Despite maintaining arterial pressure with phenylephrine, cerebral desaturation occurred with upright positioning. Cerebral oxygen saturation can provide a valuable endpoint when evaluating the effect of vasopressor therapy on cerebral perfusion.

The sitting upright or 'beachchair' position is commonly used for shoulder arthroscopic surgery. There is a theoretical concern that anaesthetised patients placed in this posture are at risk of reduced cerebral blood flow (CBF), especially if there is associated hypotension. This study investigated the effect of anaesthetic-induced hypotension on estimated cerebral blood flow in patients placed in the beachchair position for shoulder surgery.Forty patients were randomised to either sedation (propofol infusion 10 to 20 mg.hour -1 , n=20) or general anaesthesia using sub minimum alveolar concentration of sevoflurane (n=20). All patients received an interscalene brachial plexus regional block. Internal carotid artery blood flow was measured using the time averaged velocity of the spectral Doppler waveform, and was then used as an estimate of global CBF. Following a pre-anaesthesia study, measurement of internal carotid artery blood flow was made before and after beachchair positioning, and at five-minute intervals during surgery.Beachchair positioning during general anaesthesia significantly decreased the mean arterial pressure (34±10 mmHg) compared to sedation (4±2 mmHg, P <0.01), and vasopressor therapy was required more often. However, CBF remained constant in both anaesthetised (P=0.83) and sedated patients (P=0.68) despite beachchair positioning, and the fall in mean arterial pressure in the anaesthetised patients. There was no significant difference in CBF between groups (P=0.91). These findings indicate that in patients in the beachchair position receiving sevoflurane anaesthesia, CBF is maintained when mean arterial pressure is above 70 mmHg, consistent with intact autoregulation.

Interscalene blockade (ISB) is commonly associated with Horner's syndrome, indicating spread of injectate to the cervical sympathetic chain. Cervical sympathetic nerve activity (SNA) is believed to influence cerebral autoregulation, and a decrease in sympathetic tone may alter cerebral blood flow (CBF). This study investigated whether ISB influenced CBF in patients undergoing shoulder surgery. Patients (n=30) scheduled for elective shoulder arthroscopy were recruited. Cerebral oxygen saturation (ScO 2 ) of the left and right frontal cortices was continuously measured during ISB administration, sedation and anaesthetic induction. Baseline ScO 2 was similar in blocked and unblocked sides (74 ± 5% and 73 ± 5% respectively, P=0.70). ScO 2 decreased with sedation (-3 ± 3% and -4 ± 3%, P=0.93), and increased with pre-oxygenation and general anaesthesia (P <0.01). Following ISB there was no change in ScO 2 between blocked and unblocked sides (P=0.18), or any difference between right-or left-sided ISB. ISB is not associated with an increase in CBF as indicated by ScO 2 , despite the presence of Horner's syndrome.