2021B09 Discuss the relative advantages and disadvantages of using morphine and fentanyl for post-operative Patient Controlled Analgesia (PCA).

 

List:

     Summary

     PCA principles

     PC

     PK

     PD

 

Summary:

 

Morphine

Fentanyl

Settings

1mg q5min

20mcg q5min

Onset

Slower

Faster

Offset

Slower

Faster

Intra-op loading dose

Not needed

Needed

Side effects

More

Less

Suitable patient

Young, large, athletic

Older, OSA, risk of sedation

 

PCA principles:

Definition

      User controlled, locked, IV analgesic pump

      Usually opioid

Indication

      High opioid requirement

      Unreliable gut function

      Risk of overdose (OSA, OHS, severe COPD, sedated)

Toxicity

      Respiratory depression

      Sedation

      Airway obstruction

      Death

 

Pharmaceutics:

        Both stable in solution

        Both unlikely to precipitate with other drugs

 

Pharmacokinetics:

 

Time course:

Parameter

Morphine

Fentanyl

Time to peak effect

30 mins

5 mins

Bolus duration

2-3 hours

5-20 mins

->Morphine provides longer-lasting pain relief, but dose stacking may occur
->Fentanyl is better suited to q5min protocol

Distribution:

Parameter

Morphine

Fentanyl

Lipid solubility

1x

600x

pKa

8

8.4

% unionized

23%

9%

t1/2ke0

17 min

7 min

% plasma protein bound

35%

83%

VD (L/kg)

3.5

4

 

Elimination:

Parameter

Morphine

Fentanyl

Speed

-Cl (mL/kg/min)

-t1/2b (mins)

-Extraction ratio

 

16

170

0.76

 

13

210

0.8

Metabolism

Liver: phase 2>1

(Glucuronidation)

Liver: phase 1>2

(CYP3A4)

Metabolites

M6G analgesic

M3G neurotoxic

Inactive

Excretion

Renal: parent and metabolites

Renal: metabolites only

->Similar offset time after saturation

->Fentanyl safer in renal failure liver failure

 

Pharmacodynamics:

Parameter

Morphine

Fentanyl

Receptors M/K/D

3:1:1

3:1:0

PONV

Histamine release

Confusion

Abuse potential

↑ (faster onset)

 

 

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