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

The venous plasma concentrations of mepivacaine were determined in 7 adult mares (420 +/- 17.1 kg) given an injection of a 2% solution of the hydrochloride at either the sacral (S2-3 to S5-C1) epidural space or the midsacral (S2-3) subarachnoid space. An average dose of 91.4 +/- 15.7 mg (4.6 +/- 0.8 ml) was needed to produce caudal epidural analgesia (CEA) and 26.7 +/- 5.4 mg (1.3 +/- 0.3 ml) to produce caudal subarachnoid analgesia (CSA). Maximal caudal analgesia extended from spinal cord segments S-1 to coccyx during CEA and CSA. The onset of analgesia as measured by response to superficial and deep muscular pinprick stimulations was significantly (P less than 0.05) faster in mares with CSA than with CEA (8.3 +/- 2.4 minutes vs 21.4 +/- 3.8 minutes). The period of analgesia was significantly (P less than 0.05) longer in mares with CEA than with CSA (80.0 +/- 11.5 minutes vs 67.4 +/- 26.3 minutes). The rate of vascular absorption of mepivacaine from the epidural space was significantly (P less than 0.05) faster than from the subarachnoid space. Maximum venous plasma concentrations of mepivacaine were similar (P greater than 0.05) after epidural and subarachnoid injections (0.05 +/- 0.03 micrograms/ml and 0.05 +/- 0.03 micrograms/ml) at the same times after mepivacaine administration (51.4 +/- 33.4 minutes and 53.6 +/- 24.3 minutes).(ABSTRACT TRUNCATED AT 250 WORDS)