Advanced Trauma Life Support ATLS Student Course Manual 2018

108
CHAPTER 6 n Head Trauma
n FIGURE 6-6 The Monro–Kellie Doctrine Regarding Intracranial Compensation for Expanding Mass. The total volume of the intracranial
contents remains constant. If the addition of a mass such as a hematoma compresses an equal volume of CSF and venous blood, ICP remains
normal. However, when this compensatory mechanism is exhausted, ICP increases exponentially for even a small additional increase in
hematoma volume. (Adapted with permission from Narayan RK: Head Injury. In: Grossman RG, Hamilton WJ eds., Principles of Neurosurgery.
New York, NY: Raven Press, 1991.)
remains below normal for days or weeks after injury.
There is increasing evidence that low levels of CBF do
not meet the metabolic demands of the brain early
after injury. Regional, even global, cerebral ischemia
is common after severe head injury for both known
and undetermined reasons.
The precapillary cerebral vasculature typically
can reflexively constrict or dilate in response to
changes in mean arterial blood pressure (MAP). For
clinical purposes, cerebral perfusion pressure (CPP)
is defined as mean arterial blood pressure minus
intracranial pressure (CPP = MAP – ICP). A MAP of
50 to 150 mm Hg is “autoregulated” to maintain a
constant CBF (pressure autoregulation). Severe TBI
can disrupt pressure autoregulation to the point
that the brain cannot adequately compensate for
changes in CPP. In this situation, if the MAP is too
low, ischemia and infarction result. If the MAP
is too high, marked brain swelling occurs with
elevated ICP.
Cerebral blood vessels also constrict or dilate in
response to changes in the partial pressure of oxygen
(PaO 2
) and the partial pressure of carbon dioxide
(PaCO2) in the blood (chemical regulation). Therefore,
secondary injury can occur from hypotension, hypoxia,
hypercapnia, and iatrogenic hypocapnia.
Make every effort to enhance cerebral perfusion and
blood flow by reducing elevated ICP, maintaining
normal intravascular volume and MAP, and restoring
normal oxygenation and ventilation. Hematomas
and other lesions that increase intracranial
volume should be evacuated early. Maintaining
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