Changes in lung tidal distribution assessed by electrical impedance tomography in a model of hemorrhagic shock with endotoxemia and resuscitation

Introduction

Electrical impedance tomography (EIT) is a non-invasive bedside imaging tool with the potential to assess regional lung ventilation reliably [1]. The purpose of this study was to monitor changes in lung impedance in a pig model of severe shock followed by fluid resuscitation.

Methods

Twelve anesthetized, mechanically ventilated (8 ml/kg, PEEP 5 cmH₂O), supine pigs were submitted to acute hemorrhagic shock with infusion of endotoxin. Animals were allocated to positive control (PC, n = 6) or treatment group with lactated Ringer's to achieve and maintain pulse pressure variation 13% and mean arterial pressure 65 mmHg (PPV, n = 6). Ventilatory and hemodynamic parameters were recorded at baseline, 1 hour after hemorrhagic shock (Tshock), and hourly for 3 hours (T1 to T3). A 16-electrode belt was placed at the level of the sixth intercostal space for EIT measurements (Dräger, Germany). Offline analysis was based on four horizontal regions of interest (ROIs) over the ventrodorsal lung area. Statistical analysis was based on two-way ANOVA followed by the Tukey test (P < 0.05).

Results

At Tshock there was hemodynamic compromise, statistical decrease in lung compliance (C_stat) and significant increases in pulmonary vascular resistance index, mean pulmonary artery pressure and peak pressure (P_peak), with no statistical difference between groups. Endpoints and hemodynamic stability were achieved in the PPV group in 117 ± 28 minutes. C_stat continued to deteriorate and P_peak continued to rise in both groups from T1 to T3. When compared with PC, the PPV group had significant impedance increases in ROIs 1 and 2 at T2 and T3 and, at T3 the increase in ROI 1 was also statistically greater than Tshock within the group. Statistical decrease in the percentage tidal distribution in ROI 3 and increase in ROI 2 of the PPV group, in relation to the PC group, were also noted.

Conclusions

Despite re-establishment of hemodynamic adequacy in PPV group and although ventilatory parameters were similar in both groups over time, resuscitation as performed in the study induced significant changes in tidal impedance toward nondependent lung regions, implying greater lung impairment in treated animals.