Volumetric analysis of aeration in the lungs during general anaesthesia

Spiral computed tomography (CT) allows volumetric analysis of formation of atelectasis and aeration of the lungs during anaesthesia. We studied 26 premedicated patients undergoing elective surgery allocated to group 1 (conscious, spontaneous breathing, investigating inspiration and expiration), group 2 (general anaesthesia with mechanical ventilation, investigating inspiration and expiration) or group 3 (general anaesthesia with mechanical ventilation, investigating changes over time). Using spiral CT, the lungs were studied either before or during general anaesthesia. CT scans were grouped into the following areas: overaeration, normal aeration, reduced aeration, poor aeration and atelectasis. The mechanism of atelectasis appeared to be both gravitational forces and a diaphragm-related force that acts regionally in caudal lung regions. Mean atelectasis formation and poorly aerated regions comprised approximately 4% of the total lung volume between the diaphragm and carina, giving a mean value of 16-20% of the normal aerated lung tissue being either collapsed or poorly aerated. The vertical ventilation distribution was more even during anaesthesia than in the awake state.