Page 56 - BSAVA Guide to Pain Management in Small Animal Practice

5 | Pharmacological treatment of pain

VetBooks.ir Morphine: Morphine is not currently licensed in morphine, usually .1 mg kg, can provide
analgesia for up to 4 hours with fewer side
veterinary species and it may be di cult to
justify its use via the Cascade since other full
MOP agonists are licensed. Morphine can cause e ects than parenterally administered opioids.
vomiting; this e ect is not dose or route Buprenorphine: Buprenorphine is a partial MOP
dependent (see above). It can also cause agonist licensed in dogs and cats for treatment
histamine release when injected intravenously, of mild to moderate pain. It has a slow onset of
and should be administered slowly to minimize action due to slow receptor binding, high a nity
the risk of histamine related adverse e ects. for MOP receptors and a long half-life, resulting
The authors frequently use morphine by in a longer duration of action than most opioids.
intravenous infusion or by epidural injection. The exact onset and duration of action is not
The use of morphine as an intravenous clear, since the many studies on this subject
infusion has been extensively researched in have con icting results, probably due to
dogs, and it is widely used in dogs and cats to di erent study designs, doses, routes of
provide a constant level of analgesia to animals administration, nociceptive stimuli, concurrent
in severe pain. An infusion rate of .1 mg kg drug administration and also marked individual
hour morphine has been shown to give variation (especially in cats). Reported times for
equivalent analgesia to that achieved with onset and duration of action in cats are 1 to 6
repeated intravenous boluses of 1 mg kg every minutes (Steagall et al., 1 , and 6 to 1 hours
4 hours in dogs Lucas et al., 1 . In that study, respectively (Robertson et al., ; Slingsby et
two of the dogs in the repeated bolus group had al., 16 . uration of action in dogs is
periods of semi consciousness; none of the considered to be around 6 hours Shih et al.,
dogs in the infusion group experienced this. The 8 , although one study reported a duration
infusion can be titrated to e ect, starting with an of action of 16 hours o et al., 11 . his
intravenous bolus to ensure adequate plasma emphasizes the importance of pain assessment
levels are reached, then beginning the infusion clinically. he high receptor a nity means that
at the low end of the dose range and increasing naloxone may not fully antagoni e its e ects.
the dose if required. After the infusion is Absorption of buprenorphine following
discontinued, the terminal half-life is less than subcutaneous injection is variable and results in
4 minutes uedes et al., 7 . poor analgesia (Steagall et al., 6 . In cats, it
Morphine is hydrophilic and does not cross can be administered by the oral transmucosal
cell membranes easily. This property is exploited route, avoiding the need for injections. This is
when morphine is administered by epidural because the alkaline pH of feline saliva
injection since the drug remains within the facilitates the absorption of the drug across the
epidural space, providing analgesia for up to 4 oral mucous membrane, with absorption similar
hours, probably by acting on MOP receptors on to that achieved by the intramuscular route.
the spinal nerve roots. Epidurally injected owever, there is con icting evidence on the
morphine also tends to spread forwards quality of analgesia when buprenorphine is
independent of the volume used, and has been given by this route. Therefore, the intramuscular
shown to provide analgesia as far cranially as the or intravenous routes are the most reliable.
forelimb (Valverde et al., 1989 . nly In dogs, buprenorphine can be used to treat
preservative-free morphine should be injected mild to moderate pain; a full P agonist is
into the epidural space, and knowledge of more suitable for treating severe pain (Hunt et
anatomy, contraindications and complications is al., 1 . In cats, buprenorphine has been
required. It can be used alone, diluted in sterile shown to be more e ective than morphine
saline to a maximum volume of about . ml kg, (Stanway et al., , although a later study
or mixed with a local anaesthetic agent. hen disagreed with this nding Robertson et al.,
used as the sole agent, it provides analgesia . It may not be suitable for treating acute,
only, not local anaesthesia. A single dose of severe pain due to its long onset of action. It has

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