3 main components within the cranial vault that can cause ICP?
1. CSF (obstruction, overproduction, decreased clearance) 2. Blood volume (hypotension, high CVP) 3. Brain tissue (edema)
What does cerebral perfusion pressure equal?
CPP = MAP - ICP (or CVP whichever is higher)
What is a normal ICP?
< 10 mmHg
What MAP range is cerebral vascular resistance auto-regulated?
70 mmHg to 150 mmHg, above that, cerebral vascular resistance cannot increase anymore to restrict pressure dependent increase in flow
How does cerebral vascular resistance change with hypercarbia?
CVR and flow increases in a linear fashion until > 80 mmHg where the curve starts to flatten, and gets flat at 120 mmHg
How does cerebral vascular resistance change with hypoxia?
Little effect until PaO2 drops below ~50 mmHg and then rises exponentially; no effect between 60 mmHg and 300 mmHg
How do volatile anesthetics affect cerebral blood flow and cerebral metabolic rate?
VA's "uncouple" flow from consumption in that O2 consumption decreases and CBF (and therefore oxygen delivery) increases
What is the normal cerebral metabolic rate (CMRO2)?
3.5 cc/ 100g/ min
What is the normal cerebral blood flow?
50 cc/ 100g/ min
How much CSF does an average adult have at any one time? How much CSF is produced per day?
~150 cc, 500cc produced per day
What happens in the brain when the ICP is 30?
CPP decreases -> decreased blood flow -> decreased O2 delivery -> ischemia -> cellular break down -> edema -> increased ICP -> decreases CPP more
What does dexamethasone do to CSF production and absorption?
Decreases production, decreases absorption in animal studies
How does mannitol work?
Increases serum osmolality -> fluid shift into intravascular space -> increased UOP
What do you worry about with mannitol and intracranial hematomas?
Decreased brain swelling can cause a hematoma that was being tamponaded by the swollen brain tissue to expand
What happens to cerebral blood flow with 1 MAC of sevoflurane?
Increases about 20%
How does propofol affect cerebral blood flow and cerebral consumption of oxygen?
What happens in reverse steal of the brain?
Normal brain vasculature vasocontricts to stimulus (i.e. propofol) while ischemic brain cannot response -> normal brain is underperfused
How does hypercarbia affect cerebral blood flow?
Changes the H+ concentration
Mechanisms of action for cerebral ischemia with a PaCO2 less than 20?
1. Decrease in cerebral blood flow 2. Left shit in the oxyhemoglobin dissociation curve
Which anesthetic agents decrease cerebral metabolic rate of O2?
Most anesthetic agents except ketamine and N2O
Does hypothermia uncouple CMRO2 and CBF?
No, it just decreases both of them
Is autoregulation affected by high doses of propofol?
No, but propofol does decrease CBF, cerebral blood volume, and ICP
How does thiopental affect CMRO2 and CBF?
Decreases it in a dose dependent manner until 50%
What IV induction agent do you worry about in patients with seizures?
Etomidate (can be epileptogenic in patients with seizure disorders)
Which narcotic can provoke seizures in patients with epilepsy?
Do volatile anesthetics impair autoregulation and if so, which ones more than the others?
Yes in a dose-dependent manner Sevoflurane impairs less than iso or des, halothane impairs it the most
How does nitrous affect CMRO2, CBF, and ICP?
Increases all three of them
What level of consciousness do you see when an EEG shows delta waves? theta waves? alpha waves? beta waves?
Delta: deep sleep Theta: first stage of sleep/drowsiness Alpha: relaxed bu alert Beta: Highly alert
Which volatile anesthetic has epileptiform discharge during induction and at 2 MAC?
Sevoflurane and Enflurane (not clinically significant)
Blood flow to the brain comes what what two main arteries?
Internal carotid (80%) and vertebrobasilar system (20%)