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

Purpose: Rapid 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 methods: Rapid shallow breathing index was measured in 3 ways: (1) CPAP 0 cm H(2)O and fraction of inspired oxygen (Fio(2)) 0.4, (2) CPAP 5 cm H(2)O and Fio(2) 0.4, and (3) ventilator disconnected and Fio(2) 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.

Results: A 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 H(2)O and by 19.1% when using CPAP 5 cm H(2)O.

Conclusions: The RSBI values measured through the ventilator with CPAP 5 cm H(2)O are much lower than the values measured with a handheld spirometer. Even the RSBI values measured with CPAP 0 cm H(2)O are significantly lower. This is attributable to the base flow delivered by some ventilators. The difference must be taken into account during weaning assessment.