Perioperative analgaesia

Question 1

What is the difference between an opiate and an opioid?

Answer 1

Opiate - any drug that is derived from the naturally occurring opium alkaloid compounds that are found in the poppy plant (heroin, codeine, morphine opium)

Opioid - any drug that produces similar effects to opiates (e.g. fentanyl)

Question 2

How does fentanyl’s potency compare to that of morphine?

Answer 2

Fentanyl is 100 x as potent

Question 3

What is the presentation of fentanyl

Answer 3

50 ug/mL in a glass ampoule

Question 4

If the anaesthetic plan includes endotracheal intubation, how can fentanyl be used to reduce the response to laryngoscopy?

Answer 4

Onset: 5 minutes

Give about 2 - 3 ug/kg 5 minutes prior to laryngoscopy

Question 5

How long does analgaesia effect of fentanyl last if a dose of 50 - 100 ug is given intra-operatively?

How long does the analgaesic effect of morphine last if a bolus of 2 - 5 mg of morphine is given?

Answer 5

Fentanyl: 15 - 30 minutes

Morphine: 30 - 40 minutes

Question 6

Describe the presentation of fentanyl

Answer 6

Glass ampoules: 2ml and 10ml
Concentration: 50ug/ml
High lipid solubility
Weak base with pKa of 8.2 
(Bases are ionized at pH below pKa) --> therefore fentanyl is ionized at 7.4 and water soluble.

Question 7

How can the potency of fentanyl and morphine be compared graphically

Answer 7

on the Log (dose) response curve by plotting the graphs for each drug and then comparing the ED50 for each drug:

ED50 Fentanyl = 0.1mg
ED50 Morphine = 10mg

Therefore fentanyl is 100 x more potent than morphine as 100 times less agent is required to elicit a clinic effect

Question 8

Compare the physicochemical properties of fentanyl to morphine

Answer 8

Property Fentanyl Morphine

pKa 8.4 8

Unionized 9 23
[at pH 7.4 (%)]

Plasma 84 35
protein
bound

Relative
Lipid
Solubility 580 1

Terminal
half life (h)           3.5                 3

Clearance
(ml/min.kg) 10 - 25 10 - 20

Vd
(L/kg) 3 - 5 2 - 3

Question 9

Describe the side effects of fentanyl and compare these to the side effects of morphine

Answer 9

Fentanyl

CVS - Bradycardia, hypotension
RSP - RR reduction, Vt increase (Vt decreases with higher doses)
GIT - PONV, Constipation
GUS - Urinary retention

Other

• Pruritis
• Chest wall rigidity (large dose during induction of anaesthesia)

Morphine

CVS - Bradycardia, hypotension
RSP - Depression
GIT - PONV, Constipation
GUS - Urinary retention

Other

• Pruritis
• Histamine release –> asthmatics –> bronschospasm

Question 10

After initial use of fentanyl, what is the usual dosing of morphine during the procedure?

Answer 10

2 - 5 mg bolus every 30 - 40 minutes

Question 11

What would happen if an IV bolus of morphine is given 10 minutes before the surgery ending

Answer 11

RSP depression may slow recovery

Question 12

What is an effective dose and timing for administration of morphine in a surgery anticipated to cause significant postoperative pain

Answer 12

Morphine bolus of 0.1 - 0.15 mg/kg 45 minutes before the end of surgery

Question 13

What is the presentation of morphine

Answer 13

Clear liquid in 1 mL ampoules

Concentration 10mg/ml

Question 14

Does morphine have active metabolites?

Answer 14

Yes. Morphine-6-glucuronide –> more potent than morphine –> only important in patients with renal failure who receive repeated doses of morphine

Question 15

How does the duration of action of NSAIDs compare with intravenous opioids

Answer 15

NSAIDs act for significantly longer

Question 16

How do NSAIDs affect intraoperative and postoperative opioid requirements?

Answer 16

Reduce fentanyl requirements intra-operatively

Reduce postoprative opioid requirements by 20 - 30 %

Question 17

How should diclofenac suppository be used

Answer 17

INFORMED CONSENT

After induction and before surgery starts (allow time for absorption)

Question 18

How should intravenous paracetamol be given

Answer 18

1 g in 100 ml over 20 minutes IV infusion

Question 19

After how long is a repeat dose of paracetamol required?

Answer 19

6 hours. Repeat dose is rarely required

Question 20

Doses for the premedication and postoperative administration of ibuprofen

Answer 20

400 mg pre-op

400 mg 8 hourly post op

Question 21

What is the mechanism of action of ibuprofen and diclofenac

Answer 21

Non-selective COX inhibitor –> inhibition of COX 1 and COX 2 and hence prostaglandin synthesis reducing inflammation and providing analgaesia

Question 22

Most common side effects of ibuprofen and diclofenac

Answer 22

Upper GI bleed

Inhibition of platelet aggregation

Question 23

What are the contraindications for ibuprofen and diclofenac

Answer 23

Previous or existing upper GI ulcers
Avoid in asthmatics intolerant of aspirin
Caution in renal impairment

Question 24

Describe the oral, suppository and Injection preparations and doses for diclofenac

Answer 24

Oral: 25mg and 50 mg tablets (Pre-med 75 -100mg; post-op 50mg/8h)

Suppository: 50 and 100 mg suppositories are available. Intraop: 100mg PR

Injection: 25mg/ml in 3 ml ampoule (75mg)
IV dose: 50 - 75mg diluted in 10 ml saline and given slowly

Question 25

Why is diclofenac no longer available OTC in UK and USA

Answer 25

2015 –> small risk of heart problems

Question 26

What is the maximum daily dose of diclofenac for an adult

Answer 26

150mg

Question 27

What is a contraindication for diclofenac

Answer 27

porphyria

Question 28

Describe appropriate morphine prescriptions in the recovery area versus in the ward

Answer 28

Recovery area:
1-2 mg IV every 5 min until patient is comfortable (maximum of 10mg)

Ward prescription
10mg IM maximally 2 hourly whilst observed in the ward area

Question 29

Describe the appropriate dosing of diclofenac

Answer 29

Intraoperative: 50 - 75 mg IV in 10 mls slowly

Prescription: 150mg daily in divided doses (orally or rectally)

Question 30

Describe the PCA system (Patient controlled analgesia system)

Answer 30

1-2 mg given on demand (self-adminsitered) with a 5 minute lockout period. Lockout period is set by the anaesthetist to mitigate the risk of respiratory depression

Question 31

When should opioids be prescribed for postoperative analgaesia in day cases

Answer 31

NSAIDS not tolerated or = stronger agent the paracetamol is required then codeine phosphate 30 - 60 mg every 4 hours for not more than 72 hours can be prescribed