The ventilatory inefficiency measured as VE/VCO2 slope is increased in patients who fail to spontaneous breathing trial

Rationale: An increase in respiratory rate (f) associated with a tidal volume (VT) reduction is observed in some patients who fail a spontaneous breathing trial (SBT). It may be measured with the rapid shallow breathing index (f/VT) and used as weaning failure screening at the onset or during the SBT. This inefficient breathing pattern is associated to rising PaCO2 despite constant minute ventilation (VE), however, direct assessment of VI along the SBT has not been reported. In cardiopulmonary exercise testing VI is usually assessed with the VE to CO2 production ratio (VE/VCO2) or its slope. We hypothesized that if VI measured as VE/VCO2 slope is increased in patients that fail to T-piece SBT. Methods: We prospectively performed a one-hour SBT with a CO2/flow sensor between the endotracheal tube and the T-piece in 77 patients. Follow variables were recorded from volumetric capnography monitor during the SBT: f, VT, VE and VCO2 (both in L/min) at 1, 15, 30, 45 and 60 minutes. From this data both VE/VCO2 slope and f/VT were calculated. Trial failure was defined according to standard criteria. Results: Twenty-three patients (30%) failed the SBT. Variables from SBT failure as compared to success patients were the following: f/VT at 1 minute 79±10 vs 52±3 (p=0.03), f/VT at 60 minutes 91±10 vs 53±3 (p=0.01), VE/VCO2 at 1 min 44±3 vs 41±2 (p=0.36), VE/VCO2 slope 56±6 vs 29±3 (p<0.001). The AUC [CI] of the ROC curves were 0.66[0.52-0.80], 0.71[0.57-0.86] and 0.81[0.70-0.93] for f/VT at 1min, f/VT at 60 min and VE/VCO2 slope, respectively (Fig.1). Conclusion: Our data suggest that increased VI along the SBT is a physiological trait of patients with difficult weaning.