Clinical Aspects of Cerebral Perfusion and ICP Flashcards
Does grey or white matter get more blood flow?
grey
What is the cerebral blood flow?
- 15% of cardiac output
- Normal cerebral blood flow averages 55 to 60 mL/100 g brain tissue per minute
- Grey matter the blood flow is 75 mL/100 g/ minute
- White matter it is around 45 mL/100 g/ minute
What level of cerebral blood flow does ischaemia occur at and when does permanent damage occur?
- Ischemia at 20 mL/100 g/minute
- Permanent damage usually results when the blood flow drops below 10 mL/100 g/minute
Most significant factor that determines cerebral blood flow at any given time is the ______________________
Most significant factor that determines cerebral blood flow at any given time is the cerebral perfusion pressure (CPP)
What is CPP?
CPP is the effective blood pressure gradient across the brain
CPP = MAP − ICP
Cerebral perfusion pressure (CPP) is the net pressure gradient that drives oxygen delivery to cerebral tissue. It is the difference between the mean arterial pressure (MAP) and the Intracranial Pressure (ICP)
How is MAP calculated?
MAP = DP + 1⁄3PP or 2⁄3DP + 1⁄3SP
(PP = pulse pressure)
__________ ICP causes the cerebral perfusion pressure to ____________
Increased ICP causes the cerebral perfusion pressure to decrease
What are some factors that regulate cerebral blood flow under physiological conditions?
- CPP
- Concentration of arterial CO2
- Arterial PO2
What is cerebral autoregulation?
The ability to maintain constant blood flow to the brain over a wide range of CPP (50-150 mm Hg) is called cerebral autoregulation
What is the range of CPP that cerebral autoregulation occurs at?
range of CPP = 50-150 mm Hg
What happens when CPP is low?
CPP is low, the cerebral arterioles dilate to allow adequate flow at the decreased pressure
What happens when CPP is high?
When CPP is high, the cerebral arterioles constrict
Can cerebral autoregulation fail?
Under certain pathological conditions cerebral blood flow cannot always be autoregulated
CPP exceeds 150 mm Hg, such as in hypertensive crisis, the autoregulatory system fails
What are examples of things that may disturb cerebral autoregulation?
Exudation of fluid from the vascular system with resultant vasogenic edema
Certain toxins such as carbon dioxide can cause diffuse cerebrovascular dilatation and inhibit proper autoregulation
During the first 4 to 5 days of head trauma, many patients can experience a disruption in cerebral autoregulation
Cerebral edema is a prominent cause of subacute to chronic _______________
intracranial hypertension
What is cerebral oedema?
State of increased brain volume as a result of an increase in water content
What are the different types of cerebral oedema?
vasogenic oedema (extracellular oedema)
cytotoxic oedema (intracellular oedema)
interstitial oedema
What is vasogenic oedema (extracellular oedema) due to?
increased capillary permeability
the blood brain barrier (BBB) is disrupte
It is an extracellular oedema which mainly affects the white matter via leakage of fluid from capillaries
What is cytotoxic oedema (intracellular oedema) due to?
cellular swelling
result of cells being unable to maintain ATP-dependant sodium/potassium (Na+/K+) membrane pumps which are responsible for high extracellular and low intracellular Na+ concentration
What is interstitial oedema due to?
increased brain water due to impairment of absorption of CSF
oedema caused by the accumulation of fluid in the extracellular spaces of a tissue
What is the blood brain barrier?
- A barrier composed of astrocytic foot process wrapping around a capillary endothelium composed of tight junctions
- Not all substances that are carried in the blood can reach the neural tissue
- Endothelial tight junctions are the barrier to the passive movement of many substances in order to protect the sensitive neural tissue from toxic materials
What are 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 is the cranium?
The cranium is a rigid structure that contains the brain, blood and CSF
When a new intracranial mass is introduced, a ___________ change in volume must occur through a reciprocal decrease in _________ or ____ to keep the total intracranial volume constant
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
When there is an intracranial mass, what is lost to keep the pressure the same?
Starts off being venous blood then CSF
What is compliance?
- Change in volume observed for a given change in pressure
- dV / dP
What is elastance?
Inverse of compliance
Change in pressure observed for a given change in volume
dP / dV
Does compliance or elastance represent the accommodation to outward expansion of an intracranial mass?
elastance
When there is an intracranial mass, how is venous blood and CSF reduced and removed from the cranium?
- 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 ________, _______ changes in volume produce significant _________ in pressure
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
What are ICP waveforms?
The ICP waveform occurs due to a pulse wave that comes from the arterial pulse wave from the heartbeat on the brain, which essentially floats in cerebrospinal fluid (in meninges).
What are lundberg waves?
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
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
What is cushing reflex (Vasopressor response)?
Increased ICP more than MAP leads to compression of cerebral arterioles so there is decreased cerebral blood flow, causing 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
Sympathetic trying to increase blood pressure to get some blood to the brain but the vagus nerve is activated slowing the heart
Very high blood pressure with a slow heart rate is cushing effects and happens when there is catastrophic head injuries
WHat does cushing reflex lead to?
Hypertension, irregular breathing and bradycardia
What locations can brain herniation occur?
What is 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
New cocepts in management - What is brain tissue oxygenation monitoring?
- Probe to monitor oxygenation of tissue
- Detect and treat low oxygenation, increasing CPP
New cocepts in management - What is micro-dialysis?
- Investigate brain metabolism
- Implantation of specially designed catheters
- To collect small-molecular-weight substances to help measure and identify neurotransmitters, peptides, and other substances
Autoregulation – pressure changes but brain maintains _________
perfusion
