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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