['To determine whether propofol affects spontaneous breathing patterns in critically ill patients recovering from respiratory failure during initial attempts at liberation from mechanical ventilation.', 'Observational cohort study.', 'Ten critically ill patients in the medical intensive care unit of a 300-bed community teaching hospital.', 'Demographic and physiologic variables, including respiratory frequency and minute volume, were recorded while patients breathed on continuous positive airway pressure (of 5 cm H(2)O) during and 20-30 min after cessation of propofol infusion. The ratio of respiratory frequency to tidal volume (rapid shallow breathing index or RSBI) was computed for both sets of measurements and values measured during and after propofol infusion were compared.', 'The mean (+/- SEM) age was 54.0 +/- 5.2 years and the mean (+/- SEM) APACHE II (Acute Physiology and Chronic Health Evaluation) score was 18.4 +/- 2.0. During propofol infusion, patients breathed with a significantly lower tidal volume (0.245 vs 0.342 L, p = 0.006), higher RSBI (155.6 vs 114.8 breaths/min/L, p = 0.004), and tended to breathe more rapidly (34.3 vs 31.7 breaths/min, p = 0.18) than at 20-30 min after stopping propofol infusion. Three of 10 patients had RSBIs that were unfavorable (> 105 breaths/min/L) for weaning while on propofol but that became favorable (< 105 breaths/min/L) 20-30 min following cessation of propofol. The weight-adjusted dose of propofol was not correlated with the changes in any of the variables.'] Is propofol infusion associated with a higher rapid shallow breathing index in patients preparing to wean from mechanical ventilation?