Antinociceptive effects of bupivacaine injected within the internal abdominis rectus sheath in standing healthy horses.

The research article focuses on evaluating the effectiveness of bupivacaine, a regional anesthetic, in blocking pain in the abdominal midline of horses. This is done through a prospective, crossover, placebo-controlled, blinded study that spans two stages and involves both cadavers and healthy horses.

Methodology

• The study uses two groups of horses for its methodology – two cadavers and six healthy animals. The work is divided into two stages.
• In the first stage, a combination of methylene blue and bupivacaine is injected into the internal rectus abdominis sheath (RAS) of the two cadavers. This injection is administered either via a one-point or a two-point technique.
• The spread of the dye (indicative of the spread of the anesthetic) is carefully studied after dissecting the abdomens of the cadavers.
• In the second stage, each horse from the group of healthy animals is injected with either a saline solution (NaCl, treatment PT as placebo) or bupivacaine (treatment BT, the actual anesthetic) through a two-point technique.
• The effectiveness of the therapy is then assessed by measuring the mechanical nociceptive threshold (MNT, a measure of pain threshold) of the abdominal midline using a 1mm blunted probe tip. Any observation of pelvic limb weakness is also recorded.

Results

• The dissection of the cadavers showed that the anesthetic successfully reached the ventral branches of the T11 to L2 nerves when using the one-point injection technique, and T9-L2 nerves when using the two-point technique.
• Baseline MNTs were similar in both treatments PT (saline) and BT (bupivacaine), but they increased to a greater extent in the BT treatment, indicative of a higher pain threshold due to successful numbing.
• No pelvic limb weakness was observed in any of the horses. This suggests that the therapy had only targeted, localized effects and didn’t lead to any harmful systemic impacts.

Conclusions

• The study concluded that using a bupivacaine block on the RAS resulted in pain relief that lasted at least 8 hours in the abdominal midline of the standing horses. This was achieved without causing any weakness in the pelvic limbs.
• The findings need to be investigated further to assess the technique’s suitability for ventral celiotomies, a type of surgical incision into the abdominal cavity.