Changes in regional ventilation determined by volume and pressure-controlled ventilation as assessed by electrical impedance tomography in an experimental model of acid-induced acute lung injury

Introduction

Electrical impedance tomography (EIT) is a promising bedside tool for monitoring regional lung ventilation during mechanical ventilation [1]. The purpose of this study was to assess regional lung ventilation by EIT in a pig model of acid-induced acute lung injury (ALI) during pressure-controlled (PCV) and volume-controlled ventilation (VCV).

Methods

Ten anesthetized and tracheotomized female pigs (25 to 30 kg) were randomized into two groups: mechanical ventilation by PCV (pressure adjusted to achieve and maintain 8 ml/kg tidal volume, n = 5) or VCV (8 ml/kg, n = 5) in FiO₂ 50%. ALI was induced in both groups by intratracheal instillation of 4 ml/kg hydrochloric acid (HCl) 0.1 N. Impedance changes (Draeger, Germany) were recorded by a 16-electrode belt placed at the level of the sixth intercostal space, and four horizontal, equally-sized regions of interest (ROIs) were defined for offline data analysis. Measurements were performed before (T-BL) and after lung injury (T-ALI). Lung tidal volume (Vt), pulmonary static compliance (C_stat), pulmonary mean artery pressure (mPAP), peak pressure (P_p), respiratory rate (RR) and PaO₂/FiO₂ ratio were also recorded. Statistical analysis was based on two-way ANOVA followed by the Tukey test for analysis of data within and between groups, and a t test was used analysis of impedance changes induced by ALI.

Results

After ALI, both PCV and VCV groups showed significant decreases in the PaO₂/FiO₂ ratio (479.6 ± 46.5 to 244.5 ± 64.7 and 428.8 ± 22.9 to 212.4 ± 26.7) and C_stat (29.2 ± 5.1 to 13.2 ± 2.3 and 26.6 ± 3.4 to 12.3 ± 3.2) and significant increases in P_p (14.2 ± 1.9 to 25.6 ± 2.6 and 13.8 ± 0.8 to 24.6 ± 4.16) and mPAP (19.6 ± 0.5 to 35.2 ± 5.8 and 19.4 ± 1.5 to 26.8 ± 4.5). Between groups, only mPAP was statistically different at T-ALI. Global impedance decreased slightly after ALI in both groups, with no statistical significance. At T-ALI, there were statistically significant differences in tidal distributions of ROIs 2 (38.9 ± 2.6% to 42.4 ± 3.9%) and 3 (39.7 ± 3.1 to 35.1 ± 4.3%) of animals in the VCV group, when compared with T-BL. No significant difference was found in the PCV group.

Conclusions

EIT assessment of ventilation revealed a ventral shift in the tidal distribution in the VCV group after ALI, while PCV preserved tidal distribution observed at baseline.