The calculations performed on the sums of values from all pixels of the 32 × 32 pixel matrix EIT image were described as 'global'. In addition, sums of values from the left and right lung regions were described separately, and the entire EIT image was divided into 64 regions-of-interest (32 left and 32 right lung) from anterior to posterior as previously described by Frerichs and colleagues [15] . Ventilation-induced tidal volume (ΔZ VT ) was quantified by measuring the relative ΔZ from the highest point at end inspiration to the lowest point at end expiration, and an average ΔZ was calculated from multiple breaths. Changes in ΔZ VT were calibrated to volume using the known V T . The relative change in end-expiratory lung volume (relative ΔZ EELV ) was determined by measuring the median impedance from the lowest point at expiration during the sampling time (Z EELV ) [16] . The relative ΔZ EELV was normalized to volume (relative Δ EELV in ml) by multiplying the median impedance with the ratio V T /ΔZ VT .