The value of electrical impedance tomography in assessing the effect of body position and positive airway pressures on regional lung ventilation in spontaneously breathing subjects.

OBJECTIVE

Functional electrical impedance tomography (EIT) measures relative impedance changes in lung tissue during tidal breathing and creates images of local ventilation distribution. A novel approach to analyse the effect of body position and positive pressure ventilation on intrapulmonary tidal volume distribution was evaluated in healthy adult subjects.

DESIGN AND SETTING

Prospective experimental study in healthy adult subjects in the intensive care unit at university hospital.

SUBJECTS

Ten healthy male adults.

INTERVENTIONS

Change in body position from supine to prone, left and right lateral during spontaneous breathing and positive pressure support ventilation.

MEASUREMENTS AND RESULTS

EIT measurements and multiple-breath sulphur hexafluoride (SF6) washout were performed. Profiles of average relative impedance change in regional lung areas were calculated. Relative impedance time course analysis and Lissajous figure loop analysis were used to calculate phase angles between dependent or independent lung and total lung (phi). EIT data were compared to SF6 data washout measuring the lung clearance index (LCI). Proposed EIT profiles allowed inter-individual comparison of EIT data and identified areas with reduced regional tidal volume using pressure support ventilation. Phase angle phi of dependent lung in supine position was 11.7+/-1.4 degrees, in prone 5.3+/-0.5 degrees, in right lateral 11.0+/-1.3 degrees and in left lateral position 10.8+/-1.0 degree. LCI increased in supine position from 5.63+/-0.43 to 7.13+/-0.64 in prone position. Measured phi showed inverse relationship to LCI in the four different body positions.

CONCLUSIONS

EIT profiles and phi of functional EIT are new methods to describe regional ventilation distribution with EIT allowing inter-individual comparison.