A METHOD FOR FREQUENT MEASUREMENT OF SEDATION AND ANALGESIA IN SHEEP USING THE RESPONSE TO A RAMPED ELECTRICAL STIMULUS

A method for the frequent, precise measurement of the analgesic and sedative (or anesthetic) effects of drugs after bolus administration to sheep was developed. A ramped pulsed DC electrical stimulus was delivered to the hind limb of sheep via subcutaneous needles by use of a peripheral nerve stimulator modified to allow control of current ramp rate and pulse frequency, and limb withdrawal was used as an endpoint. The optimal stimulus pattern was found to be a pulse frequency of 20 Hz, with a 5-sec ramp time and measurement intervals of 30 sec. The effects of a range of analgesic and sedative drugs on the threshold current to produce limb withdrawal were examined. Administration of the sedative/anesthetic drugs propofol and thiopentone intravenously and of the analgesic xylazine both intravenously and intramuscularly resulted in a reproducible dose-dependent rise in the threshold current required to produce limb withdrawal. Administration of the opioids alfentanil and pethidine produced agitation, making measurements unreliable. It is concluded that this device allows repeated reproducible measurements of analgesia and sedation to be made in sheep at a frequency sufficient to characterize the initial effects of analgesic and sedative drugs, particularly after intravenous administration.