Indications In critically ill, unstable patients, especially those with refractory shock and respiratory failure, or in patients undergoing complex surgery with fluid shifts or blood loss: Continuous blood pressure measurement Repeated arterial blood gas measurements Repeated blood sampling for laboratory tests Contraindications Absolute contraindications An artery that is neither palpable nor detectable by ultrasonography (never try to cannulate a site simply because the artery is expected to be there) Unsuitable artery (eg, dialysis fistula at same limb, thrombosed or inaccessible artery, generalized limb ischemia) Inadequate collateral blood flow from ulnar artery circulation Full-thickness burns Local infection at the insertion site Relative contraindications Coagulopathy (including therapeutic anticoagulation *) or recent / pending thrombolysis: The radial artery is the preferred arterial cannulation site in these situations; prolonged pressure (eg, 10 minutes or more) may be needed to stop bleeding / hematoma at the site.