Anesthesia Student Survival Guide.pdf - Index of

286 ● AnesthesiA student survivAl Guide
is usually targeted, although in the presence of severe intracranial disease, the
target must be individualized to patient physiology.
Increase in brain mass (tumor, edema, traumatic brain injury), overproduction
of CSF, or obstruction to outflow (e.g., tumor, hemorrhage or clot) or increased
blood volume (¯venous drainage, arterial blood flow) all increase ICP. The
normal physiologic response to increased ICP, in the absence of severe pathology,
is diversion of CSF to the spinal canal.
As ICP continues to increase, mental status decreases, focal neurologic
signs (e.g., dilated pupils, cranial nerve defects) appear, and herniation of brain
contents occurs. Eventually, manifestations of the Cushing’s response (Cushing
Triad = hypertension, bradycardia, irregular respiration) are present due to
brainstem compression. These signs herald a neurosurgical emergency.
Management of ICP/brain volume is a critical part of anesthetic management
for the neurosurgical patient. ICP can be measured by direct catheter insertion
into a CSF-containing space or via a surgically placed subarachnoid bolt.
Interventions to reduce ICP include:
1. Head elevation to 30 degrees
2. Optimization of jugular venous drainage
3. Direct drainage via a lumbar drain or intraventricular catheter
4. Hyperventilation (P a CO 2 25–30 mmHg) to decrease CBF
5. Osmotic diuresis (mannitol, hypertonic saline)
6. Deep intravenous anesthesia (propofol/barbiturate infusion).
Cerebral Blood Flow
Under normal conditions cerebral blood flow is autoregulated in the range of
MAP 50–150 mm Hg. As cerebral metabolic rate increases, blood flow increases
proportionally. Autoregulation is assumed to be disrupted in patients with
chronic hypertension or pathologic conditions including traumatic brain injury
and stroke or by inhaled anesthetic agents. Figure 18.1 shows a relationship
between CBF and arterial O 2 content, CO 2 content, as well as CPP.
Blood Brain Barrier (BBB)
Brain capillaries contain tight-junctions that limit the passive diffusion of many
substances into the brain tissue. The physiology of the BBB facilitates reduction
of brain volume by osmotic agents such as mannitol and hypertonic saline,
which are not freely permeable. Many pathologic states, including trauma,
sepsis, and hemorrhage disrupt the BBB.