Technique and the rapid shallow breathing index.

No practice has changed more dramatically during my career than ventilator weaning. My training in the late 1980s and early 1990s was often spent at the bedside, debating whether to wean, when to do it, and which technique to use. Experts touted diverse modes and testing parameters, some rational, some not. Fortunately, critical care has evolved since then. The dangers of prolonged intubation are widely recognized, along with mandates to “liberate” patients quickly once respiratory failure resolves.1 The focus on gradual weaning has shifted to using spontaneous breathing trials for promptly identifying extubation candidates.2 Weaning protocols administered by non-physicians have achieved a status close to standard of care.3,4 A diverse array of screening tests have long played a role in weaning.5 Perhaps the most extensively investigated is the rapid-shallow-breathing index (RSBI), which reflects the respiratory pattern adopted by many patients who fail weaning trials.6,7 In its classic description,7 weaning candidates in the medical intensive care unit (MICU) were disconnected from the ventilator and breathed through a T-piece. Using a Wright spirometer, the RSBI was calculated by dividing the respiratory rate (in breaths/min) by the tidal volume (measured in liters). Compared to other parameters, an RSBI 105 breaths/min/L demonstrated a superior combination of sensitivity (97%), specificity (64%), positive predictive value (78%), and negative predictive value (95%) when predicting weaning success. The RSBI was also attractive because it was unaffected by patient effort, simple to measure, and easy to remember, especially when rounded off to 100. The RSBI has since been applied to diverse populations and settings, incorporated into weaning protocols, and used to predict outcomes ranging from tolerance of decreased ventilator support to weaning to extubation.5 Although many embrace the RSBI, others have questioned its value, leading to substantial debate about its role.4,5,8,9 The calculation and utility of the RSBI vary significantly with the population studied, the question asked, and the measurement technique used. With its high sensitivity and negative predictive value, the index is most accurate when used to identify patients likely to fail weaning, as opposed to those likely to succeed. The positive predictive value decreases after a week of mechanical ventilation.7 The RSBI’s value is limited when respiratory failure is caused by problems besides pulmonary strength-load imbalance,1 such as upper-airway obstruction or central-nervous-system disease.10,11 High RSBIs occur more commonly in the elderly and women with small endotracheal tubes, suggesting that different thresholds for success might be considered.12,13 Finally, practice variation may undermine the RSBI: some respiratory therapists use a T-piece, some use continuous positive airway pressure (CPAP), and some use pressure support to measure the RSBI, all of which could impact the measurements obtained.14 Unfortunately, variation in the way the RSBI has been used since its original description precludes a simple summary statement regarding its value and role.8