Comparison of 3 different methods used to measure the rapid shallow breathing index

Abstract

PurposeRapid shallow breathing index (RSBI) is conveniently measured through the ventilator. If continuous positive airway pressure (CPAP) is used, it may change the RSBI value. We measured the RSBI with a handheld spirometer and through the ventilator, with and without CPAP, to assess differences. Materials and MethodsRapid shallow breathing index was measured in 3 ways: (1) CPAP 0 cm H2O and fraction of inspired oxygen (Fio2) 0.4, (2) CPAP 5 cm H2O and Fio2 0.4, and (3) ventilator disconnected and Fio2 0.21. Tidal volume and respiratory frequency were recorded from ventilator monitor values in methods 1 and 2, and from a handheld spirometer and observed respiratory frequency, in method 3. ResultsA total of 170 measurements, each using all 3 methods, were obtained from 80 patients admitted to a medical intensive care unit. The mean RSBI values for methods 1, 2, and 3 were 98.1 ± 58.7, 87.6 ± 51.2, and 108.3 ± 65.3, respectively (P < .001). The RSBI decreased by 9.4% when using CPAP 0 cm H2O and by 19.1% when using CPAP 5 cm H2O. ConclusionsThe RSBI values measured through the ventilator with CPAP 5 cm H2O are much lower than the values measured with a handheld spirometer. Even the RSBI values measured with CPAP 0 cm H2O are significantly lower. This is attributable to the base flow delivered by some ventilators. The difference must be taken into account during weaning assessment.