Objectives: To compare the rapid shallow breathing
index (RSBI) under different ventilatory support settings prior to
Design: Prospective study.
Setting: Cardiac surgery unit at a university
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 cmH2O 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.
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.