2016B06 Discuss the determinants and control of spinal cord perfusion.

 

List:

·        Intro

·        Flow dynamics

·        Regular of vascular resistance

·        Anatomical factors

 

Intro:

Physiology

·     Similar to brain tissue

·     Highly active: O2 consumption 3.3mL/min/100g

·     Highly perfused: 58mL/min/100g

Pathology

·      Interruption -> ischaemia -> paralysis

 

Flow dynamics:

Ohm’s law

SBF = mAP – SCP or CVP / SVR

·     SBF: spinal blood flow

·     SCP: spinal cord pressure

·     SVR: spinal vascular resistance

 

Hence factors ↓CBF:

·     ↓ mAP

·     ↑ SCP / ↑ CVP (Starling resistor – whichever is higher)

·     ↑ SVR

Poiseuille’s law

Resistance = (8 x length x viscosity) / (π x radius4) – assuming laminar flow

 

Hence factors ↑resistance:

·     ↓Radius (note power of 4, most important)

·     ↑Length (not under control)

·     ↑Viscosity

 

 

Regulation of vascular resistance:

Autoregulation

Myogenic:

·     Global blood flow constant 58mL/min/100g

·     ↑flow -> ↑stretch -> reflex contraction -> ↓radius -> ↓flow

·     Effective for perfusion pressure 50-150mmHg

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Metabolic:

·     Regional blood flow ∝ metabolic rate (MR)

·     ↓MR -> ↓H+/K+/adenosine/lactate/pCO2 and ↑pO2 -> local vasoconstriction -> ↓radius -> ↓flow

Physiological variables

Oxygen:

·     ↓PaO2 <50mmHg -> vasodilate -> ↑radius -> ↑SBF

·     Non-linear response. Doubles at 30mmHg.

Carbon dioxide:

·     ↑PaCO2 -> vasodilate

·     Linear response 20-80mmHg

Temperature:

·     ↓Temp: ↓MR -> ↓SBF via metabolic autoregulation

·     Near linear response: ↓7% per 1°C

 

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Other

·     Neural: noradrenaline at α1 -> vasoconstriction (minor)

·     Hormonal: adrenaline -> mixed effects at α1, β1 (minor)

·     Rheologic: ↑Hct -> ↑viscosity -> CVR

 

Anatomical factors:

Arterial supply

·      Anterior spinal artery – anterior 2/3 (from vertebral arteries)

·      Posterior spinal artery x 2 – posterior 1/3

·      Segmental reinforcement by thoracic and lumbar arteries

·      Major reinforcement by the artery of Adamkiewicz (usually left T11)
i.e. vulnerable during aortic surgery

Venous drainage

·      Radicular veins

·      Anterior and posterior spinal veins

·      Internal vertebral venous plexus

 

 

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