Clinical Aspects of Cerebral Perfusion and ICP Flashcards
what percentage is cerebral blood flow of cardiac output?
15%
what is the most significant factor that determines cerebral blood flow at any given time?
cerebral perfusion pressure
what is cerebral perfusion pressure?
effective blood pressure gradient across the brain
increased ICP causes the cerebral perfusion pressure to ….
decrease
what are the factors that regulate cerebral blood flow under physiological conditions
CPP concentration of arterial CO2 arterial PO2
what is cerebral auto regulation?
the ability to maintain constant blood flow to the brain over a wide range of CPP
what happens to the arterioles when CPP is low?
arterioles dilate to allow adequate flow at decreased pressure
what happens to arterioles when CPP is high
the cerebral arterioles constrict
can cerebral blood flow always be auto regulated?
no, when CPP exceeds 150mm hg, hypertension, the system fails
what is a prominant cause of subacute to chronic intracranial hypertension?
Cerebral edema
learn
a barrier composed of astrocytic foot process wrapping around a capillary endothelium is composed of what?
tight junctions
can all substances that are carried in the blood reach the neural tissue?
no
what is the function sof endothelial tight junctions?
barrier to the passive movement of many substances in order to protect the sensitive neural tissue form toxic materials
look
what are the mechanisms by which materials may be transported naturally across the endothelial cells?
- 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
what happens 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
what are the homeostatic mechanisms 8-15mmHG
- 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
Characteristics of A waves?
- 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
characteristics of B waves
•Frequency of 0.5 to 2 waves per minute, are related to rhythmic variations in breathing
characteristics of C waves
•Rhythmic variations related to waves of systemic blood pressure and have smaller amplitude
what is cushings reflex?
- 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
whats the management of increased ICP?
- Head end elevation: facilitate venous return
- Mannitol/ Hypertonic saline
- Hyperventilation: decrease CBF (temporary measure)
- Barbiturate coma: decrease cerebral metabolism, CBF
- Surgical decompression
