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 (use CVC)	N*	N*	N*

(*internet says yes to all, examiner says yes to metaraminol)

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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