Use of methadone in young animals, geriatric animals, and those with concurrent disease
Professor Derek Flaherty BVMS, DVA, DipECVAA, MRCA, MRCVS
RCVS and European Specialist in Veterinary
Anaesthesia
Paediatric animals
• various definitions but usually considered as puppies and kittens less than 12 weeks old
“safety of methadone not demonstrated in dogs
<8wks or cats <5months”
Paediatric animals
...this does not mean that methadone is unsafe in these patients, merely
that appropriate testing has not been
carried out
Are paediatric animals different?
• cardiovascular system
• respiratory system
• hepatic function
• temperature regulation
Cardiovascular considerations
• stroke volume relatively fixed in paediatrics
• rely on high heart rate to maintain cardiac output
• opioids HR
• combine with anticholinergic (atropine/glycopyrrolate) in animals <12 weeks
Methadone in paediatric animals
• may see:
– greater sedative effects
– longer duration
with opioids in paediatrics
Methadone in paediatric animals
• initial dose should be no greater than
0.2 mg/kg** • IM route preferable • supplemental doses
should be administered as required
• may require increased dose interval (pain score)
** less than SPC dose
Methadone in hepatic disease
“Do not use in animals with severe liver dysfunction”
• protein binding of 60-90% • 96-97% of the administered dose is metabolised by the liver • expect potential exaggerated effect and increased duration
Methadone in hepatic disease
• start off with low doses (0.1-0.2 mg/kg)**
• titrate upwards to effect
• anticipate increased dose interval (but high individual variability)
** less than SPC dose
Opioids in renal disease
• decreased excretion of parent drug or metabolites (active)
• altered drug binding
• increased permeability of blood-brain barrier
Opioids in renal disease
• exaggerated opioid effects (and side-effects)
• potential drug accumulation over-dose
Methadone in renal disease
“Do not use in animals with severe renal
dysfunction”
“A small amount (3-4% in the dog) of the
administered dose is excreted unchanged in
the urine…”
Methadone in renal disease
• methadone is probably preferable to other full agonist opioids in renal disease
• morphine and pethidine both have active metabolites that are renally excreted
Methadone has no active metabolites
Methadone in renal disease
“Do not use in animals with severe renal
dysfunction”
• start with low doses (0.1-0.2 mg/kg)** to assess sensitivity
• titrate upwards as required
** less than SPC dose
Opioids in geriatric animals
• impaired organ function
• increased sensitivity to opioids
• expect exaggerated effect and longer duration
Opioids in geriatric animals
• start with low doses (0.1-0.2 mg/kg)** to assess sensitivity
• titrate upwards as required
** less than SPC dose
Opioids and intracranial disease
• if there is a significant
chance the animal may require neurosurgical intervention, best to use a short-acting opioid such as pethidine IM
• if this is unlikely / not an option, a longer-acting opioid is more practical
• avoid morphine (vomiting; unlicensed)
Image courtesy of Dr. Elisa Mazzaferro
Methadone and intracranial disease
• start with low doses and titrate upwards
– 0.1 mg/kg methadone slowly IV, repeated as required q 10 min
– N.B. this is less than the licensed dose and the IV route is off licence in cats
The ‘generic’ sick animal
• methadone has minimal
cardiovascular depressant effects (bradycardia with high doses esp. IV) and causes minimal respiratory depression
• reduces anaesthetic requirements – anaesthetics are marked
cardiopulmonary depressants
The recurring theme...
• start with low doses (0.1-0.2 mg/kg)** to assess sensitivity
• titrate upwards as required
** less than SPC dose
In summary...
• many of the issues highlighted on the Comfortan® SPC merely relate to the absence of controlled studies in these patient groups
• ...in common with most other opioid drugs
• methadone can generally be used ‘safely’ in these animals provided due consideration is given to both the dose and dosing interval