Opioids

Question 1

Morphine dose

Answer 1

Approx 0.1mg/kg for 1-2 hour operation (always reduce dose for elderly/sick patients)

Question 2

Onset of action
Morphine
Oxycodone
Fentanyl
Alfentanil
Remifentanil

Answer 2

Morphine - 15 minutes
Oxycodone - 15 minutes
Fentanyl - 5 minutes
Alfentanil - 30-60 seconds
Remifentanil - 30-60 seconds

Question 3

Duration of action
Morphine
Oxycodone
Fentanyl
Alfentanil
Remifentanil

Answer 3

Morphine - 3-4 hours
Oxycodone - 3-4 hours
Fentanyl - 30 minutes *
Alfentanil - 10-20 minutes
Reminfentanil - 10-20 minutes **

• Fentanyl is highly lipid soluble therefore after administration it rapidly redistributes away from the circulation in to other tissues. Therefore, the offset of action is due to redistribution, not metabolism (in the short term). If tissues become saturated with fentanyl there will be minimal redistribution and its duration will now depend on hepatic metabolism

** Remifentanil has a rapid offset due to metabolism by esterases that are abudant in the body.

Question 4

Morphine special features

Answer 4

Predictable and familiar

Larger therapeutic range = safer

Longer duration of action = useful for post-op analgesia

Question 5

Morphine side effects

Answer 5

• Active metabolites may accumulate in renal failure causing seizures and hyperalgesia
• Histamine release causing bronchospasm and hypotension
• Slow onset/offset compared to others

Question 6

Fentanyl dose

Answer 6

1-2mcg/kg for induction, 5mcg/kg for cardiac induction (always reduce dose for elderly/sick patients)

Use ~50mcg q15-30 minutes of the operation

Question 7

Fentanyl special features

Answer 7

Fast onset, fast offset

Cardiostable

No relevant active metabolites

Question 8

Fentanyl unique side effects

Answer 8

Accumulates in tissues with large doses/infusions

Question 9

Alfentanil dose

Answer 9

Induction 10-20mcg/kg

Question 10

Alfentanil special features

Answer 10

Very fast onset and offset

Question 11

Alfentanil unique side effects

Answer 11

Bradycardia and hypotension

Question 12

Remifentanil dose

Answer 12

Infusion at 0.1-0.2mcg/kg/min

Bradycardia and hypotension signify patient is ready for intubation

Question 13

Remifentanil special features

Answer 13

Very fast onset and offset

Rapid metabolism by plasma and tissue esterases

Very effective at maintaining muscle paralysis without traditional muscle relaxant

Question 14

Remifentanil unique side effects

Answer 14

Bradycardia

Hypotension

Chest wall rigidity

Hyperalgesia

Question 15

Common special features for all opioids

Answer 15

Analgesia

Sedation

Decrease hypnotic requirements/”MAC” sparing

Antitussive

Question 16

Common side effects for all opioids

Answer 16

Resp depression/apnoea

Bradycardia and hypotension (histamine)

Sedation

Euphoria

N&V

Constipation

Urinary retention

Question 17

How do opioids work

Answer 17

Naturally occuring/synthetic substance that interacts with opioid receptors (Mu, kappa, delta opioid receptors)

Question 18

General principles for onset of action

Answer 18

A lower pKa = faster onset of action

A low pKa means more unionised molecules which can cross membranes faster than ionised (charged) molecules

Alfentanil (pKa 6.5) and remifentanil (pKa 7.1) have the lowest values therefore the fastest onset of action