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Pulmonary Atelectasis in General Anaesthesia: Clinical Studies on the Counteracting Effects of Positive End-Expiratory Pressure
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Medicinska och farmaceutiska vetenskapsområdet, centrumbildningar mm, Centre for Clinical Research, County of Västmanland.ORCID iD: 0000-0002-7449-3907
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 [en]
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: urn:nbn:se:uu:diva-380317ISBN: 978-91-513-0635-3 (print)OAI: oai:DiVA.org:uu-380317DiVA, id: diva2:1303045
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
List of papers
1. Preserved oxygenation in obese patients receiving protective ventilation during laparoscopic surgery: a randomized controlled study
Open this publication in new window or tab >>Preserved oxygenation in obese patients receiving protective ventilation during laparoscopic surgery: a randomized controlled study
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2016 (English)In: Acta Anaesthesiologica Scandinavica, ISSN 0001-5172, E-ISSN 1399-6576, Vol. 60, no 1, p. 26-35Article in journal (Refereed) Published
Abstract [en]

BACKGROUND: Venous admixture from atelectasis and airway closure impedes oxygenation during general anaesthesia. We tested the hypothesis that continuous positive airway pressure (CPAP) during pre-oxygenation and reduced fraction of inspiratory oxygen (FIO2 ) during emergence from anaesthesia can improve oxygenation in patients with obesity undergoing laparoscopic surgery.

METHODS: In the intervention group (n = 20, median BMI 41.9), a CPAP of 10 cmH2 O was used during pre-oxygenation and induction of anaesthesia, but no CPAP was used in the control group (n = 20, median BMI 38.1). During anaesthesia, all patients were ventilated in volume-controlled mode with an FIO2 of 0.4 and a positive end-expiratory pressure (PEEP) of 10 cmH2 O. During emergence, before extubation, the control group was given an FIO2 of 1.0 and the intervention group was divided into two subgroups, which were given an FIO2 of 1.0 or 0.31. Oxygenation was assessed perioperatively by the estimated venous admixture (EVA).

RESULTS: The median EVA before pre-oxygenation was about 8% in both groups. During anaesthesia after intubation, the median EVA was 8.2% in the intervention vs. 13.2% in the control group (P = 0.048). After CO2 pneumoperitoneum, the median EVA was 8.4% in the intervention vs. 9.9% in the control group (P > 0.05). One hour post-operatively, oxygenation had deteriorated in patients given an FIO2 of 1.0 during emergence but not in patients given an FIO2 of 0.31.

CONCLUSIONS: A CPAP of 10 cmH2 O during pre-oxygenation and induction, followed by PEEP after intubation, seemed to preserve oxygenation during anaesthesia. Post-operative oxygenation depended on the FIO2 used during emergence.

National Category
Anesthesiology and Intensive Care
Research subject
Physiology; Anaesthesiology and Intensive Care
Identifiers
urn:nbn:se:uu:diva-264212 (URN)10.1111/aas.12588 (DOI)000368139400005 ()26235391 (PubMedID)
Available from: 2015-10-07 Created: 2015-10-07 Last updated: 2019-04-08
2. Minimizing atelectasis formation during general anaesthesia-oxygen washout is a non-essential supplement to PEEP
Open this publication in new window or tab >>Minimizing atelectasis formation during general anaesthesia-oxygen washout is a non-essential supplement to PEEP
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2017 (English)In: Upsala Journal of Medical Sciences, ISSN 0300-9734, E-ISSN 2000-1967, Vol. 122, no 2, p. 92-98Article in journal (Refereed) Published
Abstract [en]

Background: Following preoxygenation and induction of anaesthesia, most patients develop atelectasis. We hypothesized that an immediate restoration to a low oxygen level in the alveoli would prevent atelectasis formation and improve oxygenation during the ensuing anaesthesia. Methods: We randomly assigned 24 patients to either a control group (n=12) or an intervention group (n=12) receiving an oxygen washout procedure directly after intubation. Both groups were, depending on body mass index, ventilated with a positive end-expiratory pressure (PEEP) of 6-8 cmH(2)O during surgery. The atelectasis area was studied by computed tomography before emergence. Oxygenation levels were evaluated by measuring blood gases and calculating estimated venous admixture (EVA). Results: The atelectasis areas expressed as percentages of the total lung area were 2.0 (1.5-2.7) (median [interquartile range]) and 1.8 (1.4-3.3) in the intervention and control groups, respectively. The difference was non-significant, and also oxygenation was similar between the two groups. Compared to oxygenation before the start of anaesthesia, oxygenation at the end of surgery was improved in the intervention group, mean (SD) EVA from 7.6% (6.6%) to 3.9% (2.9%) (P=.019) and preserved in the control group, mean (SD) EVA from 5.0% (5.3%) to 5.6% (7.1%) (P=.59). .Conclusion: Although the oxygen washout restored a low pulmonary oxygen level within minutes, it did not further reduce atelectasis size. Both study groups had small atelectasis and good oxygenation. These results suggest that a moderate PEEP alone is sufficient to minimize atelectasis and maintain oxygenation in healthy patients.

Place, publisher, year, edition, pages
TAYLOR & FRANCIS LTD, 2017
Keywords
Atelectasis, computed tomography, general anaesthesia, oxygenation, PEEP, protective ventilation, ventilator settings
National Category
Anesthesiology and Intensive Care
Identifiers
urn:nbn:se:uu:diva-323496 (URN)10.1080/03009734.2017.1294635 (DOI)000401756500004 ()28434271 (PubMedID)
Available from: 2017-06-22 Created: 2017-06-22 Last updated: 2019-04-08Bibliographically approved
3. Positive End-expiratory Pressure Alone Minimizes Atelectasis Formation in Nonabdominal Surgery: A Randomized Controlled Trial
Open this publication in new window or tab >>Positive End-expiratory Pressure Alone Minimizes Atelectasis Formation in Nonabdominal Surgery: A Randomized Controlled Trial
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2018 (English)In: Anesthesiology, ISSN 0003-3022, E-ISSN 1528-1175, Vol. 128, no 6, p. 1117-1124Article in journal (Refereed) Published
Abstract [en]

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. Hie 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 (Pao(2)/Fio(2) 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. Tire difference in medians was 2.8% (95% CI, 1.7 to 5.7%; A = 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. Titus, routine recruitment maneuvers seem unnecessary, and the authors suggest that they should only be utilized when clearly indicated.

Place, publisher, year, edition, pages
LIPPINCOTT WILLIAMS & WILKINS, 2018
National Category
Anesthesiology and Intensive Care
Identifiers
urn:nbn:se:uu:diva-363070 (URN)10.1097/ALN.0000000000002134 (DOI)000441172900012 ()29462011 (PubMedID)
Available from: 2018-10-12 Created: 2018-10-12 Last updated: 2019-04-08Bibliographically approved
4. Positive end-expiratory pressure and postoperative atelectasis: A randomized controlled trial
Open this publication in new window or tab >>Positive end-expiratory pressure and postoperative atelectasis: A randomized controlled trial
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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.

National Category
Anesthesiology and Intensive Care
Research subject
Anaesthesiology and Intensive Care
Identifiers
urn:nbn:se:uu:diva-381346 (URN)
Available from: 2019-04-08 Created: 2019-04-08 Last updated: 2019-04-08

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