Perioperative analgaesia Flashcards
(31 cards)
What is the difference between an opiate and an opioid?
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)
How does fentanyl’s potency compare to that of morphine?
Fentanyl is 100 x as potent
What is the presentation of fentanyl
50 ug/mL in a glass ampoule
If the anaesthetic plan includes endotracheal intubation, how can fentanyl be used to reduce the response to laryngoscopy?
Onset: 5 minutes
Give about 2 - 3 ug/kg 5 minutes prior to laryngoscopy
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?
Fentanyl: 15 - 30 minutes
Morphine: 30 - 40 minutes
Describe the presentation of fentanyl
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.
How can the potency of fentanyl and morphine be compared graphically
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
Compare the physicochemical properties of fentanyl to morphine
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
Describe the side effects of fentanyl and compare these to the side effects of morphine
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
After initial use of fentanyl, what is the usual dosing of morphine during the procedure?
2 - 5 mg bolus every 30 - 40 minutes
What would happen if an IV bolus of morphine is given 10 minutes before the surgery ending
RSP depression may slow recovery
What is an effective dose and timing for administration of morphine in a surgery anticipated to cause significant postoperative pain
Morphine bolus of 0.1 - 0.15 mg/kg 45 minutes before the end of surgery
What is the presentation of morphine
Clear liquid in 1 mL ampoules
Concentration 10mg/ml
Does morphine have active metabolites?
Yes. Morphine-6-glucuronide –> more potent than morphine –> only important in patients with renal failure who receive repeated doses of morphine
How does the duration of action of NSAIDs compare with intravenous opioids
NSAIDs act for significantly longer
How do NSAIDs affect intraoperative and postoperative opioid requirements?
Reduce fentanyl requirements intra-operatively
Reduce postoprative opioid requirements by 20 - 30 %
How should diclofenac suppository be used
INFORMED CONSENT
After induction and before surgery starts (allow time for absorption)
How should intravenous paracetamol be given
1 g in 100 ml over 20 minutes IV infusion
After how long is a repeat dose of paracetamol required?
6 hours. Repeat dose is rarely required
Doses for the premedication and postoperative administration of ibuprofen
400 mg pre-op
400 mg 8 hourly post op
What is the mechanism of action of ibuprofen and diclofenac
Non-selective COX inhibitor –> inhibition of COX 1 and COX 2 and hence prostaglandin synthesis reducing inflammation and providing analgaesia
Most common side effects of ibuprofen and diclofenac
Upper GI bleed
Inhibition of platelet aggregation
What are the contraindications for ibuprofen and diclofenac
Previous or existing upper GI ulcers
Avoid in asthmatics intolerant of aspirin
Caution in renal impairment
Describe the oral, suppository and Injection preparations and doses for diclofenac
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
