Fig. 2. Densitometry of the whole lungs from spiral computed tomography at end-expiration. Histogram shows 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).