. However, another study comparing the same methods of weaning showed the best results with PSV as the weaning mode in terms of highest rates of successful weaning, shortest duration of weaning, and shortest ICU length of stay, with little difference between SIMV and spontaneous breathing trials.6 Finally, a third study comparing low levels of pressure support against a T-piece system in patients undertaking a 2-hour spontaneous breathing trial prior to considering extubation found both methods to be equally effective in terms of predicting successful extubation with similar reintubation rates, although the weaning trials themselves failed more often in the T-piece group (22% versus 14%).7 Whilst the reason for this discrepancy in findings is perhaps not easy to explain, the heterogeneity in the evidence base is not uncommon and demonstrates that there is still much that needs clarification on this subject. Many research studies set the duration of spontaneous breathing trials to be 2 hours. Esteban et al compared T-piece trials of 2 hours versus 30 minutes and reported that a 30-minute trial was as useful as a 2-hour trial in predicting weaning.8 Subscribe to ATOTW tutorials by visiting www.wfsahq.org/resources/anaesthesia-tutorial-of-the-week ATOTW 372 — Evidence-Based Practice of Weaning from Ventilator: A Review (6 February 2018) Page 2 of 6 In summary, there appears to be little difference between the different modes of ventilation when weaning but the evidence that does exist would suggests that PSV or intermittent T-piece trials are effective and superior to SIMV as a weaning mode. DIFFICULT WEANING Factors responsible for difficult weaning may be grouped under the following headings: 1. Respiratory a. Poor lung compliance (e.g., oedema, consolidation, fibrosis, atelectasis, pulmonary secretions) b. Poor chest wall compliance (e.g., pleural effusion, obesity) c. Increased resistive load (e.g., bronchoconstriction, dynamic hyperinflation in COPD, blocked artificial airway, airway swelling or obstruction) 2. Neuromuscular a. Decreased central respiratory drive (e.g., coma, obesity hypoventilation syndrome, myxoedema) b. Decreased airway reflexes (e.g., toxin- or drug-related, bulbar neurological dysfunction) c. Neuromuscular weakness (e.g., critical illness, neuromyopathy, myasthenia) 3. Neuropsychiatric a. Delirium, anxiety, sleep disturbance 4. Metabolic a.