COMMENTARY

Managing Pain: Determining the Right Maintenance Dose

Charles F. von Gunten, MD, PhD

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October 13, 2011

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I'm Charles von Gunten, Provost of The Institute for Palliative Medicine at San Diego Hospice. We're going to talk about establishing a maintenance dose for opioids once you've found the dose that controls the patient's pain.

Just to review, let's imagine a patient who has severe cancer pain and the first dose you gave of morphine was 2 mg IV push. But the patient's pain wasn't relieved. So after 8 minutes, you doubled the dose to 4 mg. The patient still said he had 10 out of 10 pain, so you doubled the dose again to 8 mg given 8 minutes later.

The patient still had 10 out of 10 pain. So then you doubled the dose again and now you gave 16 mg. Now the patient says "Oh, the pain is better." Now you want to establish the maintenance dose.

What I want you to do is think of a person as if they were a bucket and this amount of morphine filled the bucket. So this is a total of 30 mg of morphine total [doses of 2 mg plus 4 mg plus 8 mg plus 16 mg = 30 mg total]. So the principle for the dose escalation was the time to the maximum concentration. Now we use the principle of the half-life.

This person took 30 mg to get themselves out of pain and we know that the half-life of morphine is 4 hours. So if you think of this bucket as now "draining out," with a half-life of 4 hours, it means half of this [drug] will be relieved or will drain out in 4 hours. So half of this means that 15 mg, half of the 30 mg, will be metabolized in 4 hours. So divide 15 mg by 4 and you have the new hourly rate, which would be about 4 mg per hour, which would be the new steady state.

The key principle is you use the dosing every time to the maximum serum concentration. Find the dose that relieves the person's pain. And then convert it to an hourly rate using the half-life. For morphine, for hydromorphone, the half-life is 4 hours. Dividing the total dose, the half-life then means the 15 mg is metabolized in 4 hours. If you divide that, it comes to about 4 mg per hour.

You can see how you can quickly get to the right maintenance dose to keep this person out of pain.

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