Effect of pressure support ventilation and positive end expiratory pressure on the rapid shallow breathing index in intensive care unit patients

Objective: We compared rapid shallow breathing index (RSBI) values under various ventilatory support settings prior to extubation. Design and setting: Prospective study in the intensive care unit at a university hospital. Patients: Thirty six patients ready for extubation. Interventions: Patients were enrolled when receiving pressure support ventilation (PSV) of 5 cmH(2)O, PEEP of 5 cmH(2)O, and FIO2 of 40% (PS). Subsequently each patient received a trial of PSV of 0 cmH(2)O, PEEP of 5 cmH(2)O, and FIO2 of 40% (CPAP), a trial of PSV of 0 cmH(2)O, PEEP of 5 cmH(2)O and FIO2 of 21% (CPAP-R/A), and a 1-minute spontaneously breathing room air trial off the ventilator (T-piece). Trials were carried out in random order. Measurements and results: Respiratory frequency (f) and tidal volume (V-T) were measured during PS, CPAP, CPAP-R/A, and T-piece in all patients. RSBI (f/V-T) was determined for each patient under all experimental conditions, and the average RSBI was compared duringPS, CPAP, CPAP-R/A, and T-piece. RSBI was significantly smaller during PS (46 +/- 8bpm/l), CPAP (63 +/- 13bpm/l) and CPAP-R/A (67 +/- 14bpm/l) vs. T-piece (100 +/- 23bpm/l). There was no significant difference in RSBI between CPAP and CPAP-R/A. RSBI during CPAP and CPAP-R/A were significantly smaller than RSBI during T-piece. In all patients RSBI values were less than 105 bpm/l during PS, CPAP, and CPAP-R/A. However, during T-piece the RSBI increased to greater than 105 bpm/l in 13 of 36 patients. Conclusions: In the same patient the use of PSV and/or PEEP as low as 5 cmH(2)O can influence the RSBI. In contrast, changes in FIO2 may have no effect on the RSBI.