2007B03 Outline the important pharmacological considerations concerning choice of
opioid and dosage when converting from intravenous morphine to oral opioid analgesia
in the post-operative period.

List:

· Opioid conversion table

· Intro: PO benefits and requirements

· Administration

· Patient considerations

· Surgical considerations

Opioid conversion:

Drug	Equipotent dose
Morphine IV	10mg
Morphine PO	30mg
Oxycodone	20mg
Tramadol	100-200mg
Tapentadol	66.5mg
Codeine	200mg
Hydromorphone	6mg
Pethidine	300mg
Methadone	10mg

Intro:

IV -> PO benefits

· Can remove IVs

· Can discharge home

IV -> PO requirements

· Adequate steady state has been reached

· Gut function intact

· Swallowing intact

· Co-operation

Administration:

Dose	· ∝ bioavailability/potency · (most important factors)
Frequency	· ≈ t_1/2_β · e.g. q3-4h for morphine, oxycodone
Adjuvants	· Paracetamol + NSAID -> ↓opioid requirement
Interactions	· Benzodiazepines: synergistic effect on respiratory depression

Patient considerations:

Age

· Elderly:

o Deficient blood-brain barrier (BBB), ↑pharmacodynamic (PD) sensitivity

o ↓Dose e.g. 2.5-5mg q4h oxycodone

· Neonate:

o Immature BBB, ↑PD sensitivity; ↑↑effect if low %unionised e.g. morphine 23%

o Dose e.g. 0.05-0.1mg/kg oxycodone

Comorbidity

· Renal failure:

o Avoid drugs with renally excreted active metabolites

o Hence prefer fentanyl, methadone

· Epilepsy: avoid tramadol, pethidine

· Sleep-disordered breathing: prefer

o Tramadol (partial agonist, ceiling effect on resp depression)

o Tapentadol (↓opioid effect)

· Chronic pain history:

o Tramadol (70% of effect via NAd and 5HT reuptake inhibition)

o Tapentadol (NAd reuptake inhibition)

o Methadone (multiple non-opioid effects)

· Opioid abuse:

o Tramadol (partial agonist, only 30% of effect via MOP, less euphoria)

o Tapentadol (↓opioid effect)

o Methadone (often already prescribed)

o Buprenorphine (partial agonist –ceiling effect, inadequate for major surgery)

2D6 poly-morphism

· Cantonese 30% reduced metabolisers (codeine ineffective)

· North African and Arabs 10% ultra-rapid metabolisers (codeine -> respiratory depression)

Surgery:

Type of pain

· e.g. hip pain: somatic (most opioids suitable)

· e.g. thoracotomy: somatic and neuropathic (prefer tramadol, tapentadol)

Severity of pain

· e.g. simple abscess incision and drainage: weak opioid may be enough (e.g. codeine, but frequent polymorphism)

· e.g. lower limb ORIF: oral oxycodone

Effect on gut function

· e.g. bowel obstruction: avoid oral analgesic until gut function established

· e.g. sleeve gastrectomy: liquid only (oxycodone, morphine)