Classification
- Hypoxemic hypoxia is a state of oxygen deficiency in which the arterial content of oxygen is low because the arterial partial pressure of oxygen is low. Hypoxemic hypoxia may be due to:
- Hypoventilation. Inadequate pulmonary minute ventilation (e.g., respiratory arrest or by drugs such as opiates)
- Shunts in the pulmonary circulation or a right-to-left shunt in the heart. Shunts can be caused by collapsed alveoli that are still perfused or a block in ventilation to an area of the lung. Whatever the mechanism, blood meant for the pulmonary system is not ventilated and so no gas exchange occurs (the ventilation/perfusion ratio is decreased).
- Normal anatomical shunt occur due to Thebesian veins which empty into the left ventricle and the bronchial circulation which supplies the bronchi with oxygen.
- Normal physiological shunts occur due to the effect of gravity. The highest concentration of blood in the pulmonary circulation occurs in the bases of the pulmonary tree compared to the highest pressure of gas in the apices of the lungs.
- V/Q mismatch. When the ventilation does not match the perfusion through the paranchyema of the lung. This can occur for a variety of reasons, the commonest being a Pulmonary embolism
- Diffusing defects such as pulmonary fibrosis where the Aa gradient has increased.
- Decreased concentration of oxygen in inspired air. Low partial pressure of atmospheric oxygen such as found at high altitude or by reduced replacement of oxygen in the breathing mix.
- Low partial pressure of oxygen in the lungs when switching from inhaled anesthesia to atmospheric air, due to the Fink effect, or diffusion hypoxia.
- Anemic hypoxia is a state in which the arterial partial pressure of oxygen is nomal, but total oxygen content of the blood is reduced due to a decreased ability for hemoglobin to carry oxygen.
- Anemic hypoxia occurs in chronic anemia and carbon monoxide poisoing.
- Ischemic hypoxiaoccurs when there is decreased blood flow.
- Ischemic hypoxia can occur in heart failure and vasodilatory shock.
- Histotoxic hypoxia in which quantity of oxygen reaching the cells is normal, but the cells are unable to use the oxygen effectively, due to disabled oxidative phosphorylation enzymes. Cyanide toxicity is one example.
Read more about this topic: Hypoxia (medical)