Spinal fluid concentrations of mepivacaine in horses and procaine in cows after thoracolumbar subarachnoid analgesia
Skarda, R.T.; Muir, W.W.; Ibrahim, A.I.
American Journal of Veterinary Research 46(5): 1020-1024
1985
ISSN/ISBN: 0002-9645 PMID: 3839114 Document Number: 261416
The CSF concentrations of mepivacaine in 10 standardbred horses and of procaine in 10 Holstein cows given the drugs by thoracolumbar subarachnoid injection were determined. Mepivacaine hydrochloride was injected into the horses (502 .+-. 60.5 kg) at an average dosage of 30 mg (1.5 ml of 20 mg/ml solution). Analgesia was produced 7.5 .+-. 4.3 min after injection, extended between spinal cord segments T13 and L3 on both sides of the spinal column, and lasted 47 .+-. 18.7 min at the T18 dermatome. Procaine hydrochloride was injected into cows (614 .+-. 51.5 kg) at a dosage ranging between 75 mg and 100 mg (1.5 ml and 2 ml of 50 mg/ml solution). Analgesia was produced 8.2 .+-. 2.0 min after injection, extended between spinal cord segments T11 and L4 on both sides of the spinal column, and lasted 47 .+-. 17.5 min at the T13 dermatome. The critical CSF concentrations of local anesthetics required to eliminate response to pinprick stimulation were 204.4 .+-. 90.3 .mu.g of mepivacaine/ml in horses and 197.0 .+-. 86.1 .mu.g of procaine/ml in cows. Average CSF concentrations at 120 min after injections were made were 16.8 .+-. 15.5 .mu.g of mepivacaine/ml and 30.6 .+-. 17.1 .mu.g of procaine/ml. In in vitro experiments to determine the rates of hydrolysis of mepivacaine and procaine in CSF, significant changes (P > 0.05) were not seen in the CSF concentrations of mepivacaine in horses and procaine in cattle after a 120 min incubation (37.degree. C). The analgesic threshold concentrations of mepivacaine in CSF of horses and procaine in CSF of cows were similar. The decline of the subarachnoid mepivacaine and procaine concentrations, which determine the duration of analgesia, was due to absorption of drug into the systemic circulation and not due to hydrolysis in CSF.