Influence of overdistension/recruitment induced by high positive end-expiratory pressure on ventilation-perfusion matching assessed by electrical impedance tomography with saline bolus

Background: High positive end-expiratory pressures (PEEP) may induce overdistension/recruitment and affect ventilation-perfusion matching (VQMatch) in mechanically ventilated patients. This study aimed to investigate the association between PEEP-induced lung overdistension/recruitment and VQMatch by electrical impedance tomography (EIT). Methods: The study was conducted prospectively on 30 adult mechanically ventilated patients: 18/30 with ARDS and 12/30 with high risk for ARDS. EIT measurements were performed at zero end-expiratory pressures (ZEEP) and subsequently at high (12-15 cmH(2)O) PEEP. The number of overdistended pixels over the number of recruited pixels (O/R ratio) was calculated, and the patients were divided into low O/R (O/R ratio < 15%) and high O/R groups (O/R ratio >= 15%). The global inhomogeneity (GI) index was calculated to evaluate the ventilation distribution. Lung perfusion image was calculated from the EIT impedance-time curves caused by 10 ml 10% NaCl injection during a respiratory pause (> 8 s). DeadSpace(%), Shunt(%), and VQMatch(%) were calculated based on lung EIT perfusion and ventilation images. Results: Increasing PEEP resulted in recruitment mainly in dorsal regions and overdistension mainly in ventral regions. Delta VQMatch(%) (VQMatch(%) at high PEEP minus that at ZEEP) was significantly correlated with recruited pixels (r = 0.468, P= 0.009), overdistended pixels (r = - 0.666, P < 0.001), O/R ratio (r = - 0.686, P < 0.001), and Delta SpO(2) (r = 0.440, P = 0.015). Patients in the low O/R ratio group (14/30) had significantly higher Shunt(%) and lower VQMatch(%) than those in the high O/R ratio group (16/30) at ZEEP but not at high PEEP. Comparable DeadSpace(%) was found in both groups. A high PEEP caused a significant improvement of VQMatch(%), DeadSpace(%), Shunt, and GI in the low O/R ratio group, but not in the high O/R ratio group. Using O/R ratio of 15% resulted in a sensitivity of 81% and a specificity of 100% for an increase of VQMatch(%) > 20% in response to high PEEP. Conclusions: Change of ventilation-perfusion matching was associated with regional overdistention and recruitment induced by PEEP. A low O/R ratio induced by high PEEP might indicate a more homogeneous ventilation and improvement of VQMatch.