Anoxia
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Definition:
There are a number of terms that are used to describe this type of brain
injury. In the clinical and research literature the total deprivation of oxygen
is called anoxia. Diminished, but not total lack of oxygen to the brain is
termed hypoxia. Where there is evidence of a combination of reduced blood flow
(i.e. ischemia) and oxygen, the term hypoxic-ischemic insult is often used.
Hypoxic-ischemic encephalopathy (HIE) is a term often used by neonatologists and
pediatricians to describe a condition that is seen in both full term and low
birth weight infants who often have a variety of medical and neurological
problems associated with pre, post or perinatal events(please see section
following on "Anoxic Injury in the Newborn"). In this section the term anoxia
will be used to describe either a condition of reduced or total deprivation of
oxygen to the brain.
Normal brain metabolism requires both oxygen and
glucose. Oxygen metabolizes glucose which in turn, provides energy for living
cells. The brain represents approximately 2% of the body's total weight yet
accounts for 20% of the body's total oxygen utilization and 65% of it's glucose
needs (Lundgren,
1986). Anoxia damages nerve cells in the cortex where cells originate (as
compared to the damage to the axons and white matter fiber pathways that are
affected in TBI). Loss of consciousness occurs within 10-15 seconds of
cardio-pulmonary arrest. Irreversible brain damage can occur within 5 minutes.
Different brain structures have different metabolic needs. The gray matter of
the brain, particularly the frontal lobes have the highest metabolic
requirements. The metabolic rates of the occipital, parietal, and temporal lobes
along with the basal ganglia and cerebellum are lower. The brainstem has the
lowest metabolic need. An example of how diminished metabolism in the frontal
lobes impacts an individual: consider the published reports of experienced high
altitude mountain climbers who make uncharacteristic errors in judgment (part of
the role of frontal lobe functioning). Errors in judgement and problem solving
are common symptoms of mild anoxic events. When there is a severe lack of oxygen
to the brainstem function, regulation of breathing, heart rate, etc may stay
relatively preserved (brainstem has the lowest metabolic requirements) but
cortical, basal ganglia and cerebellum functions may be severely
compromised. A discussion of the neurological and general physiological
mechanisms of anoxia is beyond the scope of this discussion. Please see Perkin and Ashwal
(1999) for a detailed description of hypoxic-ischemic brain damage in
infants and older children.