Discussion
Bradycardia, bradycardia, iatrogenic sympathetic block, hypotension, hypotensive
HYPOTENSIVE
This randomized controlled trial showed that HRV dynamics changed through spinal anesthesia and sedation, however, dexmedetomidine and propofol exhibited similar trends in HRV dynamics. Although dexmedetomidine and propofol did not result any significant difference in the HRV dynamic, the difference in hemodynamic change was obvious between the two groups. Dexmedetomidine induced more bradycardic and less hypotensive in hemodynamic changes compared to propofol. In intragroup analysis compared to each group’s baseline, the two sedatives showed distinctive features. Both sedatives decreased sympathetic activity, but dexmedetomidine did so at a later time, and only dexmedetomidine increased the parasympathetic activity. The LF/HF ratio was decreased only in dexmedetomidine.Spinal anesthesia has known to cause an iatrogenic sympathetic block, which leads to hypotension and bradycardia, requiring intervention. However, our data showed that spinal anesthesia itself had no effect on HRV dynamics. In this study, HRV data measured at T1 (10 min after spinal anesthesia) could be considered as the pure effect of spinal anesthesia, because it was measured prior to sedative administration, and the HRV data at T1 showed no statistically significant change compared to that at T0. A study by Hidaka et al. [Although no statistically significant difference was noted in the overall HRV dynamics between dexmedetomidine and propofol, we found variation in the patterns of HRV dynamics. Intragroup analysis of each group showed that LF, which represents sympathetic heart rate modulation, decreased with both sedatives, compared to that at baseline. In contrast, HF, which represents parasympathetic heart rate modulation, increased only with dexmedetomidine, compared to that at baseline (Fig. Another hypothesis could explain the increase in HF power with only dexmedetomidine. This may be related to the timing of HRV measurement. Tarvainen et al. [In this study, dexmedetomidine showed favorable hemodynamics compared with propofol in patients who underwent spinal anesthesia. Dexmedetomidine showed a delayed decrease in sympathetic activity compared with propofol (T3 in the dexmedetomidine group and T2 in the propofol group). If sympathetic activity correlates well with BP, it can provide useful information for cardiovascular high-risk patients. By adjusting the initiation timing of sedative administration, it is possible to achieve more favorable results for BP maintenance in high-risk patients under spinal anesthesia.In the second analysis, we examined whether the LF/HF ratio predicted spinal anesthesia-induced hypotension, as reported in previous studies [It is assumed that the prediction of hypotension using the LF/HF ratio cannot be applied to all anesthetics. Most of the anesthetics decreased the sympathetic heart rate modulation, but for the parasympathetic heart rate modulation, the change pattern varied (increased or decreased), and the degree of the decrease differed depending on the agent. Previous studies have shown that various anesthetics induce dynamic changes in HRV. With sevoflurane, LF decreased with a reduction in the BIS value, but HF decreased after induction, and no further decreases were observed despite the reduction in the BIS value [The current study, as a preliminary observational study, showed how the autonomic nervous system is altered under spinal anesthesia and sedation. The results of this study have the potential to be used as a reference in various fields. For example, a delayed decrease in sympathetic activity induced by dexmedetomidine could help determine the timing of sedation during spinal anesthesia. But at the same time, care must be taken when judging and applying the results from HRV measurement. This study showed that sympathetic activity and BP are not always correlated, as other compound factors such as peripheral vasoconstriction may play a role. Bradycardia is caused not only by an increase in parasympathetic tone but also by sympathetic withdrawal or sympathetic-parasympathetic interaction. HRV detects only information from the heart-brain interactions, interference from other components is not detected. Interpreting LF and HF component of HRV as sympathetic and parasympathetic tone is oversimplified and could lead to a wrong conclusion [This study had a few limitations. First, the individual variations in HRV values were larger than the sample size. In previous study, the challenges of conducting research due to inter-individual variation have been mentioned [