Assessment of the changes in cardiac sympathetic nervous activity using the pupil size changes measured in seated patients whose stellate ganglion is blocked by interscalene brachial plexus block.

Background: As a side effect of interscalene brachial plexus block (ISBPB), stellate ganglion block (SGB) causes reductions in pupil size (Horner's syndrome) and cardiac sympathetic nervous activity (CSNA). Reduced CSNA is associated with hemodynamic instability when patients are seated. Therefore, instantaneous measurements of CSNA are important in seated patients presenting with Horner's syndrome.