Page 48 - BSAVA Guide to Pain Management in Small Animal Practice

5 | Pharmacological treatment of pain

VetBooks.ir individual pain assessment). Animals with anaesthesia when a fentanyl infusion is used to
provide analgesia. In the author’s [CJ] opinion,
hepatic encephalopathy may be more sensitive
administration of opioids to dyspnoeic animals
to the sedative e ects of opioids, and a lower
than normal dose may be appropriate. is not contraindicated, and can be helpful due
to analgesic and sedative e ects.
Opioids depress the cough centre and are
These are mediated by opioid receptors in the antitussive. They have been licensed for this
central nervous system and throughout the purpose (e.g. butorphanol tablets for cough
body. Adverse e ects are less likely to occur if prevention in dogs). Prevention of cough is
the animal is experiencing pain at the time of useful for some procedures such as
opioid administration. bronchoscopy, but it should be remembered
that coughing is sometimes required to aid
Respiratory system: Opioids can increase the clearance of the respiratory tract, so coughing
threshold at which arterial CO levels trigger should not be prevented for more than a few

ventilation, and decrease the threshold at which hours in animals with wet/productive coughs.
hypoxaemia triggers ventilation. This respiratory Opioids often cause panting in dogs (see
depression has historically prevented the ‘Thermoregulation’).
prescription of opioids to animals, and it is a real
problem in human anaesthesia. Fortunately, Circulatory system: Opioids increase vagal tone
veterinary patients are less prone to respiratory and may cause bradycardia, respiratory sinus
depression than humans, and it is very unlikely arrhythmia and even second-degree
to happen when conscious dogs or cats are atrioventricular block or other bradyarrhythmias.
given opioids at clinical doses. Respiratory In the author’s [CJ] experience, this is most likely
depression may occur with overdose, or when to occur in animals with existing high vagal tone,
opioids are combined with other respiratory such as t animals or brachycephalic breeds,
depressant drugs, for example, during sedation during general anaesthesia when full MOP
or general anaesthesia. Monitoring (pulse agonists are used. An example would be an
oximetry, capnography) will detect this problem, anaesthetized working Springer Spaniel that has
and supplementation of oxygen and controlled received methadone as part of the pre-
ventilation may be necessary to counteract the anaesthetic medication. Usually, the reduction in
respiratory depression. For example, controlled heart rate is compensated for by an increase in
ventilation may be necessary during general stroke volume, maintaining cardiac output. If the

Opioids and intracranial disease
Respiratory depression may occur after administration of opioids in mentally depressed animals
with intracranial disease, for example, following head trauma, or as a result of space-occupying
intracranial lesions. In these patients, decreased ventilation and the resulting increased arterial
CO levels may cause a further increase in intracranial pressure, worsening neurological status
2
and potentially lead to brain herniation and Cushing’s triad (hypertension, bradycardia and
respiratory disturbances due to pressure on the brainstem and catecholamine release). Opioids
can be used in patients with intracranial disease, but low doses should be used and titrated
to effect, and the patient s neurological status and vital signs should e carefully monitored. f
respiratory depression occurs, the opioid could be antagonized, or the patient anaesthetized,
the trachea intubated and the lungs ventilated to achieve normocapnia. Sedation due to opioids
may also interfere with neurological assessment. Patients with head trauma or intracranial
disease may experience severe pain so other forms of analgesia, including multimodal
analgesia, should be considered, especially if opioids cannot be used.

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