hypoxia
Meaning of hypoxia: Oxygen can fail tissue at several points along its journey, and where the failure happens defines the type of hypoxia.
Definition of hypoxia
Hypoxia is a state in which body tissue does not receive enough oxygen, or cannot use the oxygen it receives, to meet its needs. It can follow impaired breathing, reduced blood flow, a shortage of the hemoglobin that carries oxygen, or a poison that blocks the way cells burn it. Effects range from fully reversible to permanent injury.
What are the main types?
| Type | Where the failure is | Example |
|---|---|---|
| Hypoxemic | Too little oxygen entering the arterial blood | Severe pneumonia, high altitude |
| Anemic | Enough oxygen, too little carrier to hold it | Marked anemia, carbon monoxide poisoning |
| Circulatory | Blood carrying oxygen does not reach the tissue | Shock, a blocked artery |
| Histotoxic | Oxygen arrives but cells cannot use it | Cyanide poisoning |
The four types explain why a normal reading on a fingertip probe does not rule hypoxia out. That probe reports how saturated hemoglobin is with oxygen, so it looks normal in anemic, circulatory and histotoxic hypoxia even while tissue is starving. In carbon monoxide poisoning it can read high, because the probe cannot tell oxygen from carbon monoxide bound to the same site.
How does the body react?
Breathing speeds up and deepens, the heart beats faster, and blood is redirected toward the brain and heart and away from skin, gut and kidney. Over days at altitude the kidney releases more erythropoietin and the marrow builds extra red cells. None of that helps if the problem is a poison inside the cell. When oxygen falls far enough, cells switch to a wasteful backup pathway that yields little energy and leaves lactic acid behind, which is why a rising lactate level is watched as a marker of tissue that is not getting what it needs.
How long can tissue survive it?
Tolerance differs enormously between tissues. Brain and heart muscle have almost no reserve and begin to fail within minutes. Skeletal muscle, skin and cartilage tolerate far longer. Mild hypoxia sustained over weeks does something different again, prompting new blood vessel growth and thickening of the vessels in the lung. Because the causes are so varied, evaluation usually combines a measurement of blood oxygen with a search for the underlying reason, and oxygen therapy corrects only the types in which the shortage begins in the lungs or the air.
Also known as
Used in a sentence
Supplemental oxygen was started because of signs of hypoxia.
Good to know
Hypoxia describes tissue, while hypoxemia describes low oxygen measured in arterial blood; tissue can be hypoxic while arterial oxygen readings look normal, as happens in severe anemia and in cyanide poisoning.
Word origin
Where “hypoxia” comes from
From hypo-, Greek for under or below, combined with oxygen.