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Original research
Determining a safe upper limit of oxygen supplementation for adult patients: a systematic review
  1. Mathilde Languille Lassen1,
  2. Bjarke Risgaard1,
  3. Josefine S Baekgaard1,
  4. Lars S Rasmussen1,2
  1. 1Department of Anaesthesia, Centre of Head and Orthopaedics, Rigshospitalet, Copenhagen, Denmark
  2. 2Department of Clinical Medicine, University of Copenhagen, Copenhagen, Denmark
  1. Correspondence to Mathilde Languille Lassen; languillelassen{at}


Objective This systematic review aimed to describe the connection between the inspired oxygen fraction and pulmonary complications in adult patients, with the objective of determining a safe upper limit of oxygen supplementation.

Methods MEDLINE and Embase were systematically searched in August 2019 (updated July 2020) for studies fulfilling the following criteria: intubated adult patients (Population); high fractions of oxygen (Intervention) versus low fractions of (Comparison); atelectasis, acute respiratory distress syndrome (ARDS), pneumonia and/or duration of mechanical ventilation (Outcome); original studies both observational and interventional (Studies). Screening, data extraction and risk of bias assessment was done by two independent reviewers.

Results Out of 6120 records assessed for eligibility, 12 were included. Seven studies were conducted in the emergency setting, and five studies included patients undergoing elective surgery. Eight studies reported data on atelectasis, two on ARDS, four on pneumonia and two on duration of mechanical ventilation. There was a non-significant increased risk of atelectasis if an oxygen fraction of 0.8 or above was used, relative risk (RR): 1.37 (95% CI 0.95 to 1.96). One study showed an almost threefold higher risk of pneumonia in the high oxygen fraction group (RR: 2.83 (95% CI 2.25 to 3.56)). The two studies reporting ARDS and the two studies with data on mechanical ventilation showed no association with oxygen fraction. Four studies had a high risk of bias in one domain.

Conclusions In this systematic review, we found inadequate evidence to identify a safe upper dosage of oxygen, but the identified studies suggest a benefit of keeping inspiratory oxygen fraction below 0.8 with regard to formation of atelectases.

PROSPERO registration number CRD42020154242.

  • adult anaesthesia
  • adult intensive & critical care
  • respiratory medicine (see thoracic medicine)
  • respiratory physiology
  • adult intensive & critical care

Data availability statement

Data are available on reasonable request. This was a systematic review and researchers can contact the authors to access the material.

This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. See:

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Strengths and limitations of this study

  • The use of predefined Population, Intervention, Comparison, Outcome and Study design to asses studies for eligibility.

  • The use of a wide search string in two databases.

  • Two independent reviewers screening and including studies, assessing risk of bias and extracting data.

  • There is a risk of publication bias that arises due to the possibility of missing unpublished studies.

  • It is possible that our search did not identify all relevant studies.


Oxygen is a molecule vital for life, as it is the cornerstone in cellular respiration in all aerobic organisms. In trauma care, during anaesthesia and in the management of respiratory failure, an oxygen fraction (FiO2) of 0.21 may not be sufficient to maintain an acceptable oxygen concentration in arterial blood and oxygen supplementation is therefore often part of standard care.1 2

Supplementary oxygen may result in hyperoxaemia, with the risk of tissue hyperoxia. An increasing amount of evidence has connected hyperoxia and hyperoxaemia with increased mortality3–6 possibly as a consequence of a variety of factors associated with hyperoxia: atelectasis in the lungs,7 8 formation of reactive oxygen species,9 impairment of the innate immune system,10 as well as vasoconstriction with paradox tissue hypoxia to follow.11

All in all, hypoxia should be avoided, but at the same time it seems that exposure to high concentrations of oxygen may have serious consequences. Therefore, it is relevant to investigate if a safe upper dosage of oxygen can be identified.

This systematic review aimed to describe the connection between the FiO2 and pulmonary complications in intubated adult patients, with the objective of determining a safe upper limit of oxygen supplementation. We defined pulmonary complications as atelectasis, pneumonia and acute respiratory distress syndrome (ARDS).


Protocol and registration

Methods of the analysis and inclusion criteria were prespecified and documented in a protocol. The protocol was completed following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines for protocols.12 a13

Eligibility criteria

Studies were selected according to following predefined Population, Intervention, Comparison, Outcome and Study design (PICOS).

Inclusion criteria

  • Population: intubated patients ≥18 years.

  • Intervention and Comparison: low inspiratory FiO2 (as defined by author) versus high FiO2 (as defined by authors).

  • Outcome: atelectasis, pneumonia, ARDS and duration of mechanical ventilation (as defined by authors).

  • Study design: original studies both interventional and observational.

Exclusion criteria

  • Hyperbaric oxygen treatment.

  • Case reports, review articles and editorials.

We had no restrictions on year of publication. The search was restricted to studies published in French, English or Danish.

Information sources and search

We searched MEDLINE and Embase using the following predefined search string (presented search strategy is from MEDLINE).

  1. (((((((oxygen [Title/Abstract]) OR oxygen[MeSH Terms]) OR hyperoxia[Title/Abstract]) OR “supplemental oxygen”[Title/Abstract]) OR “oxygen supplementation”[Title/Abstract]) OR fio2[Title/Abstract])))

  2. ((((((((((((((atelectasis[Title/Abstract]) OR pulmonary atelectasis[MeSH Terms]) OR pneumonia[Title/Abstract]) OR pneumonia[MeSH Terms]) OR “lung collapse”[Title/Abstract]) OR “collapsed lung”[Title/Abstract]) OR “acute lung injury”[Title/Abstract]) OR acute lung injury[MeSH Terms]) OR ARDS[Title/Abstract]) OR “acute respiratory distress syndrome”[Title/Abstract]) OR respiratory distress syndrome, adult[MeSH Terms])))

  3. (intub*) OR ”mechanical ventilation”

  4. #1 AND #2 AND #3.

The search was done the 6 August 2019. The search was updated the 6 July 2020. Modifications were made to fit Embase.

We identified one additional record14 by obtaining the full-text article of an abstract identified through the search string. Another record15 was identified by screening the reference list of an article.

Selection process

Two independent reviewers (MLL and BR) screened all titles and abstracts yielded by the search against the inclusion criteria using Covidence (an online programme facilitating the production of systematic reviews developed by the Cochrane group).16 A Cohen’s Kappa for inter-rater reliability was calculated. The same reviewers obtained full text articles for all titles that appeared to meet the inclusion criteria or where there was any uncertainty. Disagreements were resolved through discussion until consensus. All full-text articles were assessed by the same two independent reviewers and those not meeting the inclusion criteria were excluded.

Data collection and data items

Data extraction was done by two authors (MLL and BR), and was facilitated by the data-extraction tool Covidence and by using predefined forms. We collected study characteristics including trial design, trial size, country, period and year of publication. From the included studies we extracted the dosage of oxygen, type of control used, duration of treatment, patient characteristics (gender, age, patient type) as well as data on the predefined outcomes (atelectasis, pneumonia, ARDS) as defined by the authors.

Risk of bias

Risk of bias for non-randomised studies were assessed by using the Newcastle-Ottawa Scale.17 Here each study can be awarded from zero to nine stars, with zero stars representing a high risk of bias, and nine stars a low risk. Each study can be judged and awarded stars on eight items, categorised into three domains: selection of the study group, comparability of cohorts, and evaluation of the outcome of interest.

For randomised studies we used the Cochrane Collaboration tool for assessing risk of bias (Table 8.5.a in the Cochrane Handbook for Systematic Reviews of Intervention) in Covidence, which covers: sequence generation, allocation concealment, blinding, incomplete data and selective outcome reporting. A judgement as to the possible risk of bias on each domain were made from the extracted information, rated as ‘high risk’, ‘low risk’ or ‘unclear’ risk of bias. These judgements were made based on the criteria for judging the risk of bias (Table 8.5.d in the Cochrane Handbook Higgins 2011).

Summary measures and synthesis of results

This systematic review was expected to be a descriptive summary of the current evidence on oxygen supplementation and pulmonary complications. Relative risk (RR) was calculated where possible and a forest plot was used to illustrate the results. RRs with 95% CIs, was calculated in studies where this information was missing and the calculation was possible. The forest plot was made with a random-effects model.

Patient and public involvement

No patient involved.


Study selection

Our initial search strategy identified 7734 records. After duplicates were removed and two additional records from other sources were added, 6120 records were screened. Of these, 6100 were excluded as they did not fulfil eligibility criteria leaving 20 records for full-text screening. Cohen’s kappa for inter-rater reliability of 0.43 (95% CI 0.26 to 0.60) was calculated, which is judged to be moderate agreement. After full-text review, 12 records fulfilled the inclusion criteria (figure 1).

Figure 1

Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) flow diagram of identification, screening, eligibility and inclusion process28 on a search for studies comparing low dose oxygen supplementation with high dose oxygen supplementation with pulmonary complications as an outcome.37

Study characteristics

Study characteristics are summarised in table 1. Eight of the 12 included studies were randomised controlled trials. Among the four remaining there were two retrospective observational studies18 19 and two prospective observational studies.20 21 About half of the studies were conducted in Europe. Seven studies were conducted in the acute care setting. Of these seven, one study22 included patients with septic shock, four studies14 18 21 23 recruited surgical, medical and trauma patients that were mechanically ventilated in the intensive care unit, one study20 included patients with acute lung injury and the last study24 recruited patients with traumatic brain injury. The remaining five studies included patients undergoing different types of elective surgery.

Table 1

Characteristics of studies comparing low FiO2 with high FiO2, with lung complications as an outcome.

The administered FiO2 varied substantially among the studies, with FiO2 ranging from 0.26 to 0.60 in the low FiO2 group and from 0.36 to 1.0 in the high FiO2 group.

Table 2 presents the outcomes of interest reported in the included studies. Eight studies reported on the incidence of atelectasis, two studies reported on ARDS, four studies reported on pneumonia and two studies reported on the duration of mechanical ventilation.

Table 2

Patient outcomes comparing low doses of oxygen supplementation with high doses of oxygen supplementation


The eight studies reporting on atelectasis, generally showed better outcomes for patients in the low FiO2 group, as two studies22 25 showed almost two-fold higher risk of atelectasis in the high FiO2, with RR: 1.875 (95% CI 0.42 to 8.37) and RR: 2.0 (95% CI 1.06 to 3.79), respectively. One study15 suggested a minor benefit of treatment with low FiO2, but this was not statistically significant, RR: 1.46 (95% CI 0.97 to 2.20). Another study24 found RR: 0.91 (95% CI 0.56 to 1.50) suggesting a benefit of treatment with high FiO2, but this was not statistically significant. These studies are illustrated in the forest plot (figure 2), which shows that in general treatment with high FiO2 was associated with higher risk of atelectasis formation, RR:1.37 (95% CI 0.95 to 1.96). The heterogeneity (I2) of the meta-analysis presented in figure 2 is 31%, which corresponds to a moderate heterogeneity (Cochrane Handbook for Systematic Reviews of Intervention, section 9.5.2 Identifying and measuring heterogeneity).

Figure 2

Forest plot of formation of atelectasis in studies comparing low FiO2 with high FiO2. FiO2, oxygen fraction; M.H, Maentel-Haentzel.

Rothen et al8 found a 16.8 times greater area of atelectasis in the high FiO2 group and similarly, the study by Benoît et al26 found a threefold larger atelectatic surface in the high FiO2 group. Suzuki et al21 estimated atelectasis as time-weighted averages, and also found a beneficial effect of a low FiO2. In the study by Ishii et al18 additional information on intubated patients were found in an abstract27 from the same study. They found a higher incidence of atelectasis in the high FiO2 group, but the total number of patients was not reported.

Acute respiratory distress syndrome

Panwar et al14 showed an increase of new-onset ARDS in the low FiO2 group, RR: 0.87 (95% CI 0.43 to 1.75), but this was not statistically significant. The study by Lång et al24 found three patients with ARDS in the low FiO2 group, while no patients with ARDS were identified in the group receiving high FiO2.


The study by Staehr-Rye et al19 showed a significant increase in the incidence of pneumonia, RR: 2.83 (95% CI 2.25 to 3.56) in the high FiO2 group. Similarly, Barrot et al23 showed a small, but non-significant, tendency to ventilator-associated pneumonias in the high FiO2 group, RR: 1.26 (95% CI 0.71 to 2.22). The two other studies, Asfar et al22 and Lång et al,24 found a non-significant tendency for pneumonia in the low FiO2 group with RR: 0.94 (95% CI 0.59 to 1.49) and RR: 0.71 (95% CI 0.26 to 1.97), respectively. These studies are illustrated in the forest plot (figure 3), which shows a non-significant tendency that treatment with high FiO2 was associated with higher risk of pneumonia, RR: 1.32 (95% CI 0.65 to 2.70).

Figure 3

Forest plot of risk of pneumonia in studies comparing low FiO2 with high FiO2. FiO2, oxygen fraction.; M.H. Random, Maentel-Haentzel.

Duration of mechanical ventilation

The two studies reporting the duration of mechanical ventilation pointed in opposite direction. Lång et al24 reported slightly more time spent on mechanical ventilation in the low FiO2 group, while Rachmale et al20 reported a twofold increase in time in the high FiO2 group.

Risk of bias assessment

Risk of bias for randomised studies are illustrated in table 3. Three studies had no blinding of participants, personal or outcome assessment, leaving them with a high risk of bias on these domains.8 14 22 In the study by Rothen et al8 it was unclear if a randomisation was performed between the low FiO2 group and the high FiO2 group, indicating a high risk of bias.

Table 3

Risk of bias assessment for randomised controlled trials comparing low dose oxygen supplementation with high dose oxygen supplementation

Lång et al24 was an open-label trial, and was therefore awarded a high risk of bias on the domain of blinding of participants and personnel, however, the outcome assessor was blinded.

The four non-randomised studies were assessed using the New-Castle Ottawa Scale.17 One study20 scored six stars, two studies18 19 scored seven stars and one study21 scored eight stars, indicating an overall high quality of the studies.


Summary of findings

In this study, we were not able to determine a safe upper limit of oxygen supplementation, due to inadequate evidence and heterogeneity as the included studies had different endpoints with varying definitions, and also different ways of defining low and high FiO2. In some studies the FiO2 in the low FiO2 group was higher than in the high FiO2 group in other studies.

Regarding atelectasis, seven of the eight studies favoured a conservative oxygen strategy with low FiO2 and an FiO2 above 0.8 seemed to be associated with higher risk of atelectasis formation. Looking at figure 2, there is an RR of 1.37, which suggests a clinically relevant difference with less atelectasis with a lower FiO2. However, the CI is wide (0.95–1.96), indicating that more information is needed before any firm conclusions can be made.

Strengths and limitations

This study was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines,28 ensuring a systematic and broadly acknowledged approach to the present literature. The strengths of this approach include predefined PICOS criteria to assess study eligibility, use of a wide search string in two databases and two independent reviewers screened and assessed studies, including risk of bias.

Our study is limited by general weaknesses of systematic reviews. This includes risk of publication bias that arises due to the possibility of missing non-published studies. Despite the systematic search with predefined search string, and screening of reference lists of included studies, there is always a possibility that our search did not identify all relevant studies. However, the heterogeneity of the 12 studies reviewed makes us believe that potentially missed studies would not change the conclusion substantially. It is possible that more studies could have been found by searching in a wider set of databases. However, we chose the most commonly used databases MEDLINE and EMBASE, where the quality is known to be best and where most studies are found.

The patient population was determined in very broad terms (intubated adult patients), resulting in more heterogeneity among the included studies.

The trials varied in patient groups, associated clinical care and disease severity. Furthermore, in some studies it is unclear when exactly the outcome of interest was measured (early or late onset of ARDS and timing of CT/X-ray for measuring the presence of atelectasis). It is also unclear how pneumonia was defined in the four studies reporting this outcome. Therefore, conclusions should be drawn with caution.

Half of the randomised controlled trials were not blinded to personnel and participants, increasing the risk of performance bias. Three of these were not blinded to outcome assessors which increase the risk of detection bias. In general, many of the studies are relatively small, increasing the risk of other bias such as publication bias (table 3).

Atelectasis was defined in different ways complicating the pooling of data and the possibility to undertake a meta-analysis. Three studies8 15 25 used CT-scans and they all considered densities between −100 and +100 Hounsfield as atelectasis. Of these three, one8 measured areas of atelectasis in cm2 whereas the two others15 25 measured if atelectases were present or not. Ishii et al18 also used CT-scans, but defined atelectases as areas with formation of more than 10 mm thick atelectasis from the first to the second scan. The study by Staehr et al25 did not define specific criteria on when densities were judged as atelectasis or not.

Asfar et al22 and Suzuki et al21 used chest X-rays, without defining atelectasis specifically, as this was decided by the individual physician. Lång et al24 used chest X-rays in the same manner, however they allowed the appliance of positive end-expiratory pressure to minimise atelectasis, which makes it hard to directly compare results with other studies. Only Suzuki et al21 used more than one radiologist to perform the outcome assessment.

In Panwar et al,14 new-onset ARDS was defined as subsequent occurrence of ARDS in those patients who did not have ARDS on day 0, and where ARDS was present according to the Berlin definition.29 Lång et al24 did not report their definition of ARDS.

Regarding pneumonia, the database study of 26 841 patients performed by Staehr-Rye et al19 found a significant, almost threefold higher risk of pneumonia in the liberal oxygen group, indicating that excess levels of oxygen may be harmful. However, this is an analysis of administrative data, with risk of misclassification bias, and therefore, direct conclusions should be drawn with caution.

Other reviews

The evidence for the use of supplemental oxygen has been investigated in recently published systematic reviews. A systematic review and meta-analysis by Damiani et al30 from 2014 suggests an association between hyperoxia and mortality in patients with stroke, traumatic brain injury and those resuscitated from cardiac arrest. However, they concluded that their results were limited by the heterogeneity of the included studies. The same conclusion was drawn in another meta-analysis from 2015 by Helmerhorst et al.31 No definite conclusions could be made due to heterogeneity in the included studies; however, the meta-analysis suggested a benefit of conservative oxygen therapy. In a Cochrane review from 2015 by Wetterslev et al,32 comparing low (FiO2 0.30–0.40) vs high (FiO2 0.60–0.90) perioperative inspiratory FiO2, they found no association between perioperative FiO2 and postoperative surgical site infection and mortality. In another Cochrane review from 2016 performed by Cabello et al,33, they focused on patients with acute myocardial infarctions. They included five studies and found no clear recommendations on the use of oxygen supplementation.

In a recent meta-analysis performed in 2018 by Chu et al,4 they included 25 randomised controlled trials on acutely ill patients and found a significant association between liberal oxygenation strategies and increased mortality in-hospital, at 30 days and at longest follow-up. Nevertheless, morbidity outcomes were similar between groups.

The available reviews are limited because of heterogeneity, including different outcome measures, overall indicate that excess oxygen is harmful, stressing the need for further investigation on this subject.

Oxygen supplementation is obviously a vital part of trauma care, practice of anaesthesia, the management of respiratory distress and treatment of a variety of other conditions. However, supplemental oxygen should be carefully considered a drug and prescribed adequately. There is a general lack of strong evidence for supplemental oxygen, and an upper limit of oxygen supplementation is not included in many guidelines.1 34–36 Our study contributes to the current evidence in a different way, by looking at the association between FiO2 and pulmonary complications, which is a highly relevant indicator in the search for a safe upper limit of oxygen supplementation.

As oxygen supplementation is so widely used, it is crucial that better evidence-based guidelines are developed. Future research is required to precisely define the oxygen therapy strategies to maximise benefits and minimise harms.


In this systematic review, we found that there was inadequate evidence to identify a safer upper dosage of oxygen, but the identified studies suggest a benefit of conservative oxygen therapy, defined as FiO2 ≤0.8 with regard to formation of atelectasis.

Data availability statement

Data are available on reasonable request. This was a systematic review and researchers can contact the authors to access the material.

Ethics statements



  • Contributors The idea for the article was formed by LSR. Literature search was performed by MLL and BR. MLL wrote the article. JSB contributed with high subject knowledge and all authors gave substantial contributions to the work. Every author revised the work critically for important intellectual content. Every author made a final approval of the version to be published. Every author agreed to be accountable for all aspects of the work.

  • Funding Departmental funding. Award/Grant number is not applicable.

  • Competing interests None declared.

  • Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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