ENIGMA - Evaluation of Nitrous Oxide In the Gas Mixture for Anaesthesia: a Randomised Controlled Trial

• The primary endpoint is hospital length of stay (LOS), defined from the start of surgery until actual hospital discharge. Patients transferred to another hospital will be tracked until final discharge to home (or other final destination). LOS is likely
  

• Secondary endpoints will be detected by a research assistant who will be masked to Group identity, using chart review up to 30 days postoperatively; confirmation of each will be sought by an independent member of the Endpoint Committee:
  
• Wound infection - if associated with purulent discharge or a positive microbial culture (46)
  
• Myocardial infarction - confirmed by ECG and/or troponin or CK-MB enzyme rise
  
• Venous thromboembolism - symptomatic DVT or PE, confirmed by venography, duplex ultrasonography, V-Q scan or spiral CT, or autopsy
  
• Stroke - a new neurological deficit persisting for 24 hours, confirmed by neurologist assessment and/or CT scan or MRI
  
• Awareness - postoperative recollection of intraoperative events
  
• Blood transfusion - any red cell transfusion within 30 days of surgery
  
• Pneumothorax, atelectasis, or pneumonia - confirmed by chest x-ray; for pneumonia: also 2 or more of temp> 38oC, white cell count >12,000/ml, positive sputum culture
  
• Severe vomiting - at least 2 episodes >6 hrs apart, or if requiring >2 doses of antiemetic medication
  
• Quality of recovery on the morning after surgery -using the validated QoR Score instrument (14), a 9-item patient-orientated measure of early postoperative health status. This is associated with patient satisfaction.
  
• 30-day mortality - for safety analysis only (study not powered for this rare event).
  

There are some compelling reasons to question the routine use of nitrous oxide (N2O), also known as "laughing gas". Despite being the first anaesthetic drug introduced, and still widely used, there is sufficient doubt as to the risk-benefit profile.

There is strong evidence that N2O is a major risk factor for postoperative nausea and vomiting. It is clear that (even) brief exposure to N2O impairs methionine synthetase, an enzyme required for DNA production, red and white blood cell formation. Tissue hypoxia may be more common. These adverse effects are enhanced in "sick" patients (ie. those at highest risk, increased hospital length of stay and healthcare expenditure), and will be more likely in longer surgery. The extent of wound infection and cardiac morbidity associated with N2O is not known.

Large outcome trial data are lacking. When considering its widespread use in about 90% of all surgery around the world, small differences in outcome would have major implications for healthcare delivery. A large randomised controlled trial is necessary to answer this question.

We have recruited 2000 patients from about 25 centres around the world (mostly Australasia), who are undergoing major surgery.