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

Arterial supply

Via circle of willis:

·   2 x internal carotid arteries -> anterior brain

·   2 x vertebral arteries -> basilar artery -> posterior brain and spinal cord

Spinal cord:

·   Anterior spinal artery to anterior 2/3 (origin: branch from each vertebral artery)

·   Two posterior spinal arteries each to posterior 1/6 (each usually from PICA)

·   Segmental reinforcement by thoracic and lumbar arteries

·   Major reinforcement by the artery of Adamkiewicz (usually left T11)

·   Major arteries -> perforating branches

Venous drainage

Similar to supply:

·   Radicular veins

·   Anterior spinal veins x 3, Posterior spinal veins x 3

·   Internal vertebral venous plexus (joining with veins from vertebral bodies)

Principle

·   Spinal cord is continuous with the brain

·   *Similar determinants to cerebral blood flow*

·   Many factors can compromise blood supply

Ohm’s law

Spinal cord blood flow (SCBF) = (mAP – spinal cord pressure (SCP) or CVP) / spinal vascular resistance (SpVR)

Hence factors ↓SCBF:

·   ↓ mAP (e.g. general anaesthesia, bleeding)

·   ↑ SCP (e.g. tumour, abscess, haematoma)

·   ↑ CVP (e.g. IPPV, heart failure)

·   ↑ SpVR (see later)

Monroe-Kellie doctrine

Vertebral column, like brain has fixed walls

Any increase in one substance causes either

a)↓Another substance (compensation)

·   CSF: high capacity, slow rate

·   Blood: low capacity, fast rate

·   Parenchyma: nil

b)↑↑pressure -> ↓blood flow -> ischaemia

Supply design

·   Chicane-like arteries supply the circle of willis

·   Turbulent flow -> ↑pressure drop -> ↓effective arteriolar pressure

·   Prevents massive rise in cerebral perfusion pressure during SNS activation

Poiseuille’s law

Resistance = (8 x length x viscosity) / (π x radius⁴)

Hence factors ↑resistance:

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

·   ↑Length (not under control)

·   ↑Viscosity

Autoregulation

Myogenic autoreg:

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

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

·   Effective for perfusion pressure 50-150mmHg

Metabolic autoreg:

·   Regional blood flow ∝ spinal cord metabolic rate (MR)

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

Physiological variables

·   ↓PaO₂ <50mmHg -> vasodilate (non-linear)-> ↑radius -> ↑SCBF (see graph above)

·   ↑PaCO₂ -> vasodilate (linear 20-80mmHg) (see graph above)

·   ↓Temperature: ↓metabolic rate -> ↓CBF via autoregulation (↓7% per 1C)

General anaesthesia

Sevoflurane:

·   Impairs metabolic autoregulation

·   <1MAC: ↓CMRO₂ predominates -> ↓CBF

·   >1MAC: vasodilatation predominates -> ↑CBF

Propofol/thiopentone:

·   Preserves myogenic autoregulation

·   ↓CMRO₂ -> ↓CBF

Other

Neural:

·   SNS noradrenaline -> α₁ adrenoceptor -> ↓radius

·   PSNS ACh: minimal innervation

Hormonal: adrenaline -> ? α₁ ↓radius; β₂ ↑radius

Rheologic: e.g. ↑Hct -> ↑viscosity -> ↑vascular resistance