Butorphanol tartrate, an analgesic opioid with κ-opioid receptor agonist and μ-opioid receptor antagonist properties,1 is considered by some veterinarians to be the opioid of choice for pain management in birds,2 although the extent of its analgesic efficacy remains controversial.3,4 In birds, the use of opioids with inhalation anesthetic agents as part of a balanced anesthetic technique may decrease the incidence of unwanted autonomic responses and hypotension and improve safety.5
Minimum anesthetic concentration is the median effective concentration for inhaled anesthetic agents.6 Increasing doses of κ- and μ-opioid receptor agonists are reported to decrease the MAC for isoflurane in chickens, although the effect of time on this response is unknown.5 Butorphanol (1 mg/kg, IM) decreases the MAC for isoflurane by 25% in cockatoos but again with an unknown duration of effect.7
The MACs for various inhalation anesthetic agents have been reported7,8 to decrease after the administration of drugs by either constant rate infusion or single-dose administration. However, the method typically used to determine such MAC-sparing effects can be problematic unless the pharmacokinetics of the drugs administered is known in the species being studied. If a drug has a short half-life and is rapidly cleared from the body, plasma concentrations of the drug will decrease over the course of inhalation anesthesia, and any MAC-sparing effect would decrease with time after drug administration. Conversely, if clearance rate is slow, a drug administered by constant rate infusion can cause an increase in the plasma concentration of the drug and may result in an increase in the MAC-sparing effect during the period of assessment. Thus, the administration of drugs by constant rate infusion or single-dose administration does not guarantee stable plasma concentrations of those drugs over the time required to measure the MAC for an inhalation anesthetic agent by use of traditional bracketing techniques.9
To our knowledge, the pharmacokinetics of butorphanol in guineafowl (Numida meleagris) is unknown; therefore, we believed that a novel study design was needed to meaningfully determine the temporal effects of butorphanol on the MAC for sevoflurane in this species. The purpose of the study reported here was to determine the individual MACs for sevoflurane in guineafowl by use of an established bracketing technique and subsequently measure the dose and temporal effects of butorphanol on the MAC for sevoflurane.
Minimum anesthetic concentration
End-tidal partial pressure of CO2
Oxygen saturation as measured by pulse oximetry
Sevocris, provided by Dr. Roberto Debom, Cristália Produtos Químicos e Farmacêuticos Ltda, São Paulo, Brazil.
Conquest 3000 ventilator, HB Hospitalar Indústria e Comércio Ltda, São Paulo, Brazil.
Tom Cat 3.5F, Ortovet, São Paulo, Brazil.
Infrared gas analyzer DX-Ajaga-1 (AGA), Dixtal, Manaus, AM, Brazil.
Sevoflurane in N2 and O2, White Martins Gases Industriais SA, Rio de Janeiro, Brazil.
BD Angiocath, BD, São Paulo, Brazil.
Medfusion 2010i syringe pump, Medex Inc, Duluth, Ga.
Dixtal 2010, Dixtal, Manaus, AM, Brazil.
Ultrasonic Doppler flow detector, model 812, Parks Medical Electronic Inc, São Bernardo do Campo, SP, Brazil.
Veterinary Thermometer, Incoterm, Porto Alegre, RS, Brazil.
T/Pump, Gaymar, Orchard Park, NY.
S48 stimulator, Astro-Med Inc, West Warwick, RI.
Torbugesic, Fort Dodge Animal Health, Campinas, SP, Brazil.
SigmaStat, version 3.0.1, Systat Software, San Jose, Calif.
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