To determine if the frequency tidal volume ratios (f/VT) obtained during spontaneous breathing on continuous positive airway pressure/pressure support (CPAP/PS) ventilation are correlated to those obtained during T-piece (TP) trials.
Medical ICU patients ventilated for at least 24 hours were eligible. On any day a given patient met criteria for weaning of mechanical ventilation, sequential 30-minute spontaneous breathing trials (SBT) were done on CPAP/PS (5 cm H2O/5 cm H2O) and TP. The order of the trials was reversed on alternating days to control for fatigue and learning effects. Respiratory frequency and tidal volume were measured at the end of each half hour period. Patients becoming tachypneic (> 35 breaths/min), tachycardic (> 120 beats/min), hemodyanmically unstable, or having new onset arrhythmias had their SBT aborted and were placed back on full ventilator support until the next day. Patients were extubated if their f/VT on CPAP/PS was < 105 breaths/minute/liter (b/m/l). Success was defined as remaining off ventilatory support for at least 48 hours.
55 paired values of f/VT were obtained; they showed a high degree of correlation (Pearson’s 0.701, p < 0.001). From these paired values, a line was extrapolated, and it was determined that a TP f/VT of 105 b/m/l corresponded to a CPAP/PS f/VT of 69 b/m/l.CONCLUSIONS: The f/VT measured using CPAP/PS (5 cm H2O/5 cm H2O) is highly correlated to that obtained during TP breathing. Our suspicion that patients’ f/VT would be lower on CPAP/PS than TP was confirmed.
An f/VT of < 70 b/m/l may be a more appropriate criterion for extubation than < 105 b/m/l when patients undergo an SBT while on low level CPAP/PS. Further studies will be necessary to determine if this more stringent f/VT is actually a better predictor of extubation success.
E.D. Anderson, None.