Page 51 - BSAVA Guide to Pain Management in Small Animal Practice

BSAVA Guide to Pain Management in Small Animal Practice

VetBooks.ir vasopressin mediated e ect. his may make merges with the pancreatic duct before
assessment of volume status using urine output
entering the duodenum, so the sphincter of
ddi controls ow of bile and pancreatic uice
more complicated in animals receiving opioids
Anderson and ay, 8 . into the duodenum. In dogs, the sphincter of
ddi only controls ow through the bile duct,
Gastrointestinal system: Opioids can stimulate which is separate from the pancreatic duct, so
the chemoreceptor trigger zone (CRTZ) that e ects on the sphincter of ddi are
situated outside the blood brain barrier , to probably irrelevant in dogs with pancreatitis.
cause emesis, and the vomiting centre (within There is little evidence in humans and none in
the blood brain barrier to have an antiemetic cats and dogs that opioids have a deleterious
e ect Blanc uaert et al., 1986 . hether the e ect when used to control pain due to
CR or the vomiting centre is a ected rst pancreatitis or biliary tract disease. There is no
depends on the lipophilicity of the drug, dose, evidence that pethidine is superior to other
and route of administration. Morphine, which is opioids for these conditions, and longer-acting
hydrophilic, administered at low doses opioids (e.g. methadone, buprenorphine), or
intramuscularly or subcutaneously frequently those that can be administered by intravenous
causes vomiting within minutes of in ection infusion (e.g. morphine, fentanyl), probably
since it crosses the blood brain barrier slowly bene t these patients more than pethidine.
and therefore in uences the CR before the
vomiting centre. The incidence of vomiting and Prevention, reduction or reversal of
signs of nausea can be reduced by prior
administration of maropitant or acepromazine any of these adverse e ects emesis,
(Koh et al., 14 . pioids that are likely to histamine release, dysphoria, panting) can be
cause vomiting should be avoided in some reduced or prevented by administration of
patients, for example, those with an acepromazine. This should be given before
oesophageal foreign body, increased the opioid to reduce the incidence of emesis.
intracranial pressure or a fragile eye at risk If necessary, adverse e ects of opioids can
of rupture. be reversed by administration of a MOP
Opioids may increase the incidence of antagonist such as naloxone at a dose of
gastro oesophageal re ux R during . . 4 mg kg intravenously,
anaesthesia due to e ects on cardiac sphincter intramuscularly or subcutaneously. This will
tone. Pethidine has been shown to reduce the also antagoni e the analgesic e ects of the
incidence of R compared with morphine opioid, so another analgesic strategy should
ilson et al., 7 . be implemented. Naloxone has a short
Opioids delay gastric emptying and duration of action, and re-administration may
increase gut transit time, which may result in be necessary after to 6 minutes. Adverse
constipation with prolonged administration. e ects of opioids may also be reduced by
Pyloric sphincter tone is increased, and this administration of a partial MOP agonist such
may a ect the ease of entering the duodenum as buprenorphine, or a mixed agonist/
during endoscopy in dogs. In the author’s [CJ] antagonist such as butorphanol, which may
experience, a clinical dose of opioid included maintain some analgesia but antagonize the
in the pre-anaesthetic medication does not unwanted e ect.
adversely a ect this procedure, although
evidence from recent studies is contradictory.
Since opioids can cause contraction of the receptors
sphincter of Oddi in humans, there is a belief i erent opioids may have di erent e ects at
that they should be avoided in veterinary the opioid receptors and some may have
patients with pancreatitis or biliary tract e ects at more than one type of opioid
obstruction. In humans and cats, the bile duct receptor igure .1 .

46

Ch05 Pain Management.indd 46 19/12/2018 10:36