Effect of retrobulbar nerve block on heart rate variability during enucleation in horses under general anesthesia

Objective Analysis of any effect of retrobulbar block during ocular surgery on heart rate variability and oculocardiac reflex. Study design Prospective study. Animal studied Horses (n=16) undergoing eye enucleation due to chronic ophthalmologic diseases. Procedure Eye enucleation was performed under general anesthesia. The horses were randomly assigned to the first (inhalation anesthesia only, n=10) or second group (inhalation and local retrobulbar anesthesia, n=6). The retrobulbar block was performed using 12mL of mepivacaine hydrochloride 2%. ECG data were taken by a Telemetric ECG before, during, and after surgery. Heart rate variability was analyzed in the time domain as mean heart rate, mean beat-to-beat interval duration, and standard deviation of continuous beat-to-beat intervals. The frequency domain analysis included the low- and high-frequency components of heart rate variability and the sympathovagal balance (low/high frequency). The low frequency represents mainly sympathetic influences on the heart, whereas high frequency is mediated by the parasympathetic tone. Results All horses without a retrobulbar block showed a significant decrease in the heart rate during traction on the globe and pressure on the orbital fat pad for homoestasis (P=0.04). Simultaneously, high-frequency power, as an indicator of vagal stimulation, increased significantly. High-frequency and low-frequency power in the retrobulbar block group increased in five horses, and heart rate decreased in only one horse. Both were not significant within the group, but there was a significant difference between both groups relating to the incidence of heart rate decrease occurring at globe traction. Conclusions and clinical relevance Heart rate variability is a sensitive, non-invasive parameter to obtain sympathovagal stimulations during general anesthesia. The retrobulbar block can prevent heart rate decrease associated with initiation of the oculocardiac reflex.