Plasma mepivacaine concentrations after caudal epidural and subarachnoid injection in the horse: comparative study.

This study explores the venous plasma concentrations of an analgesic called mepivacaine in adult horses, administered through two different methods: caudal epidural analgesia and caudal subarachnoid analgesia. The results indicate that the onset of analgesia was faster, and the absorption rate of mepivacaine in blood was slower via the subarachnoid space. However, the duration of analgesia was longer when applied via the epidural space.

Study Design and Methods

• The experiment was carried out on 7 adult mares with an average weight of around 420 kg.
• An injection of a 2% solution of mepivacaine hydrochloride was given at two different points: the sacral epidural space and the midsacral subarachnoid space.
• While an average dose of 91.4 mg was necessary for caudal epidural analgesia (CEA), around 26.7 mg was satisfactory for caudal subarachnoid analgesia (CSA).
• The analgesic effect in either injection method extended from the first spinal cord segment to the coccyx.

Results and Observations

• The onset of analgesia, gauged by the horse’s reaction to superficial and deep muscular pinprick stimulations, was quicker in the CSA method than the CEA (averaging 8.3 minutes compared to 21.4 minutes).
• However, the duration of analgesia was significantly longer in the CEA method, lasting an average of 80 minutes as opposed to CSA’s 67.4 minutes.
• The speed at which mepivacaine got absorbed into the bloodstream was noticeably quicker when it was administered through the epidural space than the subarachnoid space.
• Yet, the maximum concentrations of mepivacaine observed in the venous plasma were about the same in both methods of administration, both measuring an average of 0.05 micrograms/ml at similar points after administration.

Implications of the Findings

• The study suggests that while the CSA method results in faster analgesia onset, the CEA method offers a longer duration of pain relief; making the choice of method dependent on the specific needs of the situation.
• Furthermore, the similarity in maximum plasma concentrations of mepivacaine for both methods implies that the net effectiveness of the drug might be independent of the route of administration.