Electrical impedance tomography monitoring in acute respiratory distress syndrome patients with mechanical ventilation during prolonged positive end-expiratory pressure adjustments

Background/purpose: The time required to reach oxygenation equilibrium after positive end-expiratory pressure (PEEP) adjustments in mechanically ventilated patients with acute respiratory distress syndrome (ARDS) is unclear. We used electrical impedance tomography to elucidate gas distribution and factors related to oxygenation status following PEEP in patients with ARDS. Methods: Nineteen mechanically ventilated ARDS patients were placed on baseline PEEP (PEEP_B) for 1 hour, PEEP_B - 4 cmH₂O PEEP (PEEP_L) for 30 minutes, and PEEP_B + 4 cmH₂O PEEP (PEEP_H) for 1 hour. Tidal volume and respiratory rate were similar. Impedance changes, respiratory parameters, and arterial blood gases were measured at baseline, 5 minutes, and 30 minutes after PEEP_L, and 5 minutes, 15 minutes, 30 minutes, and 1 hour after PEEP_H. Results: PaO₂/fraction of inspired oxygen (P/F ratio) decreased quickly from PEEP_B to PEEP_L, and stabilized 5 minutes after PEEP_L. However the P/F ratio progressively increased from PEEP_L to PEEP_H, and a significantly higher P/F ratio and end-expiratory lung impedance were found at 60 minutes compared to 5 minutes after PEEP_H. The end-expiratory lung impedance level significantly correlated with P/F ratio (p < 0.001). With increasing PEEP, dorsal ventilation significantly increased; however, regional ventilation did not change over time with PEEP level. Conclusion: Late improvements in oxygenation following PEEP escalation are probably due to slow recruitment in ventilated ARDS patients. Electrical impedance tomography may be an appropriate tool to assess recruitment and oxygenation status in patients with changes in PEEP.