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 H₂O at fixed ventilation were evaluated by EIT images. DRRS was calculated as (V_T-EIT/EELI)/(V_{T-EIT[15PEEP]}/EELI_[15PEEP]), where the tidal volume (V_T)_-EIT 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 H₂O). 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 H₂O and highest value at 15 PEEP 16 ± 6 mL/cm H₂O; P < .001), whereas driving pressure decreased with PEEP (highest value at zero PEEP 34 ± 6 cm H₂O and lowest value at 15 PEEP 21 ± 4 cm H₂O; P < .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 H₂O of PEEP; P < .001. EELI increased with a slope of 0.05 (0.02–0.07) (arbitrary units) for each cm H₂O of PEEP; P < .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 H₂O increase of PEEP; P < .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.