Fig. 4. Density histograms of defined slices located at lung apex (A ) and close to the diaphragm (B ) taken from end-expiratory spiral computed tomography of each animal at end-expiration. Histograms show normalized lung volume in Hounsfield units (HU) ranging from −1,000 to 100 plotted as mean ± SEM of all animals. Aeration categories (hyperinflated, normally aerated, poorly aerated, nonaerated; see text for details) are marked and were statistically compared between airway pressure release ventilation (APRV) with and without spontaneous breathing (SB).