Effects of two ventilator-weaning methods on lung volume and ventilation distribution by electrical impedance tomography in post-cardiac surgery patients: a prospective cohort study

Abstract Background The effect of different spontaneous breathing trial (SBT) methods on lung volume and ventilation distribution has not been well clarified in post-cardiac surgery patients. Methods In this prospective observational study, patients underwent 30 min of pressure-support ventilation (PSV)-SBT [PS 8 cmH2O, zero positive end-expiratory pressure (ZEEP)], followed by a 30-min T-piece trial if tolerated. Electrical impedance tomography (EIT) was used to continuously monitor regional lung ventilation and end-expiratory lung volume (EELV) at baseline, PSV-SBT 3 min, PSV-SBT 30 min, T-piece SBT 3 min and T-piece SBT 30 min. EELVloss = [VTbaseline/tidal impedance variation (TIV)baseline] × ΔEELI. EELVloss PSV was defined as volume loss at 30 min of PSV-SBT and EELVloss T-piece was defined as volume loss during T-piece SBT. Results In 60 patients who complied with both SBT steps, 43 succeeded (71.7%) and 17 failed (28.3%) the T-piece SBT. Compared to the success group, the failure group exhibited a higher incidence of pendelluft (52.9% vs. 23.3%, p = 0.045) and significantly greater EELVloss at T-piece SBT 30 min (623[459,746] ml vs. 511[376,702]ml, p = 0.003). However, the success group showed greater EELVloss PSV than the failure group (322[247,459] ml vs. 199[166, 269] ml, p < 0.001), which was an abnormal pattern. Notably, the failure group had lower TIV (2102[1769,2562] vs. 2742[2153,3872], p = 0.005) and a higher respiratory rate (RR) than baseline at PSV-SBT 30 min (20[17,24] vs. 16[12,18], p < 0.001). Furthermore, we classified all patients into two groups based on the predominant reduction of volume loss: P-volume loss group (N = 37, EELVloss PSV > EELVloss T-piece) and T-volume loss group (N = 23, EELVloss T-piece > EELVloss PSV). In addition, the T-volume loss group had a higher weaning failure rate than the P-volume loss group (52.2% [12/23] vs. 13.5% [5/37], p < 0.001) and was associated with reduced baseline dorsal ventilation (39%[37%,43%] vs. 44%[41%,50%], p = 0.023). ROC analysis suggested that a dorsal ventilation threshold of 40.5% was associated with T-volume loss. Conclusions The successful weaning patients had a higher reduction of EELVloss PSV and a lower reduction of EELVloss T-piece. In the weaning failure patients, the paradox of lower EELVloss PSV that was accompanied by a high RR and low VT might be associated with air trapping. Attention should be paid to using EELVloss PSV to identify weaning failure.