In the last minutes of each phase , we measured arterial blood gases , changes in end-expiratory lung volume of nondependent and dependent regions , tidal volume reaching nondependent and dependent lung ( Vtnondep and Vtdep ) , dynamic intratidal ventilation heterogeneity , defined as the average ratio of Vt reaching nondependent/Vt reaching dependent lung regions along inspiration ( VtHit ) .