Higher age and obesity limit atelectasis formation during anaesthesia: an analysis of computed tomography data in 243...

General anaesthesia is increasingly common in elderly and obese patients. Greater age and body mass index (BMI) worsen gas exchange. We assessed whether this is related to increasing atelectasis during general anaesthesia.

This primary analysis included pooled data from previously published studies of 243 subjects aged 18-78 yr, with BMI of 18-52 kg m^-2. The subjects had no clinical signs of cardiopulmonary disease, and they underwent computed tomography (CT) awake and during anaesthesia before surgery after preoxygenation with an inspired oxygen fraction (FIO₂) of >0.8, followed by mechanical ventilation with FIO₂ of 0.3 or higher with no PEEP. Atelectasis was assessed by CT.

Atelectasis area of up to 39 cm² in a transverse scan near the diaphragm was seen in 90% of the subjects during anaesthesia. The log of atelectasis area was related to a quadratic function of (age+age²) with the most atelectasis at ∼50 yr (r²=0.08; P<0.001). Log atelectasis area was also related to a broken-line function of the BMI with the knee at 30 kg m^-2 (r²=0.06; P<0.001). Greater atelectasis was seen in the subjects receiving FIO₂ of 1.0 than FIO₂ of 0.3-0.5 (12.8 vs 8.1 cm²; P<0.001). A multiple regression analysis, including a quadratic function of age, a broken-line function of the BMI, and dichotomised FIO₂ (0.3-0.5/1.0) adjusting for ventilatory frequency, strengthened the association (r²=0.23; P<0.001). PaO₂ decreased with both age and BMI.

Atelectasis during general anaesthesia increased with age up to 50 yr and decreased beyond that. Atelectasis increased with BMI in normal and overweight patients, but showed no further increase in obese subjects (BMI ≥30 kg m^-2). Therefore, greater age and obesity appear to limit atelectasis formation during general anaesthesia.