Anoxia is a condition characterized by an absence of oxygen supply
to an organ or a tissue.
Description
Anoxia results when oxygen is not being delivered to a part of the
body. If the condition does not involve total oxygen deprivation,
it is often called hypoxia, although the two terms have been used
interchangeably. A related condition, anoxemia, occurs when the
blood circulates but contains a below normal amount of oxygen.
The five types of anoxia or hypoxia include hypoxemic, anemic,
affinity, stagnant, and histotoxic. Hypoxemic anoxia happens when
the oxygen pressure outside the body is so low that the
hemoglobin, the chemical which carries oxygen in the red blood
cells (RBCs), is unable to become fully loaded with the gas. This
results in too little oxygen reaching the tissues and can occur in
suffocation when a person is at high altitude, where the pressure
of oxygen in the air is much less than at sea level.
Anemic anoxia results from a decrease in the amount of
hemoglobin or RBCs in the blood, which reduces the ability to get
oxygen to the tissues. Anemia may result from lack of production
of red blood cells (iron deficiency), blood loss (hemorrhage), or
shortened lifespan of red blood cells (autoimmune disease).
Affinity anoxia involves a defect in the chemistry of the
blood such that the hemoglobin can no longer pick up as much
oxygen from the air, even though the quantities are normal,
reducing how much is delivered to the tissues.
Stagnant anoxia occurs when there is interference with the
blood flow, although the blood and its oxygen-carrying abilities
are normal. A common cause of general stagnant anoxia is heart
disease or interference with the return of blood flow through the
veins. Examples of local stagnant anoxia include exposure to cold,
diseases that restrict circulation to the extremities, and ergot
poisoning. When the tissue or organ itself has a reduced ability
to accept and use the oxygen, it is called histotoxic anoxia. The
classic example is cyanide poisoning, where the chemical
inactivates a cellular enzyme necessary for the cell to use
oxygen. Thus, tissue exposed to cyanide cannot use the oxygen even
though it is in normal amounts in the bloodstream. Histotoxic
anoxia can also be caused by exposure to narcotics, alcohol,
formaldehyde, acetone, toluene, and certain anesthetic agents.
Causes and symptoms
Anoxia and hypoxia can be caused by any number of disease states
of the blood, lungs, heart and circulation including heart attack,
severe asthma, or emphysema. It can also result from smoke or
carbon monoxide inhalation, improper exposure to anesthesia,
poisoning, strangulation, near-drowning, or high altitude exposure
through mountain climbing or travel in an insufficiently
pressurized airplane. Anoxia, and the resultant brain damage, is a
particular problem with newborns during difficult births.
No matter what the cause of anoxia, the symptoms are similar. In
severe cases, the patient is often confused and commonly stuperous
or comatose (in a state of unconsciousness). Depending on the
severity of the injury to the brain, the organ most sensitive to
reduced oxygen intake, this condition can persist for hours, days,
weeks, or even months or years. Seizures, myoclonic jerks
(involuntary muscle spasms or twitches), and neck stiffness are
some other symptoms of the anoxic condition.
Symptoms of more localized or less complete oxygen deprivation
(hypoxia) include increased breathing rate, lightheadedness,
dizziness, tingling or warm sensation, sweating, reduced field of
vision, sleepiness, a bluish tint to skin, particularly the
fingertips and lips, and behavior changes, often an inappropriate
sense of euphoria.
Diagnosis
Diagnosis of anoxia and hypoxia is commonly made through the
appearance of clinical symptoms. However, suspected reduction in
oxygen reaching the tissues can be confirmed using laboratory
tests. The exact test that is performed is dependent on the
suspected cause of the anoxia. One systemic measure of tissue
anoxia is the serum lactate (lactic acid) test. When cells are
forced to produce energy without oxygen, as would happen during
anoxia, lactic acid is one of the byproducts. Thus, an increase in
lactic acid in the blood would indicate that tissues were starved
for oxygen and are using non-oxygen pathways to produce energy.
Normally, the blood contains less than 2mmol/L of lactic acid.
However, some forms of anoxia do not increase lactic acid
concentrations in the blood and some increases in lactic acid
levels are not associated with anoxia, so an elevated value for
this test is only suggestive of an anoxic or hypoxic condition.
Treatment
The exact treatment for anoxia is dependent on the cause of the
reduced oxygen reaching the tissues. However, immediate
restoration of tissue oxygen levels through supplementing the
patient's air supply with 100% oxygen is a common first step.
Secondary steps often include support of the cardiovascular system
through drugs or other treatment, treatment of lung disease,
transfusions, or administration of anecdotes for poisoning, as
appropriate.
Prognosis
A good prognosis is dependent on the ability to treat the
underlying cause of the low oxygen levels. If cardiovascular and
respiratory systems can be supported adequately, recovery from the
injury to the tissue is possible, although extent of injury to the
brain can be difficult to assess. The exact amount of recovery
varies with the amount of injury sustained, where significant
injury brings a poorer prognosis. As recovery occurs, both
psychological and neurological abnormalities may appear, persist,
and can improve. Some problems seen after anoxia include mental
confusion, personality changes, amnesia or other types of memory
loss, hallucinations, and persistent myoclonus (involuntary
contractions of the muscles).
Prevention
Hypoxemic anoxia can be avoided by utilizing supplemental oxygen
when in high altitudes and being aware of the early symptoms of
altitude sickness and reducing altitude once recognized. Iron
supplements can avoid anemic hypoxia, although more severe anemic
states are usually caused by disease or bleeding. Maintaining good
cardiovascular health through proper diet and exercise is a good
first step to avoiding the most common cause of stagnant anoxia.
Avoiding exposure to the toxic chemicals that cause the condition
can prevent histotoxic anoxia.
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