Hypoxemia refers to the low level of oxygen in blood, and the more
general term hypoxia is an abnormally low oxygen content in any tissue
or organ, or the body as a whole. Hypoxemia can cause hypoxia
(hypoxemic hypoxia), but hypoxia can also occur via other mechanisms,
such as anemia.
Hypoxemia is usually defined in terms of reduced partial pressure of
oxygen (mm Hg) in arterial blood, but also in terms of reduced content
of oxygen (ml oxygen per dl blood) or percentage saturation of
hemoglobin (the oxygen-binding protein within red blood cells) with
oxygen, which is either found singly or in combination.
While there is general agreement that an arterial blood gas
measurement which shows that the partial pressure of oxygen is lower
than normal constitutes hypoxemia, there is less agreement concerning
whether the oxygen content of blood is relevant in determining
hypoxemia. This definition would include oxygen carried by hemoglobin.
The oxygen content of blood is thus sometimes viewed as a measure of
tissue delivery rather than hypoxemia.
Just as extreme hypoxia can be called anoxia, extreme hypoxemia can be
called anoxemia.
Signs and symptoms
In an acute context, hypoxemia can cause symptoms such as those in
respiratory distress. These include breathlessness, an increased rate
of breathing, use of the chest and abdominal muscles to breathe, and
lip pursing.
Chronic hypoxemia may be compensated or uncompensated. The
compensation may cause symptoms to be overlooked initially, however,
further disease or a stress such as any increase in oxygen demand may
finally unmask the existing hypoxemia. In a compensated state, blood
vessels supplying less-ventilated areas of the lung may selectively
contract, to redirect the blood to areas of the lungs which are better
ventilated. However, in a chronic context, and if the lungs are not
well ventilated generally, this mechanism can result in pulmonary
hypertension, overloading the right ventricle of the heart and causing
cor pulmonale and right sided heart failure. Polycythemia can also
occur. In children, chronic hypoxemia may manifest as delayed growth,
neurological development and motor development and decreased sleep
quality with frequent sleep arousals.
Other symptoms of hypoxemia may include cyanosis, digital clubbing,
and symptoms that may relate to the cause of the hypoxemia, including
cough and hemoptysis.
Serious hypoxemia occurs when the partial pressure of oxygen in blood
is less than 60 mm Hg, (the beginning of the steep portion of the
oxygen–haemoglobin dissociation curve, where a small decrease in the
partial pressure of oxygen results in a large decrease in the oxygen
content of the blood)or when hemoglobin oxygen saturation is less than
90%.[medical citation needed] Severe hypoxia can lead to respiratory
failure
Causes
Hypoxemia refers to insufficient oxygen in the blood. Thus any cause
that influences the rate or volume of air entering the lungs
(ventilation) or any cause that influences the transfer of air from
the lungs to the blood may cause hypoxemia. As well as these
respiratory causes, cardiovascular causes such as shunts may also
result in hypoxaemia.
Hypoxemia is caused by five categories of etiologies: hypoventilation,
ventilation/perfusion mismatch, right-to-left shunt, diffusion
impairment, and low PO2. Low PO2 and hypoventilation are associated
with a normal A-a gradient whereas the other categories are associated
with an increased A-a gradient.
