Effect of interscalene anaesthesia on cerebral oxygen saturation.

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.