Effect of retrobulbar block on heart rate variability during exenteration in cows

Objective. To evaluate the effect of retrobulbar block during orbital exenteration on heart rate variability (HRV). Materials and methods. Fourteen adult cows with ocular squamous cell carcinoma were used. Ocular exenteration is performed under multimodal anesthesia. The cows were sedated (xylazine 2%) and the auriculopalpebral nerve was blocked (lidocaine 2%). They were randomly assigned to two groups, one with a four-point block (n=7) and the second with a retrobulbar block (n=7). Electrocardiograms were recorded during two hours of the surgery in six moments by means of a Holter monitor (Cardio Trak Digital Holter Recorder, Model CT-085, BENEWARE). The data obtained were analyzed using linear methods in the frequency and time domain. Mean heart rate (HR) and mean beat-to-beat interval (RR) duration were evaluated in the time domain. The frequency domain included low frequency (LF), high frequency (HF) and sympathovagal balance (LF/HF) of HRV. Results. In both groups there was a significant increase in heart rate during ocular traction. There were no significant changes in HRV between the different moments of the procedure. Conclusions. During manipulation and exenteration of the eyeball, no decrease in HR associated with the oculocardiac reflex was detected, but sympathetic activation was detected as a result of painful surgical stimuli. No central nervous system (CNS) toxicity from local anesthetics occurred in the animals.