4.6 Cerebral Circulation

Question 1

During which surgeries is cerebral circulation likely to be affected?

Answer 1

1. • Surgeries performed in the head-up position;
     e.g. posterior fossa craniectomy,
     cervical laminectomy,
     sometimes thyroid operations
2. • Beach chair position;
     e.g. shoulder surgery
3. • Trendelenburg position;
     e.g. in laparoscopic colorectal surgery,
     gynaecological operations
4. • Surgery needing cardiopulmonary bypass,
     as aortic cannulation can lead
     to cerebral embolism

Question 2

Describe the arterial supply.

Answer 2

This can be divided into the anterior and posterior

cerebral circulations that are connected

via the anterior and posterior communicating arteries

forming the Circle of Willis.

Two thirds of the cerebral arterial supply is via the internal
carotid arteries and one third via the vertebral arteries.

See Figure 4.2

• The anterior cerebral artery supplies the medial portion of the frontal lobe
  and the superior medial parietal lobe
• The middle cerebral artery supplies the lateral cerebral cortex. It also
  supplies the anterior temporal lobe and the insular cortices
• The posterior cerebral artery supplies the occipital lobe and medial side
  of temporal lobe

Question 3

Draw the Circle of willis

Answer 3

Question 4

What does each artery supply

Answer 4

1. • The anterior cerebral artery
     supplies the medial portion of the frontal lobe
     and the superior medial parietal lobe
2. • The middle cerebral artery
     supplies the lateral cerebral cortex.
     It also supplies the anterior temporal lobe
     and the insular cortices
3. • The posterior cerebral artery
     supplies the occipital lobe and
     medial side of temporal lobe

Question 5

What factors affect cerebral blood flow?

Answer 5

1. • Arterial pCO2:
     Hypercapnia increases blood flow
     hypocapnia decreases it
2. • Arterial pO2:
     Does not affect it until the po2 reaches 6.7 kPa.
     The cerebral blood flow increases below this
3. • Cerebral metabolic rate of oxygen (CMRO2):
     There is a linear correlation
     between cerebral blood flow and CMRo2
4. • Cerebral perfusion pressure:
     Autoregulation occurs
     between a MAP of 60 and 160 mmHg.

The mean arterial pressure at which autoregulation
occurs in hypertensive population is in a higher range
and is impaired in pathology (e.g. traumatic brain injury)

5. • Drugs used in anaesthesia:
     Intravenous induction agents except ketamine
     decrease cerebral blood flow.

Inhalational anaesthetic agents and nitrous
oxide increase cerebral blood flow as they cause vasodilatation.

Opiates cause very little change in blood flow

6. • Temperature:
     Decrease in temperature decreases cerebral blood flow

Question 6

Factors affecting CBF

Answer 6

Question 7

What is the mechanism of cerebral autoregulation?

Answer 7

The autoregulatory vessel caliber changes are mediated by interplay
between myogenic, neurogenic, and metabolic mechanisms.

1. • Metabolic control:
     balance between demand and supply of oxygen
     (i.e. between cerebral metabolism and oxygen delivery

mediated by vasoactive substances such as No, H+, etc.)

2. • Myogenic control:
     Sensing mechanisms in smooth muscle of the
     arterioles detect changes in transmural pressure.

The calibers of vessels
are changed to maintain blood flow

3. • Neurogenic control:

The vascular smooth muscle resistance is controlled
via autonomic innervations

Question 8

What is the management of raised intracranial pressure (icP)?

Answer 8

Raised ICP may be due to an increase
in any compartment of the brain
(i.e. blood, brain tissue, or CSF).

The treatment of raised ICP involves
the reduction in any of them and
may be achieved in the following ways:

1. Blood
2. Brain Tissue
3. CSF

Question 9

Blood

Answer 9

The cerebral blood flow
can be decreased by

1. hyperventilation thus a decrease in PaCo2.

Attention should be paid to not affect venous
drainage by

2. nursing the patient in a head-up position and
3. avoiding compressing the jugular veins
   by not tying endotracheal tubes too tight.
4. Coughing should be avoided,
   adequate muscle relaxation and sedation must be used
5. • Therapeutic hypothermia to decrease CBF
6. • Use of barbiturate infusions
     (e.g. thiopentone to decrease CBF and CMRO2)

Question 10

Brain tissue

Answer 10

1. • Osmotic diuretics such as
     mannitol and hypertonic saline draw out water
     from the extracellular and intracellular spaces
2. • Surgery (e.g. frontal lobectomies and removal of tumours)

Question 11

CSF

Answer 11

• Drainage via shunts and catheters