Hypoxia (medical)

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Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole (generalized hypoxia) or a region of the body (tissue hypoxia) is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise. A mismatch between oxygen supply and its demand at the cellular level may result in a hypoxic condition. Hypoxia in which there is complete deprivation of oxygen supply is referred to as anoxia.

Hypoxia differs from hypoxemia. In the latter, the oxygen concentration within the arterial blood is abnormally low.[1] It is possible to experience hypoxia and have a low oxygen content (e.g., due to anemia) but maintain high oxygen partial pressure (pO2). Incorrect use of these terms can lead to confusion, especially as hypoxemia is among the causes of hypoxia (in hypoxemic hypoxia).

Generalized hypoxia occurs in healthy people when they ascend to high altitude, where it causes altitude sickness leading to potentially fatal complications: high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE).[2] Hypoxia also occurs in healthy individuals when breathing mixtures of gases with a low oxygen content, e.g. while diving underwater especially when using closed-circuit rebreather systems that control the amount of oxygen in the supplied air. A mild and non-damaging intermittent hypoxia is used intentionally during altitude trainings to develop an athletic performance adaptation at both the systemic and cellular level.[3]

Contents

Classification

  • Hypoxemic hypoxia is a generalized hypoxia, an inadequate supply of oxygen to the body as a whole. The term "hypoxemic hypoxia" specifies hypoxia caused by low partial pressure of oxygen in arterial blood. In the other causes of hypoxia that follow, the partial pressure of oxygen in arterial blood is normal. 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 (decrease in FiO2). Low partial pressure of atmospheric oxygen such as found at high altitude[4] or by reduced replacement of oxygen in the breathing mix.
      • Low partial pressure of oxygen in the lungs when switching from inhaled anaesthesia to atmospheric air, due to the Fink effect, or diffusion hypoxia.
  • Anaemia in which arterial oxygen pressure is normal, but total oxygen content of the blood is reduced. This is due to a decreased total carrying capacity.[5]
  • Hypoxia when the blood fails to deliver oxygen to target tissues.
  • Histotoxic hypoxia in which quantity of oxygen reaching the cells is normal, but the cells are unable to effectively use the oxygen due to disabled oxidative phosphorylation enzymes. Cyanide toxicity is one example.

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