How Mechanical Ventilation Measurement, Cutoff and Duration Affect Rapid Shallow Breathing Index Accuracy: A Randomized Trial

BACKGROUND: Decreased accuracy of the rapid shallow breathing index (RSBI) can stem from 1) the method used to obtain this index, 2) duration of mechanical ventilation (MV), and 3) the established cutoff point. The objective was to evaluate the values of RSBI determined by three different methods, using distinct MV times and cutoff points. METHODS: This prospective study included 40 subjects. Before extubation, three different methods were employed to measure RSBI: pressure support ventilator (PSV) (PSV = 5 - 8 cm H(2)O; positive end-expiratory pressure (PEEP) = 5 cm H(2)O) (RSBI_MIN), automatic tube compensation (ATC) (PSV = 0, PEEP = 5 cm H(2)O, and 100% tube compensation) (RSBI_ATC), and disconnected MV (RSBI_SP). The results were analyzed according to the MV period (less than or over 72 h) and to the outcome of extubation (< 72 h, successful and failed; > 72 h successful and failed). The accuracy of each method was determined at different cutoff points (105, 78, and 50 cycles/min/L). RESULTS: The RSBI_MIN, RSBI_ATC, and RSBI_SP values in the group < 72 h were 38 ± 18, 45 ± 26 and 55 ± 22; in the group > 72 h, RSBI_SP value was higher than those of RSBI_ATC and RSBI_MIN (78 ± 29, 51 ± 19 and 39 ± 14) (P < 0.001). For patients with MV > 72 h who failed in removing MV, the RSBI_SP was higher (93 ± 28, 58 ± 18 and 41 ± 10) (P < 0.000), with greater accuracy at cutoff of 78. CONCLUSION: RSBI_SP associated with cutoff point < 78 cycles/min/L seems to be the best strategy to identify failed extubation in subjects with MV for over 72 h.