Assessment of breath by breath recruitment by electrical impedance tomography in saline lavage lung injury

Introduction

Alveolar recruitment and maintenance of lung volume are important goals in the treatment of acute lung injury (ALI) and essential for improving oxygenation. The most usual employed strategy to achieve this goal is the use of positive end-expiratory pressure (PEEP). Recruitment and collapse are highly dynamic phenomena that are difficult to monitor. Dynamic effects of regional ventilation can be monitored by electrical impedance tomography (EIT) at the bedside [1]. We investigated the ability of EIT for providing a useful tool to detect dynamic changes of regional breath by breath recruitment at the bedside during an incremental and decremental PEEP trial in experimental lung injury. In addition, we analyzed pressure–volume (P–V) curves computed by EIT data.

Methods

ALI was induced in six pigs by repetitive lung lavage. After stabilization of the lung injury model (> 1 hour) a stepwise PEEP trial was performed consisting of 2-minute steps of tidal ventilation (10–30 cmH₂O; 30–5 cmH₂O). During the PEEP trial subjects were ventilated pressure-controlled. Global ventilatory and gas exchange parameters were continuously recorded. Offline we analysed EIT data by computing the amount of breath by breath recruitment (ΔV EIT) at each pressure level before and after lung lavage. Nondependent and dependent regions of interest were defined in the tomograms. ΔV EIT was defined as the mean increase or decrease in end-expiratory global impedance per breath.

Results

Ventilatory parameters clearly showed a recruitment of nonaerated lung areas at the descending part of the pressure ramp. The shape of the P–V curve from EIT data, in particular the increasing slope (lower level > upper level), reflected the recruitment of poorly ventilated lung regions. The flattening of the curve at higher pressures, especially at the upper level, reflected less amount of recruitment but more overdistension. Regional pulmonary recruitment/derecruitment was very high in the lower level. These phenomena were more impressive after induced lung injury.

Conclusion

Stepwise PEEP recruitment maneuvers can open collapsed lungs and certain PEEP levels are necessary to keep the lungs open. Monitoring of ΔV EIT is capable of detecting the dynamic process of recruitment and derecruitment at bedside. Plotting regional P–V curves from EIT data provides continuous information that may be of use in determining the PEEP level to maintain recruitment in acute lung injury.