Cerebral Physiology and Head Injuries

Question 1

What is the normal cerebral blood flow?

Answer 1

12-15% of cardiac output
50ml/100g of brain tissue
~170ml/min

Autoregulation (via myogenic mechanism and vasodilator washout) allows cerebral blood flow to be maintained within a range of MAPs

Question 2

What is the cerebral perfusion pressure?

Answer 2

The pressure gradient that drives cerebral blood flow (CPP = MAP - ICP)

This must remain above 70mmHg (normally 70-90mmHg)

Question 3

What is a normal ICP?

Answer 3

5-15mmHg

Question 4

What is the Monroe-Kelly doctrine?

Answer 4

It describes the cranium as a fixed box with a fixed volume.

To prevent a rise in the pressure within this fixed box, the volume of its contents (brain tissue - 80%, CSF - 10%, blood - 10%) must remain constant.

If either of these components change, there needs to be a compensation to prevent ICP from changing.

Question 5

What is the normal volume of CSF?

Answer 5

150ml

(500ml is produced daily)

Question 6

What does CSF contain?

Answer 6

Glucose
Protein (in higher amounts than in plasma)
Electrolyes (Na, K, Cl, Ca, Urea)

Question 7

How to classify head injury?

Answer 7

Minor (GCS 13-15)

Moderate (GCS 9-12)

Severe (GCS < 8)

Question 8

How to classify brain injury?

Answer 8

Primary - occurs at the time of injury e.g. direct cortex injury, contusions

Secondary - occurs after the injury e.g. due to hypoxia, hypercapneoa, raised ICP and hypotension

Question 9

What causes a raised ICP?

Answer 9

Problems with CSF:
Hydrocephalus - communicating or non-communicating

Problems with blood:
Traumatic or spontaneous

Problems with brain tissue:
Oedema
Abscess
Tumour

Question 10

Which cranial nerves are most vulnerable from brain injury when there is mass effect?

Answer 10

Oculomotor
Abducens

Question 11

How to manage traumatic brain injury?

Answer 11

Follow the ATLS protocol and guidance to stabilise and resuscitate the patient, do a primary survey, identify life-threatening injuries, and prevent secondary injuries.

Perform imaging and discuss with neurosurgery for intervention.

Question 12

What is the target PCO2 in traumatic brain injuries?

Answer 12

<4.5 (to reduce hypercapnoea-driven vasodilation)

Question 13

What medical interventions may be performed to reduce cerebral oedema?

Answer 13

Mannitol
Sodium control