Effects of sympathomimetic agents on heart rate variability during general anesthesia in Thoroughbred horses

Heart rate variability (HRV) is widely used as a noninvasive index of autonomic nervous system activity. The objective of this study was to determine whether the administration of sympathomimetic agents alters HRV indices during general anesthesia in Thoroughbred horses. Five healthy Thoroughbred horses were anesthetized with sevoflurane in oxygen. During 2-5 hr of general anesthesia, five treatments were administered to each horse in a randomized order. Two doses (µg·kg^-1·min^-1) of epinephrine (0.1 and 0.5), norepinephrine (0.5 and 1), dobutamine (1 and 5), dopamine (5 and 15), and saline (as a control) were infused for 7 min each. Electrocardiograms were recorded to assess HRV, and mean arterial pressure (MAP) was measured to evaluate hemodynamic responses. Data were analyzed using repeated-measures ANOVA. During general anesthesia, epinephrine at 0.5 µg·kg^-1·min^-1 significantly increased low-frequency (LF) power, the LF/high-frequency (HF) ratio, and MAP. Norepinephrine at both doses significantly decreased heart rate and increased HF power and MAP. Dobutamine at 5 µg·kg^-1·min^-1 significantly increased heart rate and MAP, without changing HRV indices, whereas dopamine did not induce significant changes in heart rate, MAP, or HRV indices. In conclusion, administration of sympathomimetic agents during general anesthesia induces distinct patterns of autonomic modulation in horses. Increases in LF power and the LF/HF ratio may reflect enhanced sympathetic modulation; however, these indices should be interpreted in the context of baroreflex-mediated autonomic regulation. Understanding the differential autonomic effects of sympathomimetic agents may aid in optimizing cardiovascular support during equine anesthesia.