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The Case Continues

Teaching Exercise 1: The case continues

Over the next few days, Mr. Smith’s pain increases and his parenteral dose is increased to palliate his pain.

The following week, when you examine Mr. Smith, you note that his oral thrush has resolved and that he is able to swallow well again. So you decide to stop his Morphine infusion and change him to oral morphine again. You note that he is currently at Morphine Sulfate sub-cutaneous infusion 4mg per hour without any breakthrough medications. Please calculate the appropriate dose of oral morphine sulfate for this patient.

Sequential steps

Process

Answer
1) First determine total 24 hour dose of current drug i.e. Morphine Sulfate subcutaneous infusion. The total dose is the product of the unit dose in milligrams and the frequency of administration in 24 hours.

In this case:

Unit dose = 4mg

Frequency of administration = 24 times in 24 hours.

Therefore total dose = Unit dose x frequency = 4 x 24 = 96mg Morphine Sulfate subcutaneous infusion.

 Patient is currently on 96mg of Morphine Sulfate subcutaneous infusion in 24 hours.
2) Convert total 24 hour dose of current drug (Morphine sulfate infusion) to equianalgesic dose of ORAL morphine Current drug in this case is Morphine sulfate infusion.

Parenteral to oral ratio of Morphine =1:3

Total 24 hour dose of equianalgesic dose of ORAL morphine = 94mg x 3 = 282mg

 24 hour equianalgesic dose = 282 mg of ORAL morphine sulfate.
3) Divide the 24 hour dose of the equianalgesic dose of the oral Morphine sulfate to determine the unit doses and frequency. Morphine sulfate sustained acting preparation is usually dosed every 12 hours.

In this case Morphine sulfate 282mg in 24 hours can be divided into a unit dose of 140mg to be given every 12 hours (rounding off 282mg to 280 mg/24 hours for the sake of convenience).

 Final order for basal analgesic should read:

Morphine sulfate sustained release 140mg every 12 hours for basal pain.
4) Calculate appropriate break through pain medication:

Rule of thumb: Total amount of break through dosage available in 24 hours = total amount of basal drug given in 24 hours.

 In this case, as patient is getting a total basal dose of 280 mg over 24 hours.

He should have available total break through dose = 280mg of short acting morphine sulfate over 24 hours.

Divide the 24 hour dose of the equianalgesic dose of the oral Morphine sulfate to determine the unit doses and frequency.

Morphine sulfate short acting forms are dosed every four hours (frequency = 6)
280 / 6 = 46.6mg

Round off 46.6mg to 45 mg of short acting morphine sulfate.

 Final order for break through analgesic should read:

Morphine sulfate short acting formulation 40mg every 4 hours for breakthrough pain.

 
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