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

The research article investigates whether the use of a retrobulbar nerve block during eye removal surgery in horses impacts heart rate variability and the start of the oculocardiac reflex. It found that heart rate variability, a non-invasive method to study the balance of stimulatory responses during anesthesia, can be safeguarded from decreases by using a retrobulbar block.

Methodology

• The study is a prospective one, involving 16 horses that were to undergo eye removal surgery due to suffering from persistent eye diseases.
• The horses were put under general anesthesia, and were divided randomly into two groups. The first group included 10 horses that were only provided with inhalation anesthesia. The second group, consisting of six horses, was given both inhalation anesthesia and a local retrobulbar anesthesia.
• The retrobulbar block, a type of local anesthesia applied directly to the nerves in the back of the eye, was implemented using 12 mL of a 2% mepivacaine hydrochloride solution.
• Electrocardiogram (ECG) data was collected before, during, and after the surgery. Heart rate variability was then studied in the time and frequency domains.

Results

• All horses brought under the first type of anesthesia demonstrated a significant decrease in heart rate when the globe, the eyeball minus the optic nerve, was pulled and pressure was applied on the fat pad surrounding the eye for maintaining stability.
• The high-frequency power, which is a representation of the stimulation of the rest and digest response mechanism (vagal stimulation), showed notable increase.
• In the group that underwent the eye removal surgery with a retrobulbar block, heart rate in just one horse decreased while the low-frequency and high-frequency power in five horses increased. However, there wasn’t a significant difference in this within the group.
• There was, nonetheless, a significant difference between the two groups in relation to the instances of decrease in heart rate at the time of the traction of the globe.

Conclusions

• Heart rate variability serves as a sensitive and non-invasive method to track the stimulations of the rest and digest and fight or flight response mechanisms during general anesthesia.
• Application of a retrobulbar block can help avert the decrease in heart rate, which is often accompanied by the triggering of the oculocardiac reflex.