How much blood flow do white and grey matter receive per minute? What level of blood flow do ischaemia and permanent damage usually occur at?
- Grey matter: 75mL/100g/minute
- White matter: 45Ml/100g/minute
- Ischaemia: 20mL/100g/minute
- Permanent damage: 10mL/100g/minute
(brain = 15% of CO)
What are the most significant factors determining cerebral perfusion pressure? What is it determined by?
- CPP: cerebral perfusion pressure
- CPP = MAP - ICP
What causes cerebral perfusion pressure to decrease?
Increased intracranial pressure (ICP)
Factors that regulate cerebral blood flow under physiological conditions?
- CPP
- Concentration of arterial CO2
- Arterial PO2
What is cerebral autoregulation?
- The ability to maintain adequate blood flow to the brain across a large range of CPP (50-150 mmHg). Done via constriction / dilation of arterioles
- CPP = MAP - ICP
What is vasogenic oedema?
Oedema in the cerebrum due to disruption of the blood brain barrier (can be caused by too high of CPP)
What is cerebral oedema often a cause of?
Intracranial hypertension
ICP is higher so MAP has to increase to maintain CPP?
What forms the basis of the blood brain barrier? (BBB)
- Tight junctions between the endothelium of cerebral arterioles
- Astrocytic foot processes surround brain capillaries and induce the formation of these tight junctions during development
What materials are usually naturally transported across the endothelial cells of the BBB?
- Lipid soluble substances (passive)
- Amino acids and sugars via carrier mediated transport
What is the Monro-Kelly Doctrine? (cerebral volume)
when a new intracranial mass is introduced, there must be an accompanying decrease in CSF or venous blood volume to keep ICP constant
What is compliance?
- Change in volume for a given change in pressure
- dV / dP
What is elastance?
- Change in pressure observed for a given change in volume
- dP / dV
Represents the accommodation to outward expansion of an intracranial mass
What are the mechanisms for maintaining homeostasis of ICP? Over what sort of pressure range are they effective?
- Venous collapse: squeezes blood out via jugular & emissary / scalp veins
- CSF displacement: out through foramina of Luschka and Magendie into spinal subarachnoid space
- Mechanisms work over pressure changes of about 8-15mmHg
Once the innate homeostatic mechanisms regulating ICP are exhausted what happens?
- Once a “critical volume” is reached, additional small changes in volume produce significant changes in pressure
- ICP rises and intracranial hypertension ensues
What is an extraventricular drain (EVD) used for?
- Treatment of hydrocephalus and elevated ICP
- Monitoring intracranial pressure
What is Cushing’s Reflex?
- AKA Vasopressor response
- It is the nervous response to raised ICP, consisting of hypertension, irregular breathing and bradycardia
- Often seen at the terminal stages of brain trauma, may indicate brain herniation
Explain the mechanism of the Cushing Reflex
- ICP > MAP, cerebral arteries compressed = ischaemia
- Sympathetic response: alpha-1 adrenergic compresses arterioles and induces tachycardia
- Baroreceptors detect increased MAP & induce bradycardia via vagus nerve
- Herniation / distortion of the medulla can result in irregular breathing
Function of the Cushing Reflex?
It is a last ditch effort to maintain adequate CPP in the brain
What are some different ways to manage increased ICP?
- Barbituate coma: decreases cerebral metabolism
- Surgical decompression (EVD)