Primary event rate far higher than estimated for sample size calculation.

A drop in blood pressure during anaesthesia for a surgical procedure has been associated with worse patient outcomes, including complications such as damage to the heart, brain and kidneys. Continuous blood pressure monitoring prior to the start of anaesthesia alerts the anaesthetist to drops in blood pressure and allows this to be treated promptly. This may help to avoid the complications described above.

Continuous blood pressure monitoring is carried out by inserting a small plastic tube (cannula) into an artery. In this study, the investigators propose inserting a cannula into the radial artery in the wrist before a patient is anaesthetised for surgery. The usual technique for insertion of this cannula is for the anaesthetist to identify the site of the radial artery by feeling for an arterial pulse with the fingertips (palpation). An alternative technique for identification is to use ultrasound. Ultrasound creates a two-dimensional image of the area under the skin on a screen, enabling the operator to visualise the artery being targeted. This may reduce the number of cannulation attempts required, reducing patient discomfort.

The investigators will test whether ultrasound guidance improves the success rate of radial artery cannulation, compared to palpation alone, in a randomised controlled trial in an NHS hospital. Patients will be over 45 years old, undergoing general or neuraxial anaesthesia for non-cardiac surgery lasting 120 minutes or more. Patients will be randomly assigned to a palpation or ultrasound-guided technique for arterial cannulation. Patients will receive numbing local anaesthetic cream to the area beforehand. A needle is used to introduce the cannula into the artery. There will be a maximum of one attempt allowed. In addition to measuring success rate, we will also compare whether the successfully sited cannula provides an adequate arterial waveform, requires resiting at any point during the surgery and any complications which arise directly related to arterial catheterisation. Patients will receive standard anaesthetic and surgical care in all other respects.

Inclusion Criteria:

1. Patients scheduled to undergo major elective or urgent (not requiring intervention in <24 hours) non-cardiac surgery under general anaesthesia and/or neuraxial anaesthesia, expected to take >120 minutes from induction of anaesthesia
2. Requiring overnight hospital stay.

Exclusion Criteria:

1. Anatomical deformity
2. Unable to consent
3. Cannulation attempt within 24 hours
4. Overlying infection