Clinical Aspects of Cerebral Perfusion and ICP Flashcards Preview

MBChB - Y2 - Systems - Nervous System > Clinical Aspects of Cerebral Perfusion and ICP > Flashcards

Flashcards in Clinical Aspects of Cerebral Perfusion and ICP Deck (30)
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1
Q

What is normal cerebral blood flow?

A

Averages around 55-60 ml/100g brain tissue per minute

2
Q

What blood rate and flow does ischaemia and parmanent damage usually occur?

A

Ischaemia - 20 ml/100g per minute

Permanent damage - 10 ml/100g per minute

3
Q

What is the largest factor in determining the cerebral blood flow?

A

Cerebral perfusion pressure

4
Q

How do we calculate the cerebral perfusino pressure? (CPP)

A

MAP - ICP

5
Q

What is the result of an increased ICP on cerebral perfusion pressure?

A

Increased ICP causes cerebral perfusino pressure to decrease

6
Q

What are the factors that regulate cerebral blood flow inder physiological conditions?

A

CPP

Concentration of arterial CO2

Arterial PO2

7
Q

What is meant by cerebral autoregulation?

A

The ability to maintain constant blood flow to the brain over a wide range of CPP (50-150 mmHg)

8
Q

How is cerebral autoregulation achieved when there is low CPP?

A

Cerebral arterioles dilate to allow adequate flow at decreased pressure

9
Q

Under what corcumstance would cerebral arterioles constrict?

A

When the CPP is high

10
Q

When does the autoregulatory system fail?

A

When the CPP exceeds 150 mm Hg - such as in the hypertensive crisis

During the first 4-5 days of head trauma

Exudation of fluid from the vascular system with resultant vasogenic oedema

Toxins such as carbon dioxide causing cerebrovascular dilation and inhibit proper vasoregulation

11
Q

What is the pathology associated with cerebral oedema?

A

Prominent cause of subacute to chronic intracranial hypertension

12
Q

Why does cerebral oedema cause increased brain volume?

A

Because of an increase in water content

Increased capillary permeability, the exudate is plasma filtrate containing plasma proteins and is mainly located in the white matter.

Extracellular fluid is increased

13
Q

What are the possible pathogenic lesions that can cause vasogenic oedema?

A

Tumour (primary or metastatic)

Absecess

Infarction (later stages)

Trauma

14
Q

What is the structure of the blood brain barrier?

A

A barrier composed of astrocytic foot process wrapping around a capillary endothelium composed of tight junctions

(Endothelial tight junctions are the barrier to the passive movement of many substances in order to protect the sensitive neural tissue from toxic materials)

15
Q

How do lipid soluble substances cross the endothelial cells?

A

Can penetrate all capillary endothelial cell membranes in a passive manner

16
Q

How do amino acids and sugars pass across the capillary endothelium?

A

By specific carrier mediated mechanisms

17
Q

What are the different compositions within the skull?

A

Brain (70%) and interstitial fluid (10%) - 1400 ml

Blood (10%) - 150 ml

Cerebrospinal fluid (10%) - 150 ml

Total 1700ml

18
Q

What is the monro - kelly doctrine?

A

When a new intracranial mass is introduced, a compensatory change in volume must occur through a reciprocal decrease in venous blood or CSF to keep the total intracranial volume constant

19
Q

What is meant by compliance?

A

Change in volume observed for a given change in pressure

dv/dp

20
Q

What is elastance?

A

The inverse of compliance, change in pressure observed for a chanfe in volume

dp / dv

It represents the accomdation to outward expansion of an intracranial mass

21
Q

How does venous blood get displaced when the intracranial pressure increases?

A

Venous system collapses easily

Squeezes out venous blood through the emissary and scalp veins or through the jugular veins

22
Q

How is CSF displaced?

A

CSF is displaced from the ventricular system through the foramina luschka and magendie into the spinal subarachnoid space

23
Q

What is the effect of an increasing pressure when the compensatory mechanisms have been exhausted?

A

Small changes in volume produce significant increase in pressure

Displacement of CSF and venous blood keeps compliance flat until a critical volume is reached

After this critical volume, small volumetric changes result in precipitous increases in pressurem and intracranial hypertension naturall ensues

24
Q

What are the P1, P2 and P3 waves indicative of?

A

P1 - Percussion wave

P2- Tidal wave

(Dicriotic notch of the trough between p2 and p3)

P3- Dicrotic wave

25
Q

What are the different types of Lundberg waves?

A

A waves - abrupt elevation in ICP for 5 - 20 minutes followed by a rapid fall in the pressure to resting levels

B waves - Frequency of 0.5 to 2 waves per minute, are related to rhythmic variations in breathing

C waves - Rhythmic variations related to waves of systemic blood pressure and have smaller amplitude

26
Q

What does the cushings reflex combat?

A

ICP

27
Q

What are the three main feeatures of cushings reflex?

A

Hypertension, irregular breathing and bradycardia

28
Q

What is the physiological response to raised ICP?

A

Compression of cerebral arteries

Decreased cerebral blood flow and therefore activation of the autonomic nervous system

Sympathetic response: Alpha - 1 adrenergic receptors - results in hypertension and bradycardia

Aortic baro-receptors stimulate vagus nerve - bradycardia

Bradycardia also due to mechanical distortion of the medulla

29
Q

What is management of increased ICP?

A

Head end elevation - facilitates venous return

Mannitol / hypertonic saline (In the case of vasogenic oedema steroids and mannitol are both effective in reducing intracranial pressure)

Hyperventilation: Decrease CBF (temporary measure)

Barbituate coma: decrease cerebral metabolism

Surgical decompression

30
Q

What are the newer concepts in terms of managing intracranial pressure?

A

Brain tissue oxygen monitoring -

  • Probe to monitor oxygenation of tissue
  • Detect and treat low oxygenation, increasing CPP

Micro-dialysis:

  • Investigate brain metabolism
  • Implantation of specially designes catheters
  • To collect small - molecular weight substances to help measure and identify neurotransmitters, peptides and other substances

Decks in MBChB - Y2 - Systems - Nervous System Class (48):