HYPOXEMIA
HYPOXEMIA: Excerpt from Differential Diagnosis in Primary Care
In developing the list of possible causes of hypoxemia, the physiologic
model of intake, absorption, transport, regulation, and excretion is most
useful.
Intake. Both upper airway obstruction (laryngotracheitis,
foreign body) and lower airway obstruction (bronchial asthma, emphysema) may
inhibit the intake of oxygen, causing hypoxemia. In addition, conditions
that affect the chest wall such as kyphoscoliosis, ankylosing, spondylitis,
and myasthenia gravis reduce the intake of oxygen by decreasing the vital
capacity.
Absorption. Absorption of oxygen in the lungs may be inhibited
by atelectasis, pneumothorax, or pneumonia where the alveolar sacs are
blocked or collapsed or by a diffusion defect as occurs in Hamman–Rich
disease, silicosis, sarcoidosis, and scleroderma. Absorption is also impeded
by a pulmonary embolism or pulmonary hemangioma, which interferes with the
perfusion of a segment of a lung. Large venous–arterial shunts such as
tetralogy of Fallot can produce the same picture.
Transport. The cardiovascular system transports oxygen to the
tissue. When blood flow is slowed because of CHF or shock, oxygen is not
picked up in the lungs and transported to the tissues fast enough to keep
pace with the demand, resulting in hypoxemia. CHF also decreases absorption
by the accumulation of fluid in the lungs, blocking the diffusion of oxygen
across the alveoli.
Regulation. Respirations are regulated by the central nervous
system. Consequently, drugs such as phenobarbital that decrease the
respiratory rate cause hypoxemia. Diseases that affect the respiratory
center such as poliomyelitis or Guillain–Barré syndrome can suppress
respiration, causing hypoxemia.
Excretion. Blocking the excretion of oxygen does not cause
hypoxemia. However, blockage of the excretion of carbon dioxide, as in
pulmonary emphysema and asthmatic bronchitis, contributes to hypoxemia by
not allowing the blood to pick up oxygen in exchange for carbon dioxide.
Approach to the Diagnosis
The clinical picture of obstructive lung disease is usually obvious.
Other causes of hypoxemia may require more extensive laboratory evaluation
to diagnose. It is most important to study the arterial blood gases.
An increased carbon dioxide
level suggests pulmonary emphysema or asthma. Pulmonary function tests can
assist in the diagnosis of these conditions as well. If the carbon dioxide
level is normal or decreased, a perfusion or defusion defect must be looked
for. A lung scan will help rule out a pulmonary embolism. A chest x-ray will
help reveal pneumothorax, atelectasis, sarcoidosis, and pulmonary fibrosis.
An arm-to-tongue circulation time will help rule out CHF. A consult with a
pulmonologist or cardiologist is always wise when faced with hypoxemia.
Other Useful Tests
-
CBC (shock, infection)
- Urinalysis (collagen disease)
- Sedimentation rate (infection)
- Chemistry panel (shock, myocardial infarction)
- Blood volume (shock, CHF)
- Sulfhemoglobin and methemoglobin (sulfhemoglobinemia,
methemoglobinemia)
- Carboxyhemoglobin (carbon monoxide poisoning)
- Serial ECGs and cardiac enzymes (myocardial infarction)
- CT scan of the chest (pulmonary aneurysm, bronchiectasis)
- Lung biopsy (neoplasm, pulmonary fibrosis)
- Bronchoscopy (neoplasm, bronchiectasis)
