Comparison of regional lung recruitment in electrical impedance tomograms and CT scans in experimental acute lung injury

Introduction

Assessment of regional alveolar recruitment and derecruitment during protective respiratory therapy in lung failure is necessary to predict the process of ventilatory therapy and to avoid pulmonary complications. Different studies showed that clinical management of ARDS can successfully be controlled by CT scan analysis [1]. Information about regional distribution of ventilation can also be assessed by the bedside method of electrical impedance tomography (EIT) [2]. To correlate the assessment of pulmonary recruitment with EIT and CT scans as a reference technique during a PEEP trial, we designed an experimental study with porcine saline-lavage-induced lung injury.

Methods

In six pigs (25–34 kg), acute lung injury was induced by repetitive lung-lavage. After stabilisation of the lung injury model (> 1 hour) a stepwise PEEP trial with 2 min at each pressure ramp was performed (10 up to 30 mbar and 30 down to 5 mbar) with an electrically controllable ventilator (Servo 300; Siemens-Elema). During the PEEP trial, the animals were ventilated with pressure-controlled ventilation (delta 8 mbar), respiratory rate (RR) 25, I:E 1:1 and FiO₂ 1.0. Ventilatory, hemodynamic and gas exchange parameters were continuously recorded during the stepwise PEEP trial. EIT measurements were realized at a juxtadiaphragmatic thoracic level. Simultaneously, a CT scanner was triggered to obtain reference images of the same slice for each PEEP level at the end of each pressure ramp. Three ROI in nondependent, middle and dependent lung areas were defined to compare the EIT data with the reference data of the CT slices. The correlation between the changes in air content between both methods was determined. To compare the amount of pulmonary recruitment/ derecruitment at each PEEP level expressed by CT (Hounsfield units) measurement and relative impedance changes, the effect size (ES) [3] was calculated. ES levels were defined: small: <0.2, medium: 0.2–0.5, high: >0.8.

Results

The measured tidal volumes and PaO₂ clearly showed a nonlinear lung hysteresis and recruitment of nonaerated lung areas at the descending part of the pressure ramp (Fig. 1). The highest but not significant correlation between EIT measurements and X-ray attenuation (HU) was found in the dependent lung areas. The effect of PEEP on pulmonary recruitment/derecruitment was very high at lower pressure levels. The display of pulmonary recruitment in the EIT in comparison with CT scans at the ascending part of the pressure ramp showed a higher ES. ES values were reduced in EIT, if the tidal volume was reduced (Table 1).

Figure 1

Development of tidal volume and PaO₂ during a stepwise PEEP trial (n = 6).

Table 1 Effect size values of alveolar recruitment/derecruitment in end-expiratory CT and EIT tomograms

Conclusion

The effect of a stepwise ascending and descending PEEP trial on alveolar recruitment/derecruitment in porcine saline-lavage-induced lung injury could be displayed in EIT analysis and may directly be helpful in titration of PEEP.