Equianalgesia and calculation of the rescue dose
New mu-opioid receptor agonist pain killers have an identical analgesic potential to morphine, however the respective doses must be adapted each time to get the same analgesic efficiency.
Although inter-individual variability exists, calculation rules are used to estimate the equianalgesic doses between the various commercially available drugs.
The analgesic strength of each opioid molecule varies because of two aspects:
- µu-opioid receptor affinity
- route of administration due to differing bioavailability
The strength conversion factor is determined by considering the average observed variations between individuals of these two items. Thus different rules can exist to calculate the equianalgesic dose conversion.
For example, the pharmaceutical company that supplies oxycodone in Belgium recommends that an oral intake of 5 mg of oxycodone is equal to 10 mg of oral morphine.In contrast, the Oxford Textbook of Palliative Medicine recommends a dose of 6.6 mg of oral oxycodone as being equivalent to an oral dose of 10 mg of oral morphine. Although it uses a more complicated method of calculation,the Oxford Textbook recommendation quite often provides a more appropriate oxycodone conversion than the one recommended by the company when used in the context of an in-patient clinic. In all cases, once the theoretical conversion has been performed, the background analgesia treatment must still be adapted as needed!
Knowledge of the equianalgesic dose of different molecules is important when changing from one background opioid analgesic to another. Going from one opioid to another for the 24 hour background analgesic treatment is known as “opioid rotation”.In opioid rotation, a 30% reduction in the analgesic value of the background analgesia is suggested to avoid over-dosing.
There is little risk of overdose when a different opioid is used for the rescue dose, as about one-tenth of the base dose should be administered.Thus when calculating the rescue dose, the prescribing physician should follow equianalgesia rules without reducing the dose by 30%.
To determine the rescue dose value in practice:
- Calculate the analgesic strength of the background 24-hour opioid treatment.The different molecules must be added while taking into consideration their respective administration routes, given that the latter influences the analgesic strength.If immediate release opioids are regularly administered, it is best to add them.With morphine per os being used as a reference standard, it is useful to know the opioid treatment equivalence in daily oral morphine doses to estimate the strength of the background treatment.
- Choose the rescue dose molecule. (preferably identical to the background analgesia molecule)
- Choose the route of administration for the rescue dose.
- Calculate the analgesic value equal to 1/6 or 1/10 of the total 24-hour analgesic strength.
- The minimum rescue dose is equal to 5 mg of morphine per os.