Can the delta Rapid Shallow Breathing Index predict respiratory failure in spontaneously breathing patients receiving positive pressure ventilation?

Introduction

The Rapid Shallow Breathing Index (RSBI) is derived by dividing the respiratory rate by the tidal volume. Previous work by this group has shown an association between an elevated RSBI (>105) and the need for noninvasive ventilation. Hypothesis: an improvement in RSBI, defined as a decrease from baseline (that is, ΔRSBI) can predict whether patients will develop respiratory failure either when receiving conventional therapy (non-CPAP) or continuous positive airway pressure (CPAP).

Methods

A secondary analysis of a prospective randomized controlled trial of patients receiving CPAP plus conventional therapy (CPAP group) versus conventional therapy alone (non-CPAP group) for undifferentiated dyspnea. The tidal volume was determined utilizing volumetric capnography with an end-tidal carbon dioxide flow sensor while receiving treatment. There were 26 patients in each group (CPAP and non-CPAP). Comparisons of ΔRSBI between the CPAP and non-CPAP groups were made. Simple t tests were performed to compare ΔRSBI values between groups. All tests were two-sided and assessed at the 0.05 type-I error rate.

Results

The mean ΔRSBI in the CPAP group at t = 0–5 minutes, 0–10 minutes, and 0–15 minutes were 79.1, 96.2, and 93.6, respectively. For the time period t = 5–10 minutes the mean ΔRSBI was 15.8, and for t = 10–15 minutes the mean was -2.5. In the non-CPAP group the mean ΔRSBI for t = 0–5, t = 0–10, t = 0–15, t = 5–10 and t = 10–15 minutes were 6.7, -30.2, -5.4, -36.9, and 11.4, respectively. Patients randomized to CPAP had a greater improvement in ΔRSBI compared with patients receiving conventional therapy. Change from 0 to 5 minutes (P = 0.01), from 5 to 10 minutes (P = 0.03) and from 10 to 15 minutes (P = 0.42). The largest improvement in RSBI was seen in the first 10 minutes. There were more intubations in the non-CPAP (n = 8) group compared with the CPAP group (n = 4).

Conclusion

ΔRSBI may be used as a noninvasive technique to predict respiratory failure in patients receiving CPAP. The largest improvement in respiratory function in this group occurred during the first 10 minutes of treatment with CPAP. Further studies are needed to compare ΔRSBI with conventional predictive techniques.