An interesting review article called Oxygen and anesthesia: what lung do we deliver to the post-operative ward? challenges much of the usual anaesthetic practice. We love 100% FiO2 for induction and extubation to get longer apnea times – but what about high FiO2 induced atelectasis? We also tinker with our respirators to get the best oxygenation peroperatively, but what about the post-op lung?
Pneumonia, the unsexy killer
Post operative respiratory failure due to atelectasis is not uncommon. Neither is post-op pneumonia. Risks for post-op pneumonia can be atelectasis, or post-op respiratory failure with micro-aspirations, or prolonged intubation, like the patient that got re-intubated after his post-op respiratory failure.
Since both respiratory failure and pneumonia occur after the anaesthesia itself, from several hours up to days later, the attention deficit anaesthetist’s mind often doesn’t see the the anaesthetic management as a contributing factor.
But we should. Even though acute hypoxia during induction or extubation is a much more intense emergency, the more mundane pneumonia is the bigger killer.
“It is easy to understand that the horror of failed tracheal intubation with severe oxygenation difficulties and the possibility of hypoxemic events during the ensuing anesthesia have prompted the use of supplemental oxygen well in excess of the need to keep arterial oxygen tension (PaO2) at a normal level. However, the use of high oxygen concentration promotes atelectasis formation that impedes the lungs’ ability to oxygenate blood and may make the lungs more vulnerable to pulmonary complications, as will be discussed in the following paragraphs. These effects of anesthesia may show up after the patient has left the surgical theatre and may not be linked to the anesthesia to the extent that they should be. Thus, post-operative pulmonary complications have been found in 2–20%, after noncardiac surgery and in a huge retrospective study of 161,000 patients after major, noncardiac surgery pneumonia was found in 1.5%. This may not seem terrifyingly high, but the mortality was 21% in this group”.
Recruitment maneuvres can be important to keep the lungs open. But there are many theories on how to perform them. There is good evidence that the only recruitment maneuvre that works well, is using 40mmH2O for at least 10 secs. In obese patients, or patients with distended abdomen, much of the force is used just to push down on the diaphragm and to lift the thorax. So they need around 55mmH2O to generate an effective alveoli recruitment.
Figure: CT in anaesthetised obese patients. The first row of images show the patients awake and spontaneously breathing, the next after induction of anaesthesia where atelectasis readily forms.
As seen in the top image series, PEEP had no effect on the amount of initial atelectasis. In the two lower image series, a recruitment maneuver (RM) was done right after induction. In the middle series where RM was followed by PEEP, atelectasis was reduced and the effect was sustained for 20 min. In the bottom image series with zero PEEP (ZEEP), the RM caused a temporary reduction of atelectasis, but this effect was gone already after 5 min.
During recruitment, the authors warn agains high FiO2 – it will ruin much of the effect by creating new atelectasis. Also, during anaesthesia, as a patient’s saturation falls slightly due to atelectasis, and you slowly ramp up the FiO2, more atelectasis may form, followed again by a fall in sats – a vicious cycle. Try lowering your FiO2 and go for recruitment.
These, and several other interesting points can be found in the full article. It’s a great primer on ventilation and respiratory physiology during anaesthesia. You might not find revolutionary new stuff there, but it’s all well structured and confronts you with some of the dilemmas of ventilation.
The next article I found focuses on the final part of anaesthesia – the actual post-op phase where we can also help keep the lungs open. In our institution, we often use PEP flutes for low risk patients, and CPAP on indication. A recent study on BiPAP vs no BiPAP on post-op CABG patients revealed a huge difference on atelectasis, and possibly on respiratory complications. They randomised CABG patients to BiPAP or conventional therapy for the first 24 hrs post-op. The BiPAP group had a 1 day shorter hospital stay, but the big difference was on atelectasis formation: 23% in the conventional group, but only 3 % in the BiPAP group. The conventional group also had a tendency for other respiratory complications, like reintubation, pneumonia and bibasal collapse, but the study group of 120 patients was much to small to be able to prove any significance in these numbers.
To me, both of these articles made me slightly rethink and tailor the ventilation to my patients in the OR. The biggest change for me was that it got me thinking more about the post-op and recovery phase of the patient, rather than just the immediate oxygenation in the OR. Also, lowering my threshold for initiating CPAP or BiPAP for patients at risk.
Here are the two articles to help you deliver the best possible lungs to the post-op ward and beyond.
Effect of adding postoperative noninvasive ventilation to usual care to prevent pulmonary complications in patients undergoing coronary artery bypass grafting: A randomized controlled trial, J Thorac Cardiovasc Surg, 2013 Apr.