Transpulmonary pressure and ventilation distribution measured with EIT during a PEEP trial in porcine acute lung injury

Introduction

Setting the optimal level of positive end-expiratory pressure (PEEP) in the ICU is still a matter of debate. Talmor and colleagues used the transpulmonary pressure calculated from the oesophageal balloon to set PEEP in a recent randomized controlled study. This strategy aims at preventing alveolar collapse by counterbalancing the gravitational force of the lung by an equal or higher PEEP. We evaluated the relation between ventilation distribution measured by EIT and transpulmonary pressure during a PEEP trial in porcine ALI.

Methods

Eight pigs (30 kg) were studied during a PEEP trial before and after the induction of acute lung injury (ALI) with oleic acid. Global lung parameters, regional compliance, and oesophageal pressure were recorded at the end of each PEEP step. Regional compliance was calculated by dividing the tidal impedance variation (EIT Evaluation Kit 2; Dräger, Lübeck, Germany) by the applied driving pressure.

Results

Transpulmonary pressures were negative at 0 cmH₂O PEEP and became positive during the stepwise increase of PEEP at 5 cmH₂O before, and 10 cmH₂O PEEP after the induction of ALI (Figure 1). Optimum regional compliance was different between the ventral (nondependent) and dorsal (dependent) regions of interest (ROI). In the healthy lung, optimum PEEP was 10 in the dorsal ROI and 5 in the ventral ROI, whereas after ALI this was 15 in the dorsal ROI and 5 in the ventral ROI.

Figure 1

Conclusions

If EIT is measured at a caudal lung level, optimal EIT PEEP in the dependent lung exceeds the PEEP required for positive transpulmonary pressures as used in the Talmor study, whereas in the nondependent lung optimal EIT PEEP is equal before ALI and lower after ALI. We speculate that this is probably influenced by the location of the EIT slice in the cranial to caudal direction.