Opioid Classification

Based on their interactions with the various receptor subtypes, opioid compounds can be divided into agonist, partial agonist, and mixed agonist-antagonist drugs (Table 4). The pure agonist drugs (Table 5) are most commonly used in clinical pain management, both in adult patients and in children (Table 6). The mixed agonist-antagonist opioids (pentazocine, nalbuphine, butorphanol and dezocine) and the partial agonist opioids (buprenorphine) play a minor role in the management of post-operative pain because of the existence of a ceiling effect for analgesia, the potential for precipitation of withdrawal in patients physically dependent to opioid agonists and, in some cases, the problem of dose-dependent psychotomi-metic side effects that exceed those of pure agonist drugs. The pure agonist opioid drugs appear to have no ceiling effect for analgesia. As the dose is raised, analgesic effects increase until either analgesia is achieved or the patient loses consciousness. This increase in effect occurs as a log-linear

Prevention and Treatment of Postoperative Pain Table 4. Opioid Classification

Agonists

Partial agonists

Mixed agonist/antagonists

Morphine Codeine Oxycodone Hydrocodone

Buprenorphine

Pentazocine Butorphanol Nalbuphine Dezocine

Dihydrocodeine Heroin

Oxymorphone

Meperidine

Levorphanol

Hydromorphone

Methadone

Fentanyl

Sufentanil

Alfentanil

Propoxyphene function: dose increments on a logarithmic scale yield linear increases in analgesia. In practice, it is the appearance of adverse effects, including confusion, sedation, nausea, vomiting or respiratory depression, that imposes a limit on the useful dose. The overall efficacy of any drug in a specific patient will be determined by the balance between analgesia and side effects that occurs during dose escalation.

The division of opioid agonists into 'weak' versus 'strong' opioids was incorporated into the original 'analgesic ladder' proposed by the WHO. This distinction was not based on a fundamental difference in the pharmacology of the pure agonist opioids, but rather reflected the customary manner in which these drugs were used. This explains the observation that some opioids that were customarily used for moderate pain (step 2 of the analgesic ladder), such as oxycodone, are also used for severe pain in selected patients. Indeed, the controlled-release formulation of oxycodone is now widely used in the management of severe pain. Conversely, low-dose formulations of controlled-release morphine are suitable for the management of pain of moderate severity. Weak opioids are indicated in mild to moderate pain, usually associated to other drugs such as paracetamol. A weak opioid should be added to, not substituted for, a non opioid and it's important not to ''kangaroo'' from weak opioid to weak opioid. If a weak

Weak' Versus 'Strong' Opioids

Table 5. Opioid Agonist Drugs

Drug

Dose (mg) equianalgesic to 10 mg morphine IM

P.O.

Half-life (hrs)

Duration of action (hrs)

Comments

Codeine

130

200

2-3

2-4

Usually combined with a non-opioids

Oxycodone

15

30

2-3

2-4

Usually combined with a non-opioids

Propoxyphene

100

50

2-3

2-4

Usually combined with a non-opioids. Norpropoxyphene toxicity may cause seizures

Morphine

10

30

2-3

3-4

Multiple routes of administration available. Controlled release available. M6G accumulation in renal failure

Hydromorphone

2-3

7.5

2-3

2-4

No known active metabolites. Multiple routes available

Methadone

10

3-5

15-190

4-8

Plasma accumulation may lead to delayed toxicity. Dosing should be initiated on a p.r.n. basis. When switching to Methadone from another opioid, potency may be much greater than expected; the dose of Methadone should be lowered by 75-90% to account for this

Meperidine

75

300

2-3

2-4

Low oral bioavailability. Normeperidine toxicity limits utility. Containdicated in patients with renal failure and those receiving MAO inhibitors

Oxymorphone

1

10 (p.r)

2-3

3-4

No oral formulation available. Less histamine release

Heroin

5

60

0.5

3-4

High-solubility morphine prodrug

Levorphanol

2

4

12-15

4-8

Plasma accumulation may lead to delayed toxicity

Fentanyl

Empirically, trans

48-72

Patches available to deliver 25, 50, 75 and 100 ^g/h

transdermal

dermal fentanyl 100 ng/h = 2-4 mg/h intravenous morphine

Table 6. Opioids Commonly Used for Postoperative Pain Relief in Children

Drug

Iv/sc starting dose

Oral starting

Notes

dose

Codeine

0.5-1 mg/kg

Nausea, vomiting

every 3-4 hr

Idromorphone

Bolus: 0.015 mg/kg

0.06 mg/kg

Nausea, vomiting,

every 2-4 hr

every 3-4 hr

urinary retention

Drip: 0.006 mg/kg/hr

Morphine

Bolus: 0.05-0.1 mg/kg

0.15-0.3 mg/kg

Nausea, vomiting,

every 2-4 hr

every 4 hr

urinary retention,

Drip: 0.03 mg/kg/hr

pruritus

Fentanyl

Bolus: 0.5-1 g/kg

Nausea, vomiting,

every 1-2 hr

urinary retention,

Drip: 0.5-3.0 g/kg/hr

pruritus, respiratory

depression

Remifentanyl

Bolus: 0.1-0.5 g/kg

Nausea, vomiting,

every 1 h

urinary retention,

Drip: 0.1-0.25 g/kg/

pruritus, respiratory

min

depression

Sufentanyl

Bolus: 0.2 g/kg

Respiratory

every 1 h

depression,

Drip: 0.1-0.5 g/kg/

haemodynamic

min

alterations

opioid is inadequate when given regularly, the right step is to change to strong opioids.

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