2016B Question 06
Discuss the determinants and control of spinal cord perfusion.
Examiner Report
51% of candidates achieved a pass in this question.
The question is primarily a physiological oriented one. Most candidates did describe the anatomical aspects of the cord arterial and venous components. Marks were gained for naming and drawing the cord's vascular supply.
Realising that the parameters of perfusion of cord tissue are akin to that of the intra -cerebral perfusion, the considerations needed for this question fall into place for candidates.
Systemic physiological measurements of MAP, CVP, ICP, vascular flow dynamics as described in terms of Pousieulle's law, Monroe- Kellie doctrine all apply and could be logged into the determinants of spinal cord perfusion. Adding the effects of local factors eg ph, pC02and pa02, a complete multifactorial physiological synopsis is outlined. Autoregulation behaves as does the intracerebral circulation.
Some mention of the peri- operative, trauma, medications and space occupying lesions indicated further applied understanding of the significance of the question and attained further credit.
Model Answer
Structure:
- Vascular anatomy: Arterial and venous
- Physiology of spinal cord blood flow
- Control of spinal vascular resistance in particular
Vascular Anatomy
| Vessel Type | Detail |
|---|---|
| Arterial supply | Via Circle of Willis: - 2x Internal Carotid Arteries → Anterior circulation - 2x Vertebral Arteries → Basilar artery → Posterior circulation 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) |
Physiology of Spinal Cord Blood Flow
| Factor | Mechanism |
|---|---|
| Principle | - Spinal cord is continuous with the brain - Similar determinants to cerebral blood flow |
| Ohm’s law | 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 |
Control of Spinal Vascular Resistance
| Factor | Mechanism |
|---|---|
| Poiseuille’s law | 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-1/100g - ↑ 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) - ↓ 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 |
