Variation in the rapid shallow breathing index associated with common measurement techniques and conditions.

BACKGROUND The rapid-shallow-breathing index (RSBI) is widely used to evaluate mechanically ventilated patients for weaning and extubation, but it is determined in different clinical centers in a variety of ways, under conditions that are not always comparable. We hypothesized that the value of RSBI may be significantly influenced by common variations in measurement conditions and technique. METHODS Sixty patients eligible for a weaning evaluation after >or=72 hours of mechanical ventilation were studied over 15 months in a medical intensive care unit. RSBI was measured while the patients were on 2 different levels of ventilator support: 5 cm H2O continuous positive airway pressure (CPAP) versus T-piece. RSBI was also calculated in 2 different ways: using the values of minute ventilation and respiratory rate provided by the digital output of the ventilator, versus values obtained manually with a Wright spirometer. Finally, RSBI was measured at 2 different times of the day. RESULTS RSBI was significantly less when measured on 5 cm H2O CPAP, compared to T-piece: the medians and interquartile ranges were 71 (52-88) breaths/min/L versus 90 (59-137) breaths/min/L, respectively (P<.001). There were no significant differences in the value of RSBI obtained using ventilator-derived versus manual measures of the breathing pattern. RSBI was also not significantly different in the morning versus evening measurements. CONCLUSIONS RSBI can be significantly affected by the level of ventilator support, but is relatively unaffected by both the technique used to determine the breathing pattern and the time of day at which it is measured.