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

Abstract

Background: As a side effect of interscalene brachial plexus block (ISBPB), stellate gangli-on block (SGB) causes reductions in pupil size (Horner's syndrome) and cardiac sympa-thetic nervous activity (CSNA). Reduced CSNA is associated with hemodynamic instabili-ty when patients are seated. Therefore, instantaneous measurements of CSNA are import-ant in seated patients presenting with Horner's syndrome. However, there are no effective tools to measure real-time CSNA intraoperatively. To evaluate the usefulness of pupillometry in measuring CSNA, we investigated the relationship between pupil size and CSNA.Methods: Forty-two patients undergoing right arthroscopic shoulder surgery under ISB-PB were analyzed. Pupil diameters were measured at 30 Hz for 2 s using a portable pupil-lometer. Bilateral pupil diameters and CSNA (natural-log-transformed low-frequency power [0.04-0.15 Hz] of heart rate variability [lnLF]) were measured before ISBPB (pre-ISBPB) and 15 min after transition to the sitting position following ISBPB (post -sit-ting). Changes in the pupil diameter ([right pupil diameter for post-sitting - left pupil di-ameter for post-sitting] - [right pupil diameter for pre-ISBPB - left pupil diameter for pre-ISBPB]) and CSNA (lnLF for post-sitting - lnLF for pre-ISBPB) were calculated.Results: Forty-one patients (97.6%) developed Horner's syndrome. Right pupil diameter and lnLF significantly decreased upon transition to sitting after ISBPB. In the linear regression model (R-2 = 0.242, P = 0.001), a one-unit decrease (1 mm) in the extent of changes in the pupil diameter reduced the extent of changes in lnLF by 0.659 ln(ms(2)/Hz) (95% CI [0.090, 1.228]). Conclusions: Pupillometry is a useful tool to measure changes in CSNA after the transi-tion to sitting following ISBPB.