Neurophysiology Flashcards
How long does it take for unconsciousness to develop after cessation of cerebral blood flow?
10 seconds
What are the determinants of cerebral blood flow?
CBF = CPP/CVR
CPP = MAP minus the greater of ICP or CVP
CVR is determined by factors governing cerebral vascular tone including autoregulation, PaCO2, CMRO2, temperature, viscosity, and the presence of vasodilators (including anesthetics)
What is autoregulation?
Autoregulation is the maintenance of CBF over a given range of CPPPs, usually a MAP between 50 - 150mmHg. Above and below, CBF is pressure-dependent.
What factors can impair autoregulation?
HTN, vasodilators (including potent inhalational agents), ischemia or vasomotor paralysis (e.g., around tumors or abscesses).
Which is more important, CBF or CPP?
Both are important but CBF is usually most important because under normal circumstances CBF is maintained by autoregulation over a wide range of CPP.
What are the potential advantages of inhalational agents for neurosurgery?
Ability to decrease CMRO2 and produce anesthesia and amnesia.
What are the potential disadvantages of inhalational agents for neurosurgery?
Action as vasodilators capable of increasing CBF and ICP especially at high concentrations. They can also cause hypotension and dysrhythmias.
What are the potential advantages of IV agents for neurosurgery?
Ability to decrease CMRO2 (like potent inhaled agents). Unlike inhaled agents, they also tend to decreased CBF and ICP (barbiturates, propofol, and etomidate).
What are the potential disadvantages of IV agents for neurosurgery?
tendency to produce hypotension which decreases CPP. Etomidate can cause seizure activity (rare). Ketamine is unique among the IV agents in its tendency to increase CBF and ICP.
Is ketamine contraindicated for neurosurgery?
Not absolutely, but it can increase CBF and ICP leading to bleeding, rupture of vascular structures, or compromise of CPP in patients with elevated ICPs, has been reported to cause seizures in patients with seizure disorders, may also impair arousal and cause postop delirium.
Is etomidate better than ketamine for neurosurgery?
No it just has certain advantages and disadvantages. Its primary advantage is the preservation of hemodynamic stability even with induction doses (because it has more cortical than brainstem effects which also explains why spontaneous respirations tend to be preserved). Its disadvantages are minor but include myoclonus, potential lowering of the seizure threshold, adrenal suppression and pain on injection.
Which narcotic is best for neurosurgery?
There is no “best” but fentanyl has the advantage of avoiding histamine release seen with morphine and meperidine, myocardial depression seen with meperidine, increases in ICP seen with sufentanil in pts with brain tumors or severe hypotension and bradycardia (alfentanil, sufentanil).
Which potent inhalational agent is best for neurosurgery?
Isoflurane offers the theoretical advantage of causing the smallest increase in CBF, causing the greatest decrease in CMRO2, achieving the lowest critical CBF, allowing CO 2 responsiveness to be preserved even after hyperventilation is begun after the agent, decreasing CSF production, increasing CSF resorption. These benefits are theoretical and not borne out with solid evidence of improved outcomes.
Is SCh contraindicated for neurosurgery?
Not if the concern is its potential to increase ICP.
How does SCh cause an increase in ICP?
The increase in ICP is due to activation of gamma motor afferents with fasciculations causing increased metabolic activity of the senosry cortex and concomitant increases in CBF. This is easily overcome by thiopental or hyperventilation and prevented by a defasiculating dose.
