Wessex ICS Blog

Anaesthesia

Sir Robert Macintosh Lecture (ESA 2013, Barcelona) - No patient has to die of anaesthesia

Anaesthesia and the Lung: Can Oxygen Be Harmful?
Presented at ESA 2013 by Professor Goran Hedenstierna, University Hospital, Uppsala, Sweden.

Summarised by Dr Adrian Wong

It is estimated that over 234 million anaesthetics are given across the world every year (Weiser
et al., 2008). Some studies have suggested that prolonged and sustained hypoxia occurs in up to 50% cases. (Moller et al., 1991).

Anaesthetics reduce functional residual capacity (FRC) and promote airway closure leading to hypoxia. This can be attributed to a loss of respiratory muscle tone, thus allowing the elastic forces of the lung to pull in the chest wall, reducing lung volume. Hence, there is significant atelecstasis and reduced lung compliance in both SV and IPPV. This has been shown using CT studies (Rusca
et al., 2001). Prof. Hedenstierna reminisced about scanning anaesthetised horses in the hospital CT scanner in the dead of night (he had to leave this particular hospital a few weeks later when his “experiments” were discovered).

Radiologically, 40cm H
2O pressure was needed to reinflate the collapsed lungs. 20cm H2O had minimal effect. Even with recruitment manoeuvres, the lungs will start to collapse immediately unless PEEP is applied.

Furthermore, these effects can last a few days and may not be evident on plain chest radiographs. Atelectasis is fertile breeding ground for infection in the post-operative period – this begs the question: are anaesthetists to blame for post-operative pneumonia? Post-operative pulmonary complications have been found in 2 - 20% of patients after non-cardiac surgery, and in a retrospective study of 161,000 patients after major non-cardiac surgery, pneumonia was found in 1.5% with a mortality of 21% in this group. “The goal of the anaesthetic regime should be to deliver a patient with no atelectasis to the post-operative ward and to keep the lungs open”. Using a lung-protective strategy of ventilation, it is possible to reduce the incidence of pneumonia in the post-operative period (Severgnini
et al., 2013).

Pre-oxygenation using an FiO
2 of 100% has become common practice to avoid hypoxia during the peri-induction period. However, oxygen is rapidly adsorbed behind closed airways, causing lung atelectasis and shunt. Patients who had been induced and pre-oxygenated with 100% O2 for 3 min had atelectasis in 10% of the lung area. Pre-oxygenation with 80% O2 for a similar time resulted in much less atelectasis - 2% of the lung area. Pre-oxygenation with 30% O2 caused almost no atelectasis on CT scans (Acka et al., 1999).

The Professor recommends pre-oxygenation with 100% O
2, followed by a recruitment manoeuvre (inflation to an airway pressure of 40 cm H2O for 10 seconds and to higher airway pressures in patients with reduced abdominal compliance e.g. obesity or abdominal disorders). Pre-oxygenation with 80% O2 may be sufficient in most patients with no anticipated difficulty in managing the airway, but time to hypoxaemia during apnoea decreases from mean 7 to 5 min (Edmark et al., 2001). An alternative, possibly more challenging procedure is induction of anaesthesia with CPAP/PEEP to prevent fall in FRC enabling use of 100% O2. A continuous PEEP of 7–10 cm H2O may not necessarily improve oxygenation but should keep the lung open until the end of anaesthesia. An inspired oxygen concentration of 30–40% or even less, should suffice if the lung is kept open.

The final disaster - post oxygenation. In preparation for extubation, the patient is put on an FiO2 of 100% and a suctioned catheter is passed down the endotracheal tube into the airway. All the good work done by the anaesthetist to keep the lungs open during the operation could be swiftly undone at the very end.

Prof. Hederstierna’s Top Tips
  • Pre-oxygenation with 100% O2 should be followed by a recruitment manoeuvre to reopen collapsed alveoli, or induction of anaesthesia can be done with CPAP/PEEP to maintain FRC. Pre-oxygenation with 80% O2 may be possible in the lung- healthy, non-obese patient with no anticipated difficulty in airway management, followed by a gentle inflation of the lung (shorter apnoea tolerance time but easier to open closed airways than collapsed alveoli).
  • Recruitment by inflation of the lung to an airway pressure of 40 cm H2O for 10 seconds in lung-healthy, normal-weight patients and to higher airway pressures in patients with reduced abdominal compliance (obese and patients with abdominal disorders) after pre-oxygenation with 100% O2 and every 30 min, or a continuous PEEP of 7–10 cm H2O.
  • Low inspired oxygen concentration (30–40% or even less) if no need of higher concentration.
  • High inspired oxygen concentration should only be given together with PEEP.
  • Post-oxygenation with or without a recruitment manoeuvre and with or without airway suctioning should not be performed routinely but on indication.
  • Deliver a patient with no atelectasis to the post-operative ward and keep the lung open.
References
  1. Hedenstierna G. Oxygen and anesthesia: what lung do we deliver to the post-operative ward? Acta Anaesthesiol Scand. 2012; 56(6): 675-85.
  2. Weiser et al. An estimation of global volume of surgery: a modelling strategy based on data available. Lancet 2008; 372: 139-144.
  3. Moller et al. Hypoxia during anaesthesia – an observational study. BJA 1991; 66: 437-444.
  4. Rusca M, Wicky S, Proietti S, et al. Continuous positive airways pressure prevents atelectasis formation during induction of general anaesthesia. Anesthesiology 2001; 95: A1331.
  5. Akca O, Podolsky A, Eisenhuber E, et al. Comparable postoperative pulmonary atelectasis in patients given 30% or 80% oxygen during and 2 hours after colon resection. Anesthesiology 1999; 91: 991–8.
  6. Edmark L, Enlund M, Kostova‐Aherdan K, Hedenstierna G. Atelectasis formation and apnoea tolerance after pre‐oxygenation with 100%, 80%, or 60% oxygen. Anesthesiology 2001; 95: A1330.
  7. Severgnini et al. Protective Mechanical Ventilation during General Anesthesia for Open Abdominal Surgery Improves Postoperative Pulmonary Function. Anaesthesiology 2013; 118: 1307-1321.

Adapted from the ESA 2013 Summary Newsletter


comments powered by Disqus

Comments