Assessing effects of PEEP and global expiratory lung volume on regional electrical impedance tomography.

OBJECTIVE

This study was performed to assess the value of electrical impedance tomography (EIT) as an indicator of tidal (V(T)) and end expiratory lung volume (EELV).

METHODS

EIT measurements were performed in seven healthy piglets during constant tidal volume ventilation at incremental and decremental positive end-expiratory pressure (PEEP) levels. Tidal impedance changes were calibrated to volume using V(T) calculated from flow at the airway opening. Simultaneously, calibrated respiratory inductive plethysmography was used to measure EELV changes, and used as a reference standard.

RESULTS

EIT systematically underestimated both V(T) and EELV changes when EELV deviated from the level at which it was calibrated. Calculated over the entire pressure-volume curve, EIT systematically underestimated V(T) by 28 ml, with a precision from -16 to 72 ml. EELV was systemically underestimated by 406 ml, with a precision of -38 to 849 ml. Nonlinear recruitment in the ventral regions of the lungs was the main cause of this underestimation.

CONCLUSIONS

Tidal and end-expiratory changes in pulmonary impedance reflect corresponding changes in lung volume, but the increasing underestimation with increasing lung volume should be taken into account in the analysis of EIT data.