Applying positive pressure at the end of each breath during anaesthesia for prevention of mortality and postoperative pulmonary complications

Review question

We reviewed the evidence on the effects of positive end-expiratory pressure (PEEP) during general anaesthesia in adult patients 16 years of age and older.


PEEP is a mechanical technique that is often used for ventilating an unconscious patient. The technique involves adding a quantity of pressure into the lungs at the end of each breath. This process causes a degree of deflation in the lungs and can collapse some areas because between breaths, the lungs contain less air than usual. By adding positive pressure at that time, we aim to reinflate the collapsed areas of the lung (atelectasis). Although PEEP can be used during general anaesthesia, some lung areas collapse at the end of the anaesthetic procedure. We do not know whether patients who receive PEEP have lower risks of postoperative mortality (approximately 3% to 5% of adult patients) or respiratory complications. In this review, we aimed to assess the postoperative benefits and harms of using PEEP during general anaesthesia.

Study characteristics

The evidence is current to October 2013. We found 10 randomized clinical trials involving 432 participants. The main limitation of our review was our inability to identify studies analysing intraoperative data.

Key results

Six trials reported mortality. We pooled these data and found no differences between the group of patients who received PEEP and those who did not, but because of the small number of patients, and the fact that this outcome may be rare, these results did not allow us to make a conclusion about the effect of PEEP on mortality. Two results suggested some benefit of PEEP. First, oxygenation was better on the day after surgery in the PEEP group. Second, radiological imaging showed less atelectasis after surgery in the PEEP group. The studies that we found did not suggest that intraoperative PEEP causes harm.

Because of the small number of studies, this finding is inconclusive. We performed calculations to predict how many more participants would be needed before reliable conclusions can be made about the effect on mortality of the application of PEEP. This number was 21,200.

Evidence is currently insufficient to allow conclusions about how intraoperative PEEP affects postoperative mortality and respiratory complications.

Quality of the evidence

The quality of the evidence is very low because of poorly conducted studies, small numbers of participants and low event rates.

Authors' conclusions: 

Evidence is currently insufficient to permit conclusions about whether intraoperative PEEP alters risks of postoperative mortality and respiratory complications among undifferentiated surgical patients.

Read the full abstract...

General anaesthesia causes atelectasis, which can lead to impaired respiratory function. Positive end-expiratory pressure (PEEP) is a mechanical manoeuvre that increases functional residual capacity (FRC) and prevents collapse of the airways, thereby reducing atelectasis. It is not known whether intraoperative PEEP alters the risks of postoperative mortality and pulmonary complications. This review was originally published in 2010 and was updated in 2013.


To assess the benefits and harms of intraoperative PEEP in terms of postoperative mortality and pulmonary outcomes in all adult surgical patients.

Search strategy: 

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) 2013, Issue 10, part of The Cochrane Library, as well as MEDLINE (via Ovid) (1966 to October 2013), EMBASE (via Ovid) (1980 to October 2013), CINAHL (via EBSCOhost) (1982 to October 2013), ISI Web of Science (1945 to October 2013) and LILACS (via BIREME interface) (1982 to October 2010). The original search was performed in January 2010.

Selection criteria: 

We included randomized clinical trials assessing the effects of PEEP versus no PEEP during general anaesthesia on postoperative mortality and postoperative respiratory complications in adults, 16 years of age and older.

Data collection and analysis: 

Two review authors independently selected papers, assessed trial quality and extracted data. We contacted study authors to ask for additional information, when necessary. We calculated the number of additional participants needed (information size) to make reliable conclusions.

Main results: 

This updated review includes two new randomized trials. In total, 10 randomized trials with 432 participants and four comparisons are included in this review. One trial had a low risk of bias. No differences were demonstrated in mortality, with risk ratio (RR) of 0.97 (95% confidence interval (CI) 0.20 to 4.59; P value 0.97; 268 participants, six trials, very low quality of evidence (grading of recommendations assessment, development and evaluation (GRADE)), and in pneumonia, with RR of 0.40 (95% CI 0.11 to 1.39; P value 0.15; 120 participants, three trials, very low quality of evidence (GRADE)). Statistically significant results included the following: The PEEP group had higher arterial oxygen pressure (PaO2)/fraction of inspired oxygen (FiO2)on day one postoperatively, with a mean difference of 22.98 (95% CI 4.40 to 41.55; P value 0.02; 80 participants, two trials, very low quality of evidence (GRADE)), and postoperative atelectasis (defined as an area of collapsed lung, quantified by computerized tomography scan) was less in the PEEP group (standard mean difference -1.2, 95% CI -1.78 to -0.79; P value 0.00001; 88 participants, two trials, very low quality of evidence (GRADE)). No adverse events were reported in the three trials that adequately measured these outcomes (barotrauma and cardiac complications). Using information size calculations, we estimated that a further 21,200 participants would have to be randomly assigned to allow a reliable conclusion about PEEP and mortality.