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.