In a systematic Cochrane review and in an overview of JAMA surgery, it was found that there is no evidence that postoperative PEP use or incentive spirometry (same method) prevents postoperative pulmonary complications (1, 2). Nevertheless, the PEP method is used in Sweden and internationally. There are no studies that investigate the effect on continuous oxygen and carbon dioxide levels below and directly after the actual PEP use. If oxygenation is improved, and the carbon dioxide content decreases with PEP, the method is likely to be safe. If oxygenation instead drops and the carbon dioxide level rises when breathing out a resistance, the use of postoperative PEP should be questioned