Purpose: Hypotensive bradycardic events (HBEs) occur in 5-28% of patients undergoing arthroscopic shoulder surgery in the sitting position after an interscalene brachial plexus block (ISBPB). The objective of this study was to investigate the effects of the sitting position following ISBPB on heart rate variability (HRV).
Methods: In this prospective case-control study, we evaluated 64 patients undergoing arthroscopic shoulder surgery under ISBPB and 49 nonsurgical control subjects. HRV power spectral analysis parameters were measured (and natural log-transformed) before ISBPB and after changing to the sitting position. The patients experiencing HBEs were assigned to the HBE group, and the remaining patients were assigned to the non-HBE group.
Results: HBEs developed in 18 patients (28.1%). Changing from the supine position to the sitting position after ISBPB did not induce a significant increase in the natural log-transformed ratio of low-frequency to high-frequency power (lnLF/HF). A significant decrease in natural log-transformed high-frequency power (lnHF) was observed compared to the control group, who presented a significant increase in lnLF/HF and an insignificant change in lnHF. lnHF was found to be significantly higher in the HBE group compared to the non-HBE group.
Conclusions: Sustained vagal activity with a failed shift in the sympathovagal balance toward sympathetic predominance in response to sitting after ISBPB is associated with the development of HBE.
Keywords: Arthroscopy; Autonomic nervous system; Bradycardia; Heart rate; Hypotension.