Electrical impedance tomography measured at two thoracic levels can visualize the ventilation distribution changes at the bedside during a decremental positive end-expiratory lung pressure trial

Ido G Bikker,Diederik Gommers,Carsten Preis,Jan Bakker,Mahamud Egal

doi:10.1186/cc10354

Abstract

IntroductionComputed tomography of the lung has shown that ventilation shifts from dependent to nondependent lung regions. In this study, we investigated whether, at the bedside, electrical impedance tomography (EIT) at the cranial and caudal thoracic levels can be used to visualize changes in ventilation distribution during a decremental positive end-expiratory pressure (PEEP) trial and the relation of these changes to global compliance in mechanically ventilated patients.MethodsVentilation distribution was calculated on the basis of EIT results from 12 mechanically ventilated patients after cardiac surgery at a cardiothoracic ICU. Measurements were taken at four PEEP levels (15, 10, 5 and 0 cm H2O) at both the cranial and caudal lung levels, which were divided into four ventral-to-dorsal regions. Regional compliance was calculated using impedance and driving pressure data.ResultsWe found that tidal impedance variation divided by tidal volume significantly decreased on caudal EIT slices, whereas this measurement increased on the cranial EIT slices. The dorsal-to-ventral impedance distribution, expressed according to the center of gravity index, decreased during the decremental PEEP trial at both EIT levels. Optimal regional compliance differed at different PEEP levels: 10 and 5 cm H2O at the cranial level and 15 and 10 cm H2O at the caudal level for the dependent and nondependent lung regions, respectively.ConclusionsAt the bedside, EIT measured at two thoracic levels showed different behavior between the caudal and cranial lung levels during a decremental PEEP trial. These results indicate that there is probably no single optimal PEEP level for all lung regions.

Highlights

Computed tomography of the lung has shown that ventilation shifts from dependent to nondependent lung regions
The total regional compliance of both region of interest (ROI) decreased at the caudal level during the decremental positive end-expiratory pressure (PEEP) trial, whereas it increased initially at the cranial level after PEEP was lowered from 15 to 10 cm H2O
The tidal impedance variation increased in the nondependent ROI and decreased in the dorsal ROI during the decremental PEEP steps

Summary

Introduction

Computed tomography of the lung has shown that ventilation shifts from dependent to nondependent lung regions. We investigated whether, at the bedside, electrical impedance tomography (EIT) at the cranial and caudal thoracic levels can be used to visualize changes in ventilation distribution during a decremental positive end-expiratory pressure (PEEP) trial and the relation of these changes to global compliance in mechanically ventilated patients. On the basis of computed tomography (CT), it is known that the dorsal and dependent lung regions are most susceptible to alveolar collapse and recruitment [4] These lung regions would benefit most from individualized positive end-expiratory pressure (PEEP) settings, and most EIT studies have been performed at the caudal thoracic level in the dependent lung regions [5,6]. In this study, we evaluated whether ventilation distribution measured by EIT, as well as the relation between EIT regional compliance and global dynamic compliance, differs between cranial and caudal thoracic levels in response to decremental PEEP steps

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Results

Discussion

Conclusion