2019B08 Briefly explain the cardiovascular effects of central neural blockade.

List:

· Summary

· Cardiovascular innervation

· Direct effects

· Compensatory effects

· Special responses

Summary:

Mechanism of action	· Antagonist at voltage-dependent Na+ channel · Epidural: spinal nerve roots (dorsal > ventral), dorsal root ganglia, paravertebral SNS chain o Causes more hypotension for a given height and depth of block · Subarachnoid: spinal nerve roots, dorsal root ganglia, spinal cord
Direct effects	· ↓SNS activity -> hypotension o ∝ Block height o ∝ Block density · ↔PSNS activity
Compensation	· ↓Hormonal response · ↓Neural response if high block
Problem	· Bilateral sympatholysis is very dangerous if o Hypovolaemic (e.g. dehydration in prolonged labour) o Fixed cardiac output (e.g. aortic stenosis) o Susceptible to LVOT obstruction (e.g. HOCM)

Cardiovascular innervation: sympathetic nervous system

Pathway	· Premotor cell: RVM medulla (releases ACh) · Pre-ganglionic cell: intermediolateral column (releases ACh) · Post-ganglionic cell: paravertebral chain (releases NAd)
Spinal cord level	· Upper thoracic: head, upper limb, thorax · Lower thoracic: abdomen, kidneys, adrenal medulla (secretes adrenaline) · Upper lumbar: pelvis, lower limb
Receptor effects	· α1: vasoconstriction, venoconstriction · β1: ↑heart rate, ↑contractility; ↑renin release

Direct effects:

Vascular	· Vasodilation -> ↓SVR -> ↓mAP (less important – intrinsic tone well maintained) · Venodilation -> ↓venous return, ↓cardiac output, ↓mAP (more important – intrinsic tone not well maintained)
Cardiac	· ↓Heart rate, ↓contractility (only if upper thoracic)

Compensation:

Neural (seconds)	· Baroreceptors: stretch-sensitive mechanoceptors in carotid and aortic sinuses · ↓mAP -> ↓stretch -> ↓afferent -> ↓SNS inhibition o ↑HR, ↑contractility o ↑Venoconstriction -> ↑preload o ↑Vasoconstriction -> ↑SVR
Hormonal (minutes)	· ↑Renin-angiotensin-aldosterone system activation o Vasoconstriction o ↑Na^+/H2O reabsorption -> ↑blood volume -> ↑venous return · ↑ADH release o Vasoconstriction o ↑H2O reabsorption -> ↑blood volume -> ↑venous return · ↓ANP release o ↓Natriuresis -> ↑blood volume -> ↑venous return

Special responses:

Bezold-Jarisch reflex	· Caused by ↓venous return and ↓afterload · Contraction of underfilled LV -> stimulation of unmyelinated PSNS C fibres -> ↑↑PSNS output -> ↓↓HR, ↓BP, coronary vasodilatation
Reverse Bainbridge reflex	· Caused by ↓venous return · ↓RA distension -> ↓activation of venoatrial stretch receptors -> ↑PSNS output and direct effect on AV node -> ↓↓HR

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