Effect of continuous positive airway pressure on the rapid breathing index in patients cardiac surgery

Objectives: To compare the rapid shallow breathing index (RSBI) under different ventilatory support settings prior to extubation trials. Design: Prospective study. Setting: Cardiac surgery unit at a university hospital. Patients: A total of 33 coronary artery bypass grafting patients ready for extubation. Interventions: Enrolled patients received a continuous positive airway pressure (CPAP) trial of 5 cm H2O and fraction of inspired oxygen (FIO2) of 40% (condition 1), a CPAP trial of 5 cm H2O and FIO2 of 21% (condition 2), and a 1-min spontaneously breathing room air trial without ventilatory support (condition 3). These trials were applied in random order. Measurements and main results: Average values of respiratory frequency and tidal volume were measured under the three experimental conditions in all patients immediately prior to extubation. The RSBIs were determined for each patient under each condition; the average RSBIs under conditions 1, 2, and 3 were compared for significance. The average RSBIs (+/- SD) were significantly smaller under condition 1 (34 +/- 13) and condition 2 (36 +/- 14) compared to condition 3 (71 +/- 24). There was no significant difference in RSBI between conditions 1 and 2. Conclusions: The administration of 5 cm H2O of CPAP can influence the determination of the RSBI. In contrast, changes in FIO2 have no effect on RSBI determination. We speculate that using the RSBI during CPAP may mislead the clinician into premature discontinuation of mechanical ventilation. Consequently, different threshold values for the RSBI should be derived for different ventilatory support levels.