Effects of two ventilator-weaning methods on lung volume and ventilation distribution by electrical impedance...

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.

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. EELV_loss = [VT_baseline/tidal impedance variation (TIV)_baseline] × ΔEELI. EELV_{loss PSV} was defined as volume loss at 30 min of PSV-SBT and EELV_{loss T-piece} was defined as volume loss during T-piece SBT.

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 EELV_loss at T-piece SBT 30 min (623[459,746] ml vs. 511[376,702]ml, p = 0.003). However, the success group showed greater EELV_{loss 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, EELV_{loss PSV} > EELV_{loss T-piece}) and T-volume loss group (N = 23, EELV_{loss T-piece} > EELV_{loss 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.

The successful weaning patients had a higher reduction of EELV_{loss PSV} and a lower reduction of EELV_{loss T-piece}. In the weaning failure patients, the paradox of lower EELV_{loss PSV} that was accompanied by a high RR and low VT might be associated with air trapping. Attention should be paid to using EELV_{loss PSV} to identify weaning failure.