The effect of flow trigger on rapid shallow breathing index measured through the ventilator.

BACKGROUND: The rapid shallow breathing index (RSBI) has the best predictive value to assess readiness for weaning from mechanical ventilation. At many institutions, this index is conveniently measured without disconnecting the patient from the ventilator, but this method may be inaccurate. Because modern ventilators have a base flow in the flow trigger mode that may provide a substantial help to the patient, we hypothesized that the RSBI is significantly decreased when measured through the ventilator with flow trigger even without continuous positive airway pressure (CPAP) and pressure support (PS). METHODS: The RSBI was calculated using the values of minute ventilation and respiratory rate obtained either through the digital display of the ventilator or from a digital ventilometer. The RSBI was measured using 3 different methods: method 1, CPAP and PS both 0 cm H2O with flow trigger; method 2, CPAP and PS both 0 cm H2O without flow trigger; and method 3, using digital ventilometer. RESULTS: A total of 165 measurements per method were obtained in 80 adult patients in the medical intensive care unit (MICU). The RSBI (breaths/min/L) values were 70.2 ± 26.5 with method 1, 85.4 ± 30.3 with method 2, and 80.1 ± 30.3 with method 3. The RSBI was significantly decreased using mechanical ventilation with flow trigger as compared with mechanical ventilation without flow trigger (P < .0001) or digital ventilometer (P < .0001). When method 1 was compared with methods 2 and 3, the RSBI decreased by 17% and 12%, respectively. CONCLUSIONS: The RSBI measurement is significantly decreased by the base flow delivered through modern ventilators in the flow trigger mode. If RSBI is measured through the ventilator in the flow trigger mode, the difference should be considered when using RSBI to assess readiness for weaning from mechanical ventilation.