ANALYSIS OF LUNG DENSITY BY COMPUTED-TOMOGRAPHY BEFORE AND DURING GENERAL-ANESTHESIA

Pulmonary structure was analysed by means of computed tomography (CT) in 20 lung-healthy patients, relating tissue density to the attenuatio n value (AV) of a picture element. Regional density of pulmonary tissu e (r(lung)) was determined using mean lung density in five regions of interest (ROI1-5) (sector method). Vertical and horizontal distributio ns 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 effec t of mechanical ventilation with positive end-expiratory pressure (PEE P, 1.0 kPa 10 cmH2O!) was studied. In the awake state, a vertical tis sue density difference between the top and the 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 com pared with the awake state. The basal lung areas (ROI5) revealed a sig nificantly increased tissue density (P less-than-or-equal-to 0.0 1), r eaching 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 enh anced r(lung) of the bottom of the lung in all patients, interpreted a s atelectasis. The amount of atelectasis accounted for 4.8 +/- 2.6% (r ight lung) and 4.7 +/- 2.1% (left lung) of the intrapulmonary area. Th ere was no evidence of ''non-gravitational'' inhomogeneity of density distribution seen in the horizontal density profiles. After applicatio n of PEEP, basal lung densities decreased significantly, although smal l basal densities remained in most patients (2.2 7 +/- 2.57% of right intrapulmonary area P less-than-or-equal-to 0.01!, 2.2 +/- 2.37% left intrapulmonary area P less-than-or-equal-to 0.01!). Calculated alveo lar recruitment was 7.7 cm2 and 8.4 cm2, whereas expansion of both lun gs was smaller (4.3 cm2 and 4.4 cm2 right and left lung!). Mean densi ty of aerated tissue had decreased by 25%, and both horizontal and ver tical attenuation profiles revealed an even distribution of r(lung). A nalysis of r(lung) provides useful information about regional pulmonar y morphology during anaesthesia and may be related to lung function.