formation of atelectasis after pre-oxygenation and induction of anaesthesia is oxygen and time dependent. The benefit of using 80% oxygen during induction of anaesthesia in order to reduce atelectasis diminished gradually with time.
Background Various methods for protective ventilation are increasingly being recommended for patients undergoing general anesthesia. However, the importance of each individual component is still unclear. In particular, the perioperative use of positive end-expiratory pressure (PEEP) remains controversial. The authors tested the hypothesis that PEEP alone would be sufficient to limit atelectasis formation during nonabdominal surgery. Methods This was a randomized controlled evaluator-blinded study. Twenty-four healthy patients undergoing general anesthesia were randomized to receive either mechanical ventilation with PEEP 7 or 9 cm H2O depending on body mass index (n = 12) or zero PEEP (n = 12). No recruitment maneuvers were used. The primary outcome was atelectasis area as studied by computed tomography in a transverse scan near the diaphragm, at the end of surgery, before emergence. Oxygenation was evaluated by measuring blood gases and calculating the ratio of arterial oxygen partial pressure to inspired oxygen fraction (Pao2/Fio2 ratio). Results At the end of surgery, the median (range) atelectasis area, expressed as percentage of the total lung area, was 1.8 (0.3 to 9.9) in the PEEP group and 4.6 (1.0 to 10.2) in the zero PEEP group. The difference in medians was 2.8% (95% CI, 1.7 to 5.7%; P = 0.002). Oxygenation and carbon dioxide elimination were maintained in the PEEP group, but both deteriorated in the zero PEEP group. Conclusions During nonabdominal surgery, adequate PEEP is sufficient to minimize atelectasis in healthy lungs and thereby maintain oxygenation. Thus, routine recruitment maneuvers seem unnecessary, and the authors suggest that they should only be utilized when clearly indicated.
Background: 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. Methods: This primary analysis included pooled data from previously published studies of 243 subjects aged 18e78 yr, with BMI of 18e52 kg m À2 . 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 (FIO 2 ) of >0.8, followed by mechanical ventilation with FIO 2 of 0.3 or higher with no PEEP. Atelectasis was assessed by CT. Results: Atelectasis area of up to 39 cm 2 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 (ageþage 2 ) with the most atelectasis at ~50 yr (r 2 ¼0.08; P<0.001). Log atelectasis area was also related to a broken-line function of the BMI with the knee at 30 kg m À2 (r 2 ¼0.06; P<0.001). Greater atelectasis was seen in the subjects receiving FIO 2 of 1.0 than FIO 2 of 0.3e0.5 (12.8 vs 8.1 cm 2 ; P<0.001). A multiple regression analysis, including a quadratic function of age, a broken-line function of the BMI, and dichotomised FIO 2 (0.3e0.5/1.0) adjusting for ventilatory frequency, strengthened the association (r 2 ¼0.23; P<0.001). PaO 2 decreased with both age and BMI. Conclusions: 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 À2 ). Therefore, greater age and obesity appear to limit atelectasis formation during general anaesthesia.
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