Clinical Aspects of Cerebral Perfusion and ICP

Question 1

what percentage is cerebral blood flow of cardiac output?

Answer 1

15%

Question 2

what is the most significant factor that determines cerebral blood flow at any given time?

Answer 2

cerebral perfusion pressure

Question 3

what is cerebral perfusion pressure?

Answer 3

effective blood pressure gradient across the brain

Question 4

increased ICP causes the cerebral perfusion pressure to ….

Answer 4

decrease

Question 5

what are the factors that regulate cerebral blood flow under physiological conditions

Answer 5

CPP concentration of arterial CO2 arterial PO2

Question 6

what is cerebral auto regulation?

Answer 6

the ability to maintain constant blood flow to the brain over a wide range of CPP

Question 7

what happens to the arterioles when CPP is low?

Answer 7

arterioles dilate to allow adequate flow at decreased pressure

Question 8

what happens to arterioles when CPP is high

Answer 8

the cerebral arterioles constrict

Question 9

can cerebral blood flow always be auto regulated?

Answer 9

no, when CPP exceeds 150mm hg, hypertension, the system fails

Question 10

what is a prominant cause of subacute to chronic intracranial hypertension?

Answer 10

Cerebral edema

Question 11

learn

Answer 11

Question 12

a barrier composed of astrocytic foot process wrapping around a capillary endothelium is composed of what?

Answer 12

tight junctions

Question 13

can all substances that are carried in the blood reach the neural tissue?

Answer 13

no

Question 14

what is the function sof endothelial tight junctions?

Answer 14

barrier to the passive movement of many substances in order to protect the sensitive neural tissue form toxic materials

Question 15

look

Answer 15

Question 16

what are the mechanisms by which materials may be transported naturally across the endothelial cells?

Answer 16

• lipid-soluble substances can usually penetrate all capillary endothelial cell membranes in a passive manner
• amino acids and sugars are transported across the capillary endothelium by specific carrier-mediated mechanisms

Question 17

what happens when a new intracranial mass is introduced

Answer 17

a compensatory change in volume must occur through a reciprocal decrease in venous blood or CSF to keep the total intracranial volume constant

Question 18

what are the homeostatic mechanisms 8-15mmHG

Answer 18

• The venous system collapses easily and squeezes venous blood out through the jugular veins or through the emissary and scalp veins
• CSF can be displaced from the ventricular system through the foramina of Luschka and Magendie into the spinal subarachnoid space
• When these compensatory mechanisms have been exhausted, small changes in volume produce significant increase in pressure
• The innate homeostatic pressure-buffering mechanism offered by 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 pressure, and intracranial hypertension naturally ensues

Question 19

Characteristics of A waves?

Answer 19

• Abrupt elevation in ICP for 5 to 20 minutes followed by a rapid fall in the pressure to resting levels
• The amplitude may reach as high as 50 to 100 mm Hg

Question 20

characteristics of B waves

Answer 20

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

Question 21

characteristics of C waves

Answer 21

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

Question 22

what is cushings reflex?

Answer 22

• Vasopressor response
• Hypertension, irregular breathing and bradycardia
• Increased ICP more than MAP
• Compression of cerebral arterioles
• Decreased CBF, activation autonomic nervous system
• Sympathetic response: alpha-1 adrenergic receptors -> Hypertension and tachycardia
• Aortic baro-receptors stimulate vagus nerve -> Bradycardia
• Bradycardia also due to mechanical distortion of medulla

Question 23

whats the management of increased ICP?

Answer 23

• Head end elevation: facilitate venous return
• Mannitol/ Hypertonic saline
• Hyperventilation: decrease CBF (temporary measure)
• Barbiturate coma: decrease cerebral metabolism, CBF
• Surgical decompression