Dynamic Relative Regional Lung Strain Estimated by Electrical Impedance Tomography in an Experimental Model of ARDS.

BACKGROUND

To analyze the role of PEEP on dynamic relative regional strain (DRRS) in a model of ARDS, respective maps were generated by electrical impedance tomography (EIT).

METHODS

Eight ARDS pigs submitted to PEEP steps of 0, 5, 10, and 15 cm HO at fixed ventilation were evaluated by EIT images. DRRS was calculated as (V/EELI)/(V/EELI), where the tidal volume (V) and end-expiratory lung impedance (EELI) are the tidal and end-expiratory change in lung impedance, respectively. The measurement at 15 PEEP was taken as reference (end-expiratory transpulmonary pressure > 0 cm HO). The relationship between EIT variables (center of ventilation, EELI, and DRRS) and airway pressures was assessed with mixed-effects models using EIT measurements as dependent variables and PEEP as fixed-effect variable.

RESULTS

At constant ventilation, respiratory compliance increased progressively with PEEP (lowest value at zero PEEP 10 ± 3 mL/cm HO and highest value at 15 PEEP 16 ± 6 mL/cm HO; < .001), whereas driving pressure decreased with PEEP (highest value at zero PEEP 34 ± 6 cm HO and lowest value at 15 PEEP 21 ± 4 cm HO; < .001). The mixed-effect regression models showed that the center of ventilation moved to dorsal lung areas with a slope of 1.81 (1.44-2.18) % points by each cm HO of PEEP; < .001. EELI increased with a slope of 0.05 (0.02-0.07) (arbitrary units) for each cm HO of PEEP; < .001. DRRS maps showed that local strain in ventral lung areas decreased with a slope of -0.02 (-0.24 to 0.15) with each cm HO increase of PEEP; < .001.

CONCLUSIONS

EIT-derived DRRS maps showed high strain in ventral lung zones at low levels of PEEP. The findings suggest overdistention of the baby lung.