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Positive end-expiratory pressure and postoperative atelectasis: A randomized controlled trial
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Surgical Sciences.ORCID iD: 0000-0002-7449-3907
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2019 (English)In: Anesthesiology, ISSN 0003-3022, E-ISSN 1528-1175Article in journal (Refereed) In press
Abstract [en]

Background

Positive end-expiratory pressure (PEEP) increases lung volume and protects against alveolar collapse during anesthesia. During emergence, safety preoxygenation preparatory to extubation makes the lung susceptible to gas absorption and alveolar collapse, especially in dependent regions being kept open by PEEP. We hypothesized that withdrawing PEEP before starting emergence preoxygenation would limit postoperative atelectasis formation.

Methods

This was a randomized controlled evaluator-blinded trial in 30 healthy patients undergoing non-abdominal surgery under general anesthesia and mechanical ventilation with PEEP 7 or 9 cm H2O depending on body mass index. A computed tomography scan at the end of surgery assessed baseline atelectasis. The study subjects were thereafter allocated to either maintained PEEP (n = 16) or zero PEEP (n = 14) during emergence preoxygenation. The primary outcome was change in atelectasis area as evaluated by a second computed tomography scan 30 min after extubation. Oxygenation was assessed by arterial blood gases.

Results

Baseline atelectasis was small and increased modestly during awakening, with no statistically significant difference between groups. With PEEP applied during awakening, the increase in atelectasis area was median (range) 1.6 (-1.1 to 12.3) cm2 and without PEEP 2.3 (-1.6 to 7.8) cm2. The difference was 0.7 cm2 (95% CI, -0.8 to 2.9 cm2; P = 0.400). Postoperative atelectasis for all patients was median 5.2 cm2 (95% CI, 4.3 to 5.7 cm2), corresponding to median 2.5% of the total lung area (95% CI, 2.0 to 3.0%). Postoperative oxygenation was unchanged in both groups when compared to oxygenation in the preoperative awake state.

Conclusion

Withdrawing PEEP before emergence preoxygenation does not reduce atelectasis formation after non-abdominal surgery. Despite using 100% O2 during awakening, postoperative atelectasis is small and does not affect oxygenation, possibly conditional on an open lung during anesthesia, as achieved by intraoperative PEEP.

Place, publisher, year, edition, pages
2019.
National Category
Anesthesiology and Intensive Care
Research subject
Anaesthesiology and Intensive Care
Identifiers
URN: urn:nbn:se:uu:diva-381346OAI: oai:DiVA.org:uu-381346DiVA, id: diva2:1303034
Available from: 2019-04-08 Created: 2019-04-08 Last updated: 2019-04-08
In thesis
1. Pulmonary Atelectasis in General Anaesthesia: Clinical Studies on the Counteracting Effects of Positive End-Expiratory Pressure
Open this publication in new window or tab >>Pulmonary Atelectasis in General Anaesthesia: Clinical Studies on the Counteracting Effects of Positive End-Expiratory Pressure
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Partial lung collapse, i.e., pulmonary atelectasis, is common during general anaesthesia. The main causal mechanism is reduced lung volume with airway closure and subsequent gas absorption from preoxygenated alveoli. Atelectasis impairs oxygenation and forms the pathophysiological basis for postoperative pulmonary complications. Positive end-expiratory pressure (PEEP) counteracts the loss in lung volume, but its role in preventing atelectasis during anaesthesia is not clear.

All studies included in this thesis were prospective randomized clinical trials. In the first study, oxygenation was used as a surrogate measure of atelectasis in obese patients undergoing laparoscopic gastric bypass. The subsequent studies used single-slice computed tomography (CT) to evaluate atelectasis in healthy patients undergoing non-abdominal surgery.

Paper I: We studied the use of continuous positive airway pressure (CPAP) and PEEP during induction of anaesthesia and a reduced inspired oxygen fraction (FiO2) during emergence. Oxygenation was maintained in the group that received CPAP during induction, followed by a PEEP of 10 cmH2O. Postoperative oxygenation was impaired in the group that received a high FiO2 during emergence.

Paper II: An early oxygen washout manoeuvre to quickly restore nitrogen levels and thus stabilize the alveoli, had no effect on atelectasis at the end of surgery. Both study groups exhibited small atelectasis after being ventilated with a moderate PEEP of 6-8 cmH2O during anaesthesia.

Paper III: The effect of PEEP versus zero PEEP on atelectasis formation and oxygenation at the end of surgery was compared. The PEEP group maintained oxygenation better and exhibited less atelectasis than the zero-PEEP group, with atelectasis involving a median 1.8% of total lung area compared with 4.6% in the zero-PEEP group (P = 0.002).

Paper IV: Postoperative atelectasis was compared between a group in which PEEP was maintained during emergence preoxygenation with FiO2 1.0 and a group in which PEEP was withdrawn just before the start of emergence preoxygenation with FiO2 1.0. The two groups had small atelectasis when fully awake at 30 min after extubation, with no statistically significant difference between them.  

In conclusion, preserved end-expiratory lung volume is the key to avoiding atelectasis, in particular when an increased oxygen reserve is required during airway manipulation. PEEP is both necessary and sufficient to minimize atelectasis in healthy patients undergoing non-abdominal surgery.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 63
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 1566
Keywords
General anaesthesia, pulmonary atelectasis, positive end-expiratory pressure, oxygen, computed tomography, continuous positive airway pressure, mechanical ventilation
National Category
Anesthesiology and Intensive Care
Research subject
Anaesthesiology and Intensive Care
Identifiers
urn:nbn:se:uu:diva-380317 (URN)978-91-513-0635-3 (ISBN)
Public defence
2019-05-31, Vårdskolans aula, ingång 21, Västmanlands sjukhus Västerås, 13:15 (Swedish)
Opponent
Supervisors
Available from: 2019-05-10 Created: 2019-04-08 Last updated: 2019-06-18

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