Opioids

Question 1

Morphine:

1. pKa
2. Percentages unionised at physiological pH

Answer 1

1. pKa - 8

2. 23% unionised

Question 2

Pethidine:
1. pKA

2. %unionised at pH 7.4

Answer 2

pKa 8.5

% unionised- 5%

Question 3

Fentanyl

1. pKa
2. % unionised
3. Which opioids have faster onset/ offset?

Answer 3

pKa - 8.4

% unionised - 9%

Question 4

Alfentanil:

1. pKa
2. % unionised

Answer 4

pKa - 6.5

% unionised - 90%

Question 5

Remifentanil:

pKa

% unionised at pH 7.4

Answer 5

pKa - 7.1

68% unionised

Question 6

Lipid solubility compared to morphine of:

1. Remifentanil
2. Alfentanil
3. Fentanyl

Answer 6

Remi - x 20

Alfent- x 90

Fentanyl- x 600

Question 7

Which is the most plasma protein bound?

Answer 7

Alfentanil - 90%

Question 8

Least plasma protein bound?

Answer 8

Morphine - 35%

Question 9

Biggest Vd?

Answer 9

Fentanyl - 4L

Pethidine - 4L

Question 10

Smallest Vd?

Answer 10

Remi - 0.3L

Alfentanil - 0.6L

Question 11

Longest elimination half life?

Answer 11

Pethidine - 210 minutes

Question 12

Elimination half lives of:

Remifentanil
Alfentanil
Fentanyl

Answer 12

Remi- 10 mins

Alf- 100 minutes

Fentanyl- 190 minutes

Question 13

Which drug can prolong elimination half life of alfentanil?

Answer 13

Midazolam - metabolised by same CYP3A3/4 so both eliminations prolonged when given together.

Erythromycin may prolong by inhibiting hepatic CYP450z

Question 14

Why is alfentanil have a faster onset/ offset than fentanyl?

Answer 14

Even though alfentanil has a lower lipid solubility than fentanyl it has a lower pKa of 6.5, 90% unionised and able to cross the lipid membranes.

Fentanyl has a pKa of 8.4 so 9% unionised. Therefore slower onset.

Alfentanil has a small Vd so is offset faster as well despite lower clearance than fentanyl.

Question 15

Which opioids can cause chest wall rigidity?

Answer 15

Remifentanil and Morphine

Question 16

Oral bioavailability of:

Oxycodone?

Pethidine?

Answer 16

Oxycodone - 85%

Pethidine - 50%

Question 17

Oral bioavailability:

Methadone?

Tramadol?

Answer 17

Methadone - 75%

Tramadol - 70% which increases to 90% with repeated doses

Question 18

Paracetamol plasma protein binding?

Answer 18

10%

Question 19

Protein binding aspirin?

Answer 19

85%

Question 20

What analgaesic is linked to Reyes Syndrome in children?

Answer 20

NSAIDS, aspirin

• encephalopathy and cerebral oedema.

Question 21

Paracetamol oral bioavailability?

Answer 21

80%