Pain control

Re: Pain control

Rob Loxley — Sun, 02 Nov 2014 15:44:24 GMT

[quote user="chris J"]the powers that be dont want two opioids to monitor (which i can understand)[/quote]

If you have a robust policy for dealing with one CD2 then isn't it easier rather than harder to introduce a second?

Re: Pain control

Anonymous — Fri, 31 Oct 2014 09:21:24 GMT

thanks for the replies. I'll check out the cri calculator link and probably stick with morphine if I can persuade them!

Re: Pain control

Andrew Kent — Thu, 30 Oct 2014 20:23:16 GMT

Chris - if it helps our anaesthetists only use methadone IM when they choose it as they find the duration of action unpredictable IV. So if its the only thing available they would give it IM every 4 hours, but they tend to use other things (for example fentanyl) as a CRI during procedures.

Re: Pain control

Andrew Kent — Thu, 30 Oct 2014 20:18:54 GMT

My source was one of our anaesthetists when I was a student.

Looking in Plumb it says "Methadone is more lipid-soluble than is morphine and approximately 1–1.5 times as potent" and doesn't list a CRI dose.

A quick VIN search showed fairly divided opinion - some people saying they have used it, some saying it should never be used.

Re: Pain control

John Flynn — Thu, 30 Oct 2014 20:02:05 GMT

[quote user="Andrew Kent"]My understanding is that because methadone is so lipid soluble it is generally not regarded as being suitable for a constant rate infusion as it will accumulate rapidly leading to an increased risk of adverse effects.[/quote]

Do you have a source for "so lipid soluble"?

I don't have Plumb's to hand to check data for morphine, but I remember it not being that different to some of the published methadone stuff:

"In dogs volume of distribution at steady state (Vss) was 4.84 and 6.11 L/kg in males and females respectively. The terminal half-life is in the range 0.9 to 2.2 hours following intramuscular administration, and is independent of dose and sex. The terminal half-life may be slightly longer following intravenous administration.... Total plasma clearance (CL) of methadone following intravenous administration is high 2.92 to 3.56 L/h/kg or ca 70% to 85% of the cardiac plasma output in dogs (4.18 L/h/kg)." (Synthadon/Comfortan EU SPC)

EDIT:

"Following i.v. [morphine] administration ... Mean±SE values for half-life, apparent volume of distribution, and clearance after i.v. administration were 1.16±0.15 h, 4.55±0.17 L/kg, and 62.46±10.44 mL/min/kg, respectively." Journal of Veterinary Pharmacology and Therapeutics 2005 Vol. 28 No. 4 pp. 371-376

So I make that not a big difference between what I'm used to with morphine CRI's - I don't think those differences would be enough to cause problems over 24hrs anyway?

I use morphine myself (I think there is just more info on its use in dogs generally), but I have spoken to practitioners using methadone as CRI's and know there are at least a few specialists across the pond that do this (including at least one prestigious US vet school), so I wouldn't be afraid to at the doses discussed, though having read what Brendan says I'd lower the dose from what I've seen written.

Re: Pain control

Andrew Kent — Thu, 30 Oct 2014 19:33:02 GMT

My understanding is that because methadone is so lipid soluble it is generally not regarded as being suitable for a constant rate infusion as it will accumulate rapidly leading to an increased risk of adverse effects.

Andy

Re: Pain control

John Flynn — Thu, 30 Oct 2014 17:45:42 GMT

[quote user="Michael Woodhouse"]There is something in my brain about methadone accumulating with a CRI[/quote]

This has been oft-cited. The evidence-base for or against seems negligible to me and really comes down to which PK/PD data for methadone you follow... there's very little data and what is there conflicts markedly - try asking Dechra where the data on their datasheet comes from... let alone ask to see a methodology...

Re: Pain control

Michael Woodhouse — Thu, 30 Oct 2014 17:32:08 GMT

There is something in my brain about methadone accumulating with a CRI. Maybe better giving methadone as injection and the rest as CRI if needed.

Re: Pain control

John Flynn — Thu, 30 Oct 2014 17:06:00 GMT

[quote user="Michael Woodhouse"]There's an online calculator for CRI dose rates for all sorts of fun things. I may have the link at home.[/quote]

Is it:

http://www.vasg.org/drug_delivery_calculators.htm

if so, doesn't contain info on methadone I think ( and as an aside, if I remember correcly bizarrely used to have butorphanol included?)

Re: Pain control

Michael Woodhouse — Thu, 30 Oct 2014 16:55:27 GMT

There's an online calculator for CRI dose rates for all sorts of fun things. I may have the link at home.

Re: Pain control

Mark Hedberg — Thu, 30 Oct 2014 16:37:24 GMT

[quote user="Braden Collins"]

[quote user="John Flynn"]

[quote user="chris J"]cant find anything regarding methadone use in place of morphine though.[/quote]

I did read this from someone somewhere @ 0.1mg/kg/hr for up to 24hrs I *think*, but don't trust that unless someone else validates it (I can hunt for where i read it if no-one else comes forward)

[/quote]

We tried 0.1mg/kg/hr methadone and found it was creating profound sedation and animals were becoming hypothermic if not actively heated, even once fully recovered from the GA. We have reduced our dose rate to 0.05mg/kg/hr and find it gives very good analgesia and much less sedation.

[/quote]

I've heard that both methadone and dexdomitor have rather high dose rates, too!

Re: Pain control

John Flynn — Thu, 30 Oct 2014 14:57:32 GMT

great info, Braden, duly noted

Re: Pain control

Braden Collins — Thu, 30 Oct 2014 13:47:11 GMT

[quote user="John Flynn"]

[quote user="chris J"]cant find anything regarding methadone use in place of morphine though.[/quote]

I did read this from someone somewhere @ 0.1mg/kg/hr for up to 24hrs I *think*, but don't trust that unless someone else validates it (I can hunt for where i read it if no-one else comes forward)

[/quote]

Re: Pain control

Anonymous — Thu, 30 Oct 2014 13:35:28 GMT

thanks for that

Re: Pain control

John Flynn — Thu, 30 Oct 2014 12:57:10 GMT

not where i read it, but:

http://www.aapma.com/resources/Small%20Animal%20Drug%20List.pdf

Re: Pain control

John Flynn — Thu, 30 Oct 2014 12:37:55 GMT

[quote user="chris J"]cant find anything regarding methadone use in place of morphine though.[/quote]

I did read this from someone somewhere @ 0.1mg/kg/hr for up to 24hrs I *think*, but don't trust that unless someone else validates it (I can hunt for where i read it if no-one else comes forward)