Cerebral oxygenation in the beachchair surgical patient

INTERVENTION: This study will investigate the effects of clinically used vasopressor drugs versus saline infusion on cerebral blood flow and oxygenation, in patients placed into beach chair position for shoulder surgery. Following interscalene brachial plexus anaesthesia with 20‐30 ml ropivicaine 0.75%, general anaesthesia will be induced with propofol 50‐200mg and maintained on sevoflurane 2‐3%. Following stabilisation an infusion of either noradrenaline (20 microg/ml), phenylephrine (100 microg/ml), ephedrine (300 microg/ml) or vasopressin (0.3 U/ml) will be commenced. Infusions are equipotent in concentration and will initially commence at a rate of 6 ml/hr. At 2.5 minutely intervals, the infusion rate will be increased stepwise to 15, 30, 45 and 60 ml/hr. Infusion is titrated until the mean arterial pressure increases to 120 % baseline/or cerebral desaturation decreases to 80% baseline. Infusion is maintained within these limits whilst patient placed into beachchair position for surgery. The duration of intervention is 1 hour. CONDITION: Beachchair position during shoulder surgery Intraoperative cerebral ischaemia PRIMARY OUTCOME: Cerebral oxygen delivery in the beach chair position. Measurement of frontal lobe cerebral oxygenation by near infra‐red spectroscopy. SECONDARY OUTCOME: An ipsilateral change in cerebral oxygen saturation following interscalene blockade. Measurement of frontal lobe cerebral oxygenation by near infra‐red spectroscopy. Change in arterial blood pressure using intra‐arterial pressure monitoring with transducer placed at tragus, and heart rate from ECG. INCLUSION CRITERIA: >18 years of age Shoulder surgery suitable for beach chair position Able to provide informed consent