Page 72 - BSAVA Guide to Pain Management in Small Animal Practice

5 | Pharmacological treatment of pain

VetBooks.ir 1 . It should be appreciated that the dose einberg et al., . If lipid rescue is
not available, symptomatic treatment (anti-
was a cumulative dose administered over a
convulsants, antimuscarinics, antiarrhythmics,
signi cant amount of time and not as a rapid
bolus. For comparison, another study reported a inotropes, vasopressors and uids can
single rapid intravenous bolus of 11 mg kg of be attempted to control clinical signs while
bupivacaine to cause severe cardiovascular the toxic drug is cleared by the body (Aprea
depression in dogs (Liu et al., 198 . et al., 11 .
A study in cats demonstrated that, in a
model of continuous intravenous infusion, Alpha-2 agonists
neurotoxicity occurred at 1 mg kg for lidocaine Alpha agonists such as medetomidine and
and 4 mg kg for bupivacaine. he cardiotoxic dexmedetomidine (its active enantiomer) are
dose was found to be 47 mg kg for lidocaine widely used in small animal practice for
and 18 mg kg for bupivacaine. hile all cats in sedation and pre-anaesthetic medication. They
the lidocaine group could be resuscitated, 1 also have analgesic properties. They bind to
cats in the bupivacaine group could not alpha adrenergic receptors in the cerebral
Chadwick, 198 . cortex and locus coeruleus of the brain, causing
It should be pointed out that these toxic sedation, analgesia and a reduction in
doses are all obtained administering the drug sympathetic tone, and in the dorsal horn of the
intravenously, thus they represent the clinical spinal cord, causing analgesia. The mechanism
scenario of accidental intravenous of analgesia is complex and not well
administration of a supra-clinical dose of local understood. Agonism of alpha receptors in
anaesthetic, not the uncomplicated use in peripheral and pulmonary blood vessels causes
locoregional anaesthesia (Aprea et al., 11 . vasoconstriction, hypertension, re ex
Published data about toxic intravenous bradycardia and decreased cardiac output.
doses should be taken with a pinch of salt, as ther e ects include hypothermia, decreased
the rate of absorption from tissues should be gastrointestinal motility, diuresis and
taken into account in a clinical setting, and hyperglycaemia. All these e ects are dose
therefore the current suggested maximum dose dependent, and can be antagonized using
is likely to be fairly conservative, as atipamezole, although this also removes any
demonstrated by clinical studies in humans. The analgesic e ect.
risk of toxicity probably is greater in smaller Because of their sedative and
animals, as a relatively small increase in the cardiovascular e ects, alpha agonists are
administered volume results in a greater unsuitable for use as the primary or sole
increase of the amount of drug administered on analgesic, but may provide additional analgesia
the basis of body mass (Rosenberg et al., 4 . when used as part of an anaesthetic protocol.
In the event of toxicity, intravenous infusion They act synergistically with opioids. The
of a lipid emulsion has been shown to dramati- analgesic e ect is of a shorter duration than the
cally improve haemodynamic side e ects of sedative e ect, and re uires higher plasma
bupivacaine overdose in experimental settings, concentrations. They may be useful for treating
and there are case reports describing its neuropathic pain (Murrell and Hellebrekers,
successful clinical use in veterinary medicine .
(Feldman et al., 1991; einberg et al., ;
O’Brien et al., 1 . It is widely used in humans, Clinical use
although recently its e cacy and safety have Inclusion of alpha agonists as part of the
been questioned (O’Brien et al., 1 ; Picard sedation or pre-anaesthetic protocol may
and eek, 16; Rosenberg, 16 . Lipid rescue provide additional analgesia for the patient.
protocols reported in veterinary medicine owever, because of the cardiovascular e ects
suggest infusing 1. ml kg Intralipid over of these drugs, they are usually reserved for
1 minutes, and repeating this if necessary healthy patients without cardiac disease.

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