Exam 4: Neurocritical Flashcards
describe the Monroe-Kelly Doctrine
o Skull is a rigid compartment, contains three structures (1450mL)
* Brain tissue (1300 mL)
* Arterial/venous blood (110mL)
* Cerebrospinal fluid (65mL)
o Normal: Static volumes in balanced state of equilibrium
* Blood and CSF can shift to compensate
o Abnormal: elevations in relative volume of one compartment without compensation = elevated ICP
* There is some injury to the brain that causes one of the three compartments to rise in volume, that causes an increase in ICP
what are causes of elevated ICP
o Increase in volume
o Increase in brain tissue
o Increase in blood
Causes of elevated ICP: increase in volume
- Disordered CSF Hydrodynamics
o CSF overproduction
o Impaired absorption, unable to drain CSF
Psuedotumor cerebri, elevated CSF protein/blood, EVD or VPS malfunction
o Increased Dural pressure (cerebral venous sinus thrombosis)
causes of elevated ICP: increase in brain tissue (3)
- Hydrocephalus
o ICH, SAH, IVH, tumor, meningitis
o Compression of the ventricle from edema, tumor, hemorrhage - Cerebral edema
o Osmotic, vasogenic, cytotoxic - Mass lesions
o Tumor (primary or metastatic)
o Abscess
o Hematoma
causes of elevated ICP: increase in blood
- Vasodilation from;
o Increase in CO2 levels
o Medications that are given
define brain compliance
- Compliance: ability of cranial content to tolerate changes in volume
o Brain can compensate for a period of time to maintain ICP
o Small change in volume significantly increases ICP - -> Leads to loss of compliance
o also is a factor in a patient course of tx
what is normal ICP
o Normal ICP 0-10 mmHg (or 5-15 mmHg)
* Generally, interventions are >20 (abnormal)
balance of intracranial pressure depends on:
- Balance is critical for normal ICP
o Balance depends on-
* Volume of the cerebral structures
o Brain tissue, CSF, blood
- Concept of autoregulation
o Via the cerebral blood vessels
o Vasodilation/vasoconstriction “protective mechanism”
[Example- Vessels in brain will dilate if you are hemorrhaging from a trauma, and during HTN crisis the vessels will constrict to decrease the volume in the brain] - Cerebral perfusion pressure (CPP)
o CPP= MAP-ICP
This is how much blood flow is the brain actually getting
Normal CPP in normal autoregulator range is 50-150mmHg
CPP can be reduced by increase in ICP - [this is why we want a MAP to be high during an elevated ICP, in order to perfuse the brain]
what is CPP?
(cerebral perfusion pressure)
- it is the balance of pressures
(e.g. when someone has an injured brain, we need MAP to perfuse the brain tissue
what are the blood vessels inside the brain going to do if blood flow is dropping?
vasodilate
what happens to blood vessels when you’re having really high pressure?
vasoconstrict d/c if they do not then it’ll be dumping at high pressure blood into the brain tissue= bad
what values defined as intracranial hypertension
> 15-20 mmHg
define stage I of intracranial hypertension
- Vasoconstriction and external compression of the venous system occur in an attempt to further decrease the intracranial pressure
- ICP may not change because of the effective compensatory mechanisms and there may be few symptoms
- Small increases in volume cause an increase in pressure, and the pressure may take longer to return to baseline
define stage II of intracranial hypertension and what are the clinical manifestions?
- The resulting increase in ICP may exceed the brain’s compensatory capacity to adjust
- The pressure begins to compromise neuronal oxygenation, and systemic arterial vasoconstriction occurs in an attempt to elevate the systemic blood pressure sufficiently to overcome the ICP
- Clinical manifestations at this stage-
o usually subtle and transient, including episodes of confusion, restlessness, and drowsiness, and slight pupillary and breathing changes
define stage III of intracranial hypertension and what are the clinical manifestations
- The ICP begins to approach arterial pressure, the brain tissue begins to experience hypoxia and hypercapnia, and the individuals’ condition rapidly deteriorates
- Clinical manifestations include-
o decreasing levels of arousal or central neurogenic hyperventilation, widened pulse pressure, bradycardia, and pupil’s that become small and sluggish - Autoregulation no longer working
define stage IV intracranial hypertension
- Brian tissue shifts (herniates) from the compartment of greater pressure to a compartment of lower pressure
Explain consequences of impaired autoregulation: intracranial hypotension
o Major cause is loss of CSF volume
o Effects on decrease of CSF volume-
* Sagging of cerebral tissue away from the skull
* Pulling and breaking of bridging veins and dura
* Which leads to Hemorrhage
Explain consequences of impaired autoregulation: intracranial hypertension
o Sustained values >15-20 mmHg
* ICP >20 warrants treatment
* ICP >40 is life threatening
Two factors that MATTER and are important to determine severity of ICP
- Onset (timing)
o Slow growing lesion versus acute cause of elevated ICP
Acute vs chronic, such as a tumor there for years which slowly increased the ICP. - Location
o ICP levels vary according to compartment of the brain and may not always be reflected by neuromonitoring
o Acute lesion in the temporal area may cause herniation at ICP levels 15-20 mmHg
[for example- increase in volume, 100ml of blood, closer to the surface of the brain will have a different exam pictured vs if it were found in the base or deeper of the brain]
what happens to autoregulation with increased ICP
- Autoregulation the compensatory alteration in the diameter of the intracranial blood vessels designed to maintain a constant blood flow during changes in cerebral perfusion pressure, is lost with progressively increased ICP
- Increase in ICP can break down–>
o Autoregulation system and blood brain barrier
Autoregulation vs Lack of autoregulation: Normal
- Cerebral blood vessels vasodilate and/or constrict to maintain constant blood flow to cerebral tissue
Autoregulation vs Lack of autoregulation: Semi-normal (danger zone)
- Acute increase in one structure volume may be compensated by a decrease in another tissue (CSF shifting or decreased blood flow)
Autoregulation vs Lack of autoregulation: Abnormal
- Either end of the autoregulatory curve
- Neurologic injury
o Impaired or loss of cerebral autoregulation - Vasodilatory cascade
o Increased ICP leads to decreased CPP leads to vasodilation to address decreased CPP leads to increased ICP
o Increased CCP results in vasoconstriction and may reduce ICP
what is cerebral edema?
- an increase in the fluid content of brain tissue
- it is key cause of increased brain tissue volume
what is the result of cerebral edema?
increased extracellular or intracellular tissue volume
when does cerebral edema occur?
o occurs after brain insult from trauma, infection, toxicity, hemorrhage, tumor, ischemia, infarction, or hypoxia.
what are the harmful effects of cerebral edema caused by?
caused by the distortion of blood vessels, the displacement of brain tissues, increase in ICP, and the eventual herniation of brain tissue from one brain compartment to another.
what are the types of cerebral edema?
- osmotic
- vasogenic
- cytotoxic
cerebral edema: Osmotic (interstitial)
due to hypo-osmolar state
* Change in oncotic pull
o i.e., Acute hyponatremia or Rapid hemodialysis
* [abnormal shifts in substrates in blood and fluid shifting to brain tissue]
*have fluid shifting into brain tissue b/c there’s not enough substrates in the blood to keep the osmotic pull inside the blood vessels
cerebral edema: vasogenic
due to altered capillary permeability
* Allows the fluid to leak into the brain and manifests as swelling
* Disruption in the brain-blood barrier
* Plasma proteins leak into the extracellular spaces, drawing water to them, and the water content of the brain parenchyma increases
o i.e., Hypertensive encephalopathy or Tumor-related edema
* [this is happening at the capillary level causing abnormal leakage of capillaries, such as a brain tumor- the edema around the tumor- the edema causes the symptoms, not the tumor itself]
o Clinical manifestations-
Focal neurologic deficits, disturbances of consciousness, and a severe increase in intracranial pressure