Analysis of lung density by computed tomography before and during general anaesthesia

Pulmonary structure was analysed by means of computed tomography (CT) in 20 lung-healthy patients, relating tissue density to the attenuation value (AV) of a picture element. Regional density of pulmonary tissue (rlung) was determined using mean lung density in five regions of interest (ROI1-5) (sector method). Vertical and horizontal distributions of x-ray attenuation were analysed by density profiles, relating AV values to evenly distributed and normalised length scales. In group I (n = 12), CT-densitometry was obtained in awake, supine patients and after induction of general anaesthesia. In group II (n = 8), the effect of mechanical ventilation with positive end-expiratory pressure (PEEP, 1.0 kPa [10 cmH2O]) was studied. In the awake state, a vertical tissue density difference between the top and bottom of the lung was found in all patients, accounting for a mean of 0.235 g.cm-3 (right lung) and 0.199 g.cm-3 (left lung). Only minor changes were seen in the horizontal lung density profiles. After induction of anaesthesia, x-ray attenuation of ROI1-4 showed no significant differences when compared with the awake state. The basal lung areas (ROI5) revealed a significantly increased tissue density (P < or = 0.01), reaching mean values of 0.94 g.cm-3 (right lung) and 0.814 g.cm-3 (left lung). Similarly, vertical density profiles showed a markedly enhanced rlung of the bottom of the lung in all patients, interpreted as atelectasis. The amount of atelectasis accounted for 4.8 +/- 2.6% (right lung) and 4.7 +/- 2.1% (left lung) of the intrapulmonary area.(ABSTRACT TRUNCATED AT 250 WORDS)