Abstract: We included 12 trials with 695 participants; five trials included participants awaiting elective cardiac surgery and seven trials included participants awaiting elective major abdominal surgery. All trials contained at least one domain judged to be at high or unclear risk of bias. Of greatest concern was the risk of bias associated with inadequate blinding, as it was impossible to blind participants due to the nature of the study designs. We could pool postoperative atelectasis in seven trials (443 participants) and postoperative pneumonia in 11 trials (675 participants) in a meta-analysis. Preoperative IMT was associated with a reduction of postoperative atelectasis and pneumonia, compared with usual care or non-exercise intervention (respectively; risk ratio (RR) 0.53, 95% confidence interval (CI) 0.34 to 0.82 and RR 0.45, 95% CI 0.26 to 0.77). We could pool all-cause mortality within postoperative period in seven trials (431 participants) in a meta-analysis. However, the effect of IMT on all-cause postoperative mortality is uncertain (RR 0.40, 95% CI 0.04 to 4.23). Eight trials reported the incidence of adverse events caused by IMT. All of these trials reported that there were no adverse events in both groups. We could pool the mean duration of hospital stay in six trials (424 participants) in a meta-analysis. Preoperative IMT was associated with reduced length of hospital stay (MD -1.33, 95% CI -2.53 to -0.13). According to the Grades of Recommendation, Assessment, Development and Evaluation (GRADE) Working Group guidelines for evaluating the impact of healthcare interventions, the overall quality of studies for the incidence of pneumonia was moderate, whereas the overall quality of studies for the incidence of atelectasis, all-cause postoperative death, adverse events, and duration of hospital stay was low or very low. We found evidence that preoperative IMT was associated with a reduction of postoperative atelectasis, pneumonia, and duration of hospital stay in adults undergoing cardiac and major abdominal surgery. The potential for overestimation of treatment effect due to lack of adequate blinding, small-study effects, and publication bias needs to be considered when interpreting the present findings.

Plain-language summary: We included 12 trials with 695 participants. Five of the 12 studies included participants awaiting planned heart surgery, and seven studies included participants awaiting planned major abdominal surgery. The evidence is current to October 2014. This review showed that training of breathing muscles before surgery reduced the risk of some lung complications (atelectasis and pneumonia) after surgery and the length of hospital stay, compared with usual care. However, the effect of this training on in-hospital death after surgery is unclear and needs further investigation. The trials did not report any undesirable effects associated with training of breathing muscles, and no study reported on costs resulting from breathing training using a device. Although the available evidence is insufficient in terms of the quality and size of trials, we can conclude that training of breathing muscles before surgery prevents lung complications after surgery. This training is easily performed at home under the supervision of a physiotherapist. The training of breathing muscles therefore appears to be a suitable option as one of the preparations for planned surgery, especially for adults awaiting high-risk heart and abdominal surgery. Other surgeries, such as oesophageal resection (removal of part of the gastrointestinal tract 'food pipe'), should be evaluated; cost-effectiveness and patient-reported outcomes should be reported. The potential for overestimation of treatment effect needs to be considered when interpreting the present findings, as the quality of evidence is low to moderate.