Neuroanesthesia and Neuromonitoring Flashcards

A 58-year-old male is undergoing surgical clipping of a ruptured anterior communicating artery aneurysm. Intraoperatively, the surgeons announce sudden, brisk bleeding from the aneurysm dome. Which of the following is the most appropriate immediate anesthetic management step?

Answer: Induce profound hypotension (MAP 40-50 mmHg) transiently.

In the event of an intraoperative aneurysm rupture, the immediate goals are to facilitate surgical control of the bleeding and maintain adequate cerebral perfusion. Transiently inducing profound hypotension (Mean Arterial Pressure 40-50 mmHg) reduces the pressure head against which the surgeon is working, decreases blood loss, and can allow for proximal control and clipping of the aneurysm. Norepinephrine would worsen bleeding. While mannitol or hypertonic saline are used to reduce brain bulk, they are not the primary immediate intervention for controlling active, brisk hemorrhage.

Which of the following statements most accurately describes the effect of propofol on cerebral physiology?

Answer: It decreases both CBF and CMRO2 while preserving the coupling between them.

Propofol is a potent cerebral vasoconstrictor that causes a dose-dependent reduction in both cerebral blood flow (CBF) and the cerebral metabolic rate of oxygen (CMRO2). Crucially, the coupling between cerebral metabolism and blood flow is maintained, meaning that the reduction in blood flow is matched to the reduced metabolic demand, which is a desirable property in neuroanesthesia. Cerebral autoregulation is generally preserved at clinical doses.

A patient is undergoing spinal surgery with monitoring of Somatosensory Evoked Potentials (SSEPs). Which anesthetic agent, when used as a primary agent, would cause the most significant dose-dependent decrease in SSEP amplitude and increase in latency?

Answer: Isoflurane

Volatile anesthetic agents like isoflurane, sevoflurane, and desflurane cause a significant dose-dependent decrease in the amplitude and increase in the latency of cortical SSEPs. High concentrations can obliterate the signals entirely. Propofol and opioids have less of an effect. Ketamine is unique in that it may actually enhance SSEP amplitude, making it a favorable agent when monitoring is crucial.

During transcranial Doppler (TCD) monitoring of a patient 7 days after a subarachnoid hemorrhage, the mean flow velocity in the middle cerebral artery (MCA) is measured at 180 cm/s. The mean flow velocity in the ipsilateral extracranial internal carotid artery (ICA) is 50 cm/s. What is the most likely interpretation of these findings?

Answer: Severe cerebral vasospasm

These findings are indicative of cerebral vasospasm. The high MCA velocity (>120 cm/s is suggestive, >160cm/s is moderate, and >200cm/s is severe) points to arterial narrowing. To differentiate vasospasm from hyperemia (which also increases velocity), the Lindegaard ratio (MCA velocity / ICA velocity) is calculated. In this case, 180/50 = 3.6. A Lindegaard ratio greater than 3 is consistent with vasospasm, and a ratio between 3 and 6 suggests mild-to-moderate vasospasm. Hyperemia would have a ratio less than 3.

Which of the following is a recognized limitation of Bispectral Index (BIS) monitoring in neuroanesthesia?

Answer: BIS values can be paradoxically elevated by ketamine.

Ketamine produces a dissociative state and can cause prominent high-frequency EEG activity, leading to a BIS value that is higher than the actual level of hypnosis would suggest. BIS is not a reliable measure of sedation with ketamine. Conversely, BIS is not reliably affected by nitrous oxide or opioids alone. A burst suppression pattern indicates deep anesthesia and corresponds to very low BIS values, not elevated ones.

A patient with a traumatic brain injury has a sustained intracranial pressure (ICP) of 25 mmHg. A bolus of mannitol is administered. What is the primary, initial mechanism by which mannitol reduces ICP?

Answer: Creating an osmotic gradient across the blood-brain barrier, drawing water from brain parenchyma.

The principal and most immediate mechanism of mannitol's effect on ICP is the creation of an osmotic gradient between the plasma and the brain tissue. As an osmotically active agent that does not readily cross the intact blood-brain barrier, it draws free water from the brain's interstitium into the vasculature, thereby reducing brain volume and lowering ICP. While osmotic diuresis and a decrease in CSF production do occur, they are secondary or slower mechanisms.