Atelectasis and chest wall shape during halothane anesthesia

Anesthesia produces atelectasis in the dependent areas of the lungs by mechanisms that remain unknown. It has been proposed that anesthesia produces a cephalad shift in the end-expiratory position of the diaphragm, which compresses the lungs and produces atelectasis. This study tested the hypothesis that the extent of atelectasis is correlated with the cephalad displacement of the dependent portion of the diaphragm produced by halothane anesthesia in healthy young human subjects.

Twelve volunteers (mean age 34 yr) were studied while awake and during approximately 1.2 minimum alveolar concentration halothane anesthesia. Chest wall configuration was determined using images of the thorax obtained by three-dimensional fast computed tomography. Functional residual capacity was measured by a nitrogen dilution technique. Measurements were performed during quiet breathing in all subjects and after paralysis with 0.1 mg/kg vecuronium and mechanical ventilation in six subjects. Atelectasis was assumed to be present in regions of the lung that showed radiographic attenuation values similar to solid organs such as the liver.

Atelectasis in dependent lung regions was not apparent in scans performed while the subjects were awake. Anesthesia with spontaneous breathing increased the volume of atelectasis measured at end-expiration by more than 1 ml in 9 of 12 subjects. For all subjects, the volume of atelectasis was 29 +/- 10 ml (M +/- SE), representing 0.67 +/- 0.23% of the total thoracic volume. The distribution of atelectasis varied along the cephalocaudal axis, with less atelectasis in more cephalad transverse sections. Paralysis and mechanical ventilation significantly decreased the volume of atelectasis present at end-expiration. There was no correlation between the average amount of cephalad displacement of the most dependent region of the diaphragm and the amount of atelectasis, nor was there any correlation between the amount of atelectasis and anesthesia-induced changes in the end-expiratory position of any chest wall structure.

The dependent lung atelectasis produced by halothane anesthesia does not appear to be related to changes in the position of any single chest wall structure in these healthy young subjects, but rather to an interaction of several factors that remain to be identified.