2006B01 Describe the use of different sympathomimetics to treat hypotension occurring as a result of subarachnoid block.
Outline the advantages and disadvantages of these agents.

 

List:

·      Intro

·      Pharmaceutics

·      Pharmacokinetics

·      Pharmacodynamics

 

Intro:

Cause of ↓BP

·  Subarachnoid block -> ↓SNS outflow -> ↓BP -> ↓organ perfusion

o Vasodilatation -> ↓SVR

o Venodilatation -> ↓preload -> ↓cardiac output (exaggerated if hypovolaemic)

Block height

·  Lower thoracic block: ↓SVR -> ↓mAP

·  Upper thoracic block: ↓SVR and ↓HR/contractility -> ↓↓mAP

·  Brainstem block: ↓↓mAP, HR at intrinsic SA node rate -> ↓↓mAP

·  (I have no evidence for brainstem level)

 

Pharmaceutics:

 

Adrenaline

Noradrenaline

Phenylephrine

Metaraminol

Ephedrine

Available pre-diluted for fast use

Y

N

Y

Y

Y

Extravasation induced necrosis

Y (use CVC)

Y

N

N

N

 

Pharmacokinetics:

 

Adrenaline

Noradrenaline

Phenylephrine

Metaraminol

Ephedrine

Route: IV and IM

Y

N

Y

Y

Y

Prefer bolus

 

 

Y

Y

Y

Prefer infusion

Y

Y

 

 

 

Onset

<1 min

<1 min

1-2 mins

1-2 mins

1-2 mins

Duration

5 mins

5 mins

10-20 mins

10-20 mins

10-20 mins

Metabolism

COMT, MAO

COMT, MAO

MAO

-

-

 

Dynamics:

 

Adrenaline

Noradrenaline

Phenylephrine

Metaraminol

Ephedrine

Receptor effects

β1, β2, α1

α1, β1

α1

α1 (weak β1)

β1, α1

Direct

Y

Y

Y

Y

Y

Indirect

N

N

N

Y

Y

HR

↑ or reflex ↓

Reflex ↓HR

Reflex ↓HR

SVR

/↑high dose

/↑

mAP

/↑high dose

/↑

Arrhythmia

Y

Rare

Rare

Rare

Rare

Use in pregnancy

 

 

Y (safer)

 

N (foetal lactataemia)

MOAi interaction

N

N

N

N

↑BP crisis

 

 

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