Morphine and body weight

Factual Questions
1

There’s an article in the paper today that’s a follow-up on a (then) high school student who was severely burned two years ago in a fire in her chemistry class lab.

The article vaguely describes something happening that sounds like her first debridement and makes this statement: “She was so thin – 5 feet, 9 and a half inches, and 105 pounds – that the amount of morphine hardly took the edge off.”

The amount of morphine is not stated in the article.

How does weight relate to efficacy of morphine?

Maybe something was edited out of the article that makes it hard to connect the dots, or maybe it was just sloppy journalism where fact is less important than pathos.

2

I agree, it doesn’t make a helluva lot of sense.

Maybe they’re alluding to the fact that the morphine couldn’t be given subcutaneously since there wasn’t enough subcutaneous (fat) tissue to allow it to work. Maybe they’re trying to say that she was so thin that the debridement went right to the bone and/or muscle (i.e. again, not enough subcutaneous tissue). That would be extremely painful.

Morphine itself is seldom prescribed on a per kg basis. It’s true you might use somewhat more or less depending on the weight, but that’s about it. There’s certainly no precise adjustment per kg. (at least in common clinical use)

3

A drug made nowadays has to have a wide safety / efficacy window according to its concentration in a patient’s plasma. If this window is narrow, meaning possible toxic side effects according to body weight, liver capacity etc then the drug is not safe and will not get past the FDA. The exception to this will be drugs that are given to seriously ill people in a controlled environment e.g. cancer patients in a hospital.

Warfarin, the blood thinning drug, is an example of a compound that has a narrow window of safe dosage. I believe that a warfarin regime requires the patient to undergo a titration to determine the right dosage, regular blood tests etc. If warfarin was invented today it could not get approval. Given that it’s an old drug, that works under controlled conditions, doctors are happy to keep using it.

I don’t know if morphine has the same narrow range of effectiveness according to plasma conc. It’s an ancient medicine, with serious side effects, so it is quite possible that you would need to hit the right dosage. Saying that, I’ve never had to take morphine, but don’t patients self-administer it in hospitals with a button by the bed?

4

well no actually, morphine has a very low toxicity, much less than tylenol, for example. that’s not to say it doesn’t have side effects, but high doses can and are given.

In my experience, wt based per kilo dosing is the norm in pediatrics. However, at 105 lbs, I would expect to use adult dosing. It sounds to me like the author of this article didn’t really understand what they were writing about.

5

[QUOTE=The Cocky Watchman]
…don’t patients self-administer it in hospitals with a button by the bed?
[/QUOTE]
Yes; I’ve self-administered morphine (after spinal surgery) via a Patient-controlled analgesia (PCA) pump.

Of course, PCA’s have a maximum amount that they will dispense, which is determined by the doctor given the particular circumstances of the patient.

6

There are other factors to consider with regard to opiates.

People who have used non-morphine opiate analgesia long-term will have a higher tolerance for morphine, so they will need higher doses for the same analgesic effect. It is not uncommon for terminally ill patients with severe pain who have been on codeine or oxycontin long term to require the kind of dosages of morphine that would kill someone who was opiate naive.

There is evidence that how sensitive you are to morphine depends a lot on genetics too.

Generally my rule of thumb with morphine (as a doctor I only administer IV morphine, oral, SC and IM opiates are given by nurses) is to get 10mg of morphine or 5mg of diamorphine, make it up to 10mls in a syringe and inject 0.5-1ml every minute until the pain is relieved. If I have to use a second syringe, I will, as long as the patient is still breathing normally and not drowsy. IV morphine is used for things like heart attacks, very bad post-op pain and traumatic injury, or for something like burns when subcut or IM injection isn’t available as a route. It’s not what you’d use in a chronic situation.

If the pain isn’t relieved after 10mls and they’re not drowsy, I have to think that they’ve got a high tolerance or are a poor responder. If the pain is no better but they’re drowsy or breathing slower, it means the pain isn’t going to respond to morphine and another agent will be better suited to get the pain under control. By titrating the dose to the pain in that way you don’t overdose people and stop them breathing, but it can takea bit longer to control the pain.

One would tend to be cautious with the young, the elderly and the very petite. The young and petite because you’re giving them more drug proportionally, the elderly because they tend to respond to higher doses with a respiratory arrest and also because their kidney function tends to be poor and the drug and metabolites can take longer to clear their systems. In practice that means giving the drug slightly slower or allowing more time between each small dose, it doesn’t mean cutting someone off earlier if they’re obviously in pain and not suffering ill effects from the dose you have given so far.

Personally, after several years of endometriosis and surviving that pain with codeine based painkillers, both my pain thresh-hold and my opiate tolerance are pretty high. I needed 10mg of IV morphine after a diagnostic laparoscopy to take my pain from “ouchy” to “niggly”. After that dose I wasn’t drowsy or euphoric, just at a pain level where I was comfortable and able to eat some toast and read my book. The medical staff were less than keen to give me that dose because of my size, but when we’d got to 8mg and there wasn’t any discernible effect, they kept on going , bless them.

7

Sorry, edit window shut.

PCAs are risky business. In one unit where I have worked no post-c-section patients get PCAs as there was an incident some time before I worked there where one was set up incorrectly and the patient overdosed and died.

Another patient of mine, who had drug and alcohol issues, broke open the lock on his PCA and threatened to self-inject the rest of the bag of opiate solution because we had set up the dosing fail-safe too low to get him high.

For severe post-op pain most of our anaesthetists prefer epidurals to PCAs if at all possible.

8

wow, we dispense it with a much freer hand in California, I wouldn’t think twice about going from 8 to 10 mg in a pt who wasn’t already snowed, or didn’t have serious co-morbidities. Nurses push undiluted morphine iv in the ER all the time out here. I really don’t mean this to sound snarky, but don’t you have monitors?

9

[QUOTE=irishgirl]
For severe post-op pain most of our anaesthetists prefer epidurals to PCAs if at all possible.
[/QUOTE]

What are the NICE guidelines on PCA?

I’ve always (the last 8 or so ops, it wasn’t available before that) been given a free choice between epidural and PCA, and I plump for PCA everytime (at UCLH).