ETIOLOGY OF EXTUBATION FAILURE AND THE PREDICTIVE VALUE OF THE RAPID SHALLOW BREATHING INDEX

Failure of weaning from mechanical ventilation is thought to result fr om an imbalance between respiratory muscle capacity and respiratory de mand. The ratio of respiratory rate to tidal volume (f/VT, rapid shall ow breathing index) during spontaneous unsupported respiration increas es when this imbalance exists, and may predict the success or failure of weaning from mechanical ventilation. Using f/VT, Yang and Tobin dem onstrated a positive predictive value (PPV) of 0.78 (f/VT less than or equal to 105 and weaning success) (1). To define the etiology of the 20% false-positive rate (FPR, f/VT less than or equal to 105 and weani ng failure), 94 patients who had an f/VT determined prior to extubatio n were studied prospectively. Of 84 patients with an f/VT < 100, 14 re quired reintubation within 72 h of extubation (FPR = 0.17, PPV = 0.83) . Extubation in 13 of these 14 cases failed because of congestive hear t failure, upper airway obstruction, aspiration, encephalopathy, or th e development of a new pulmonary process. Only one patient needed rein tubation solely because of the original respiratory process. Of 10 pat ients extubated with an f/VT greater than or equal to 100, four requir ed reintubation, all because of the underlying respiratory process. Th is study confirms the high PPV for an f/VT < 100. The FPR of approxima tely 0.20 is best explained by extubation failure caused by processes for which f/VT is physiologically or temporally unlikely to predict su ccess or failure. The negative predictive value (f/VT greater than or equal to 100 but extubation success) for f/VT may be lower than previo usly reported.