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.
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.