The ENIGMA II Trial:Nitrous Oxide Anaesthesia and Cardiac Morbidity After Major Surgery: a Randomised Controlled Trial

Large, Randomised, Parallel-group, Controlled Trial, With Patients Randomly Allocated to Either N2O-containing (70% N2O in Oxygen [FiO2 0.3]) or N2O-free (70% Nitrogen in Oxygen [FiO2 0.3]).

To investigate the safety of nitrous oxide (N2O) anaesthesia in patients with risk factors for coronary artery disease undergoing major surgery.

Study Overview

• Status: Completed
• Conditions: Coronary Artery Disease
• Intervention / Treatment: Other: Nitrous Oxide; Other: No Nitrous Oxide

Detailed Description

Approximately 25% of patients undergoing major surgery have known coronary artery disease (CAD) or risk factors for CAD.

N2O interferes with vitamin B12 and folate metabolism. This impairs production of methionine (from homocysteine), used to form tetrahydrofolate and thymidine during DNA synthesis. It has been repeatedly demonstrated that N2O anaesthesia increases postoperative homocysteine levels. Chronic hyperhomocysteinaemia is associated with cardiovascular disease, and acute hyperhomocysteinaemia is known to cause endothelial dysfunction. One small trial has demonstrated an increased incidence of postoperative myocardial ischaemia in patients receiving N2O anaesthesia. Reducing postoperative myocardial infarction and death are important aims for those with CAD undergoing major surgery.

Our previous trial (ENIGMA) studied 2050 patients and identified some serious adverse effects, but most patients were not at risk of CAD and so we could not reliably assess serious cardiac complications. We propose a large simple randomized clinical trial of 7,000 patients to provide a definitive evaluation of the safety of N2O anaesthesia.

Updated statistical analysis plan can be found at www.enigma2.org.au.

• Study Type: Interventional
• Enrollment (Actual): 7112
• Phase: Phase 4

Contacts and Locations

Study Locations

• Australia
  • Victoria
    • Melbourne, Victoria, Australia, 3004
      • Alfred Hosptial

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: 18 years and older (Adult, Older Adult)
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria

1. Adult males and females age ≥ 45 years, undergoing noncardiac surgery and general anaesthesia expected to exceed two hours.

2. At increased risk of cardiac events, defined as any of

   1. history of coronary artery disease as defined by a history of any one of the following: i. angina ii. MI iii. segmental wall motion abnormality on echocardiography or a fixed defect on radionuclide imaging iv. a positive exercise stress test for cardiac ischaemia v. a positive radionuclide exercise, echocardiographic exercise, or pharmacological cardiovascular stress test for cardiac ischaemia vi. coronary revascularization (CABG or PTCA) vii. angiographic evidence of atherosclerotic stenosis > 50% of the diameter of any coronary artery viii. ECG with pathological Q waves in two contiguous leads
   2. heart failure
   3. cerebrovascular disease thought due to atherothrombotic disease
   4. aortic or peripheral vascular disease

   5. or three or more of the following risk factors:

      • age ≥70 years
      • any history of congestive heart failure
      • diabetes and currently on an oral hypoglycaemic agent or insulin therapy
      • current treatment for hypertension
      • preoperative serum creatinine >175 micro mol/L (> 2.0 mg/dl)
      • current or previous high cholesterol ≥6.2 mmol/L (> 240 mg/dl)
      • history of a transient ischemic attack (TIA) (i.e. a transient focal neurological deficit that lasted less than 24 hours and thought to be vascular in origin)
      • emergency/urgent surgery (i.e. surgery which must be undertaken within 24 hours of acute presentation to hospital)
      • high-risk type of surgery (i.e. intrathoracic or intraperitoneal)

Exclusion Criteria

1. having cardiac surgery
2. marked impairment of gas-exchange expected to require Fi02> 0.5 intraoperatively
3. specific circumstances where N2O is contraindicated (eg. volvulus, pulmonary hypertension, raised intracranial pressure) or the anaesthetist plans to use supplemental oxygen (eg. colorectal surgery)
4. N2O unavailable for use.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: Triple

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Other: 70% Nitrous Oxide; General anaesthesia using 70% Nitrous Oxide with fraction of inspired oxygen at 30%	Other: Nitrous Oxide; General anaesthesia with the inclusion of Nitrous Oxide at 70%
Other: No Nitrous Oxide; General anaesthesia not containing Nitrous oxide with fraction of inspired oxygen at 30%	Other: No Nitrous Oxide; General anaesthesia with no use of Nitrous Oxide

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Time Frame
The Primary Endpoint is a Composite of Death and Cardiovascular Events (Clinical and Silent MI, Cardiac Failure, Cardiac Arrest, Pulmonary Embolism, and Stroke) Measured at 30 Days After Surgery.	30 days post op

Secondary Outcome Measures

Outcome Measure	Time Frame
Myocardial Infarction (MI)	30 days post op
Cardiac Arrest	30 days
Pulmonary Embolism	30 Days Post op
Stroke	30 Days Post op
Wound Infection	30 Days Post op
Hospital Stay (Days)	30 Days Post Op

Collaborators and Investigators

• Sponsor: Bayside Health
• Collaborators: National Health and Medical Research Council, Australia
• Investigators: Principal Investigator: Paul S Myles, MBBS MPH MD, The Alfred

Publications and helpful links

General Publications

• Beattie WS, Wijeysundera DN, Chan MTV, Peyton PJ, Leslie K, Paech MJ, Sessler DI, Wallace S, Myles PS, Galagher W, Farrington C, Ditoro A, Baulch S, Sidiropoulos S, Bulach R, Bryant D, O'Loughlin E, Mitteregger V, Bolsin S, Osborne C, McRae R, Backstrom M, Cotter R, March S, Silbert B, Said S, Halliwell R, Cope J, Fahlbusch D, Crump D, Thompson G, Jefferies A, Reeves M, Buckley N, Tidy T, Schricker T, Lattermann R, Iannuzzi D, Carroll J, Jacka M, Bryden C, Badner N, Tsang MWY, Cheng BCP, Fong ACM, Chu LCY, Koo EGY, Mohd N, Ming LE, Campbell D, McAllister D, Walker S, Olliff S, Kennedy R, Eldawlatly A, Alzahrani T, Chua N, Sneyd R, McMillan H, Parkinson I, Brennan A, Balaji P, Nightingale J, Kunst G, Dickinson M, Subramaniam B, Banner-Godspeed V, Liu J, Kurz A, Hesler B, Fu AY, Egan C, Fiffick AN, Hutcherson MT, Turan A, Naylor A, Obal D, Cooke E; ANZCA Clinical Trials Network for the ENIGMA-II Investigators. Implication of Major Adverse Postoperative Events and Myocardial Injury on Disability and Survival: A Planned Subanalysis of the ENIGMA-II Trial. Anesth Analg. 2018 Nov;127(5):1118-1126. doi: 10.1213/ANE.0000000000003310.
• Corcoran T, Kasza J, Short TG, O'Loughlin E, Chan MT, Leslie K, Forbes A, Paech M, Myles P; ENIGMA-II investigators. Intraoperative dexamethasone does not increase the risk of postoperative wound infection: a propensity score-matched post hoc analysis of the ENIGMA-II trial (EnDEX). Br J Anaesth. 2017 Feb;118(2):190-199. doi: 10.1093/bja/aew446.
• Chan MT, Peyton PJ, Myles PS, Leslie K, Buckley N, Kasza J, Paech MJ, Beattie WS, Sessler DI, Forbes A, Wallace S, Chen Y, Tian Y, Wu WK; and the Australian and New Zealand College of Anaesthetists Clinical Trials Network for the ENIGMA-II investigators. Chronic postsurgical pain in the Evaluation of Nitrous Oxide in the Gas Mixture for Anaesthesia (ENIGMA)-II trial. Br J Anaesth. 2016 Dec;117(6):801-811. doi: 10.1093/bja/aew338.
• Myles PS, Leslie K, Chan MT, Forbes A, Peyton PJ, Paech MJ, Beattie WS, Sessler DI, Devereaux PJ, Silbert B, Schricker T, Wallace S; ANZCA Trials Group for the ENIGMA-II investigators. The safety of addition of nitrous oxide to general anaesthesia in at-risk patients having major non-cardiac surgery (ENIGMA-II): a randomised, single-blind trial. Lancet. 2014 Oct 18;384(9952):1446-54. doi: 10.1016/S0140-6736(14)60893-X.
• Myles PS, Leslie K, Peyton P, Paech M, Forbes A, Chan MT, Sessler D, Devereaux PJ, Silbert BS, Jamrozik K, Beattie S, Badner N, Tomlinson J, Wallace S; ANZCA Trials Group. Nitrous oxide and perioperative cardiac morbidity (ENIGMA-II) Trial: rationale and design. Am Heart J. 2009 Mar;157(3):488-494.e1. doi: 10.1016/j.ahj.2008.11.015.

Helpful Links

• Related Info

Study record dates

Study Major Dates

• Study Start: April 1, 2007
• Primary Completion (Actual): September 1, 2013
• Study Completion (Actual): September 1, 2013

Study Registration Dates

• First Submitted: January 31, 2007
• First Submitted That Met QC Criteria: February 1, 2007
• First Posted (Estimated): February 2, 2007

Study Record Updates

• Last Update Posted (Actual): January 20, 2026
• Last Update Submitted That Met QC Criteria: December 29, 2025
• Last Verified: December 1, 2025

More Information

Terms related to this study

• Keywords: Coronary artery disease; Anaesthesia; Nitrous oxide; Major Surgery; Effects of Nitrous Oxide following Anaesthesia; Anaesthesia and coronary artery disease
• Additional Relevant MeSH Terms: Vascular Diseases; Cardiovascular Diseases; Heart Diseases; Arteriosclerosis; Arterial Occlusive Diseases; Coronary Disease; Myocardial Ischemia; Coronary Artery Disease; Inorganic Chemicals; Nitrogen Compounds; Oxides; Oxygen Compounds; Gases; Nitrogen Oxides; Nitrous Oxide
• Other Study ID Numbers: 6/07

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO