Pulmonary Densities during Anesthesia with Muscular Relaxation—A Proposal of Atelectasis

TLDR

Twenty patients were examined by CT before and after induction of general anesthesia with muscular relaxation, and four subjects were studied awake for a longer period to assess changes in transverse thoracic area and lung tissue structure. Within five minutes of induction, all patients developed crest‑shaped high‑density areas in dependent lung regions (largest caudal 4.8% of transverse area), which persisted unchanged after 20 minutes, were unaffected by oxygen fraction, disappeared in the non‑dependent lung when repositioned, were reduced by 10 cmH₂O PEEP, and were absent in awake subjects after 90 min, indicating that these densities represent atelectases caused by compression rather than gas resorption.

Abstract

Twenty patients (23-76 yr) were studied with regard to lung tissue changes prior to and following induction of general anesthesia with muscular relaxation, and another four subjects were studied for a longer period awake. The transverse thoracic area and the structure of the lung tissue were determined by computerized tomography. No abnormalities in the lung tissue were noted before anesthesia. Within 5 min after induction, including muscular relaxation, all subjects had developed crest-shaped changes of increased density in the dependent regions of both lungs. They were largest in the most caudal segment (4.8 +/- 0.8% of the transverse lung area, mean +/- SE) and smaller in the cephalad exposures (3.4 +/- 0.7% of the transverse area). The size of the densities showed no correlation to age. The densities did not increase after a further 20 min of anesthesia and were not affected by the inspiratory oxygen fraction. When the subjects were moved from the supine to the lateral position, the crest-shaped densities disappeared in the nondependent lung and remained in the dorsal part of the dependent lung. The application of positive end-expiratory pressure of 10 cmH2O eliminated or reduced the densities. The four awake subjects showed no lung densities after 90 min in the supine position. It is suggested that these crest-shaped densities represent atelectases, which develop by compression of lung tissue rather than by resorption of gas.