2019B Question 02

Describe the normal regulation of cerebral blood flow and outline the physiological factors which may alter it. DO NOT discuss the effect of medications or pathology.

Examiner Report

37.4% of candidates achieved a pass in this question.

The major domains assessed in this question were:

  • The normal values for cerebral blood flow and the importance of consistency of supply
  • Local autoregulatory mechanisms
  • Physiological factors altering flow

Credit was given for other correct relevant material.

Notably this question focused on normal regulation, and physiology – and specifically asked candidates not to discuss medications or pathology. Candidates who digressed into factors which impact intracranial pressure were commonly not able to explore the breadth and depth of the other material considered central to this question.

Model Answer

Structure:

  • Introduction
  • Cerebral blood flow dynamics
  • Regulation of cerebral vascular resistance
  • Regulation of arterial pressure
  • Anatomical factors

Introduction

The brain is:

  • Highly metabolically active
    • CMRO2 46mL.min-1 = 3.3mL.min-1/100g
  • Well perfused
    • CBF 15% cardiac output = 750mL.min-1 = 58mL.min-1/100g
  • Minimal anaerobic capacity
    • Interruption → Loss of consciousness, head injury, ischaemia

Cerebral Blood Flow Dynamics

Laws Detail
Ohm’s law

Cerebral blood flow (CBF) = (MAPICP or CVP) / cerebral vascular resistance

- Starling resistor: Whichever of ICP or CVP is higher

Hence factors ↓ CBF:

- ↓ MAP

- ↑ ICP

- ↑ CVP

- ↑ CVR

Poiseuille’s law

– assuming laminar flow

Hence factors ↑ resistance:

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

- ↑ Length (not under control)

- ↑ Viscosity

Regulation of Cerebral Vascular Resistance

Factor Details
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 → ↑ CBF (see graph above)

- ↑ PaCO2 → Vasodilate (linear 20-80mmHg) (see graph above)

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

Other

- Neural (insignificant)

 - SNS noradrenaline → Α1 adrenoceptor → ↓ Radius

 - PSNS ACh: Minimal innervation

- Hormonal: (insignificant)

 - Adrenaline at α1: ↓ radius

 - Adrenaline at β2: ↑ radius

- Rheologic: e.g. ↑ Hct → ↑ Viscosity → ↑ Vascular resistance

Regulation of Arterial Pressure

Factor Details
Baroreceptor Response

- Stretch-activated mechanoreceptors in walls of aortic arch and carotid sinuses

- ↓ MAP → ↓ Stretch → ↓ Activation → ↓ Inhibition of SNS →

 - Vasoconstriction → ↑ SVR

 - Venoconstriction → ↑ Preload

 - ↑ HR, ↑ contractility

→ ↑ MAP

- Important for maintaining CBF during posture change

Central Ischaemic Response

- ↓↓ CBF → Brainstem ischaemia → ↑↑ SNS activity

 - ↑ BP

 - ↓ HR (reflex)

Anatomical Factors

Factor Details
Arterial supply

Circle of Willis:

- 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

Venous drainage

*Dural venous sinuses:*

- No valves: Allows equilibration of venous pressure

- Elastic and distensible: Minimizes resistance to flow


Last updated 2021-08-23

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