Blood carries oxygen to every cell in your body to allow them to live and function properly. Low blood oxygen levels -- known as hypoxemia -- typically refer to an oxygen level in your arteries less than 80 millimeters of mercury (mm Hg). If the oxygen level in your blood is too low, your cells cannot function normally. The effects of hypoxemia depend on whether it is mild or severe and how long it has been present. Shortness of breath and an increased breathing rate are common, and changes in energy levels, heart function and brain function may occur as well. If left untreated, severe hypoxemia can be fatal.
The most common effects of low blood oxygen levels are related to the respiratory system. In an effort to increase the amount of oxygen in the blood, breathing increases above the normal rate of around 12 to 16 breaths per minute. Rates of 24 breaths per minute or even higher may occur. As such, feeling short of breath is generally one of the first and most obvious symptoms. The heart rate also increases to help circulate oxygen-containing blood throughout the body in an attempt to meet the cells' needs. Anxiety or restlessness, fatigue and headaches are other common effects of mild hypoxemia.
If hypoxemia worsens, brain function can become impaired, creating symptoms such as a decreased attention span, confusion and disorientation. Breathing may become irregular, with alternating cycles of deep and shallow breaths called Cheyne-Stokes breathing. Physical activity becomes difficult because of fatigue and shortness of breath, and movements, particularly fine movements of the hands, can also become impaired. Cyanosis -- a bluish discoloration of the skin and mucous membranes -- becomes visible. With severe hypoxemia, the heart rate and blood pressure will start to fall. Chest pain may occur if the heart is not receiving enough oxygen, which is especially likely if the arteries leading to the heart are narrowed by coronary artery disease. Ultimately, severe untreated hypoxemia leads to coma or death.
A low blood oxygen level lasting for several days or longer is called chronic hypoxemia. The effects of chronic hypoxemia will vary, depending on its severity and duration. Fatigue, lethargy and irritability are common symptoms, as is impaired judgment. Breathing may be irregular, and abnormal heart rhythms are often present. Polycythemia, an increase in the number of red blood cells, develops more slowly. It causes a reddish complexion. Clubbing of the fingers, characterized by enlargement of the fingertips and nail changes, may also occur. Pulmonary hypertension -- high blood pressure within the lungs -- can result from long-term untreated hypoxemia. It causes the right ventricle of the heart, which pumps blood into the lungs, to work harder, eventually leading to right ventricular enlargement and failure.
When to Seek Medical Attention
If you feel more short of breath than expected for your level of activity, see your doctor to determine whether you have hypoxemia. Excessive fatigue, decreased mental function and the presence of cyanosis, clubbing, a red complexion or other manifestations of hypoxemia should also be evaluated by a physician. Sudden or severe shortness of breath for any reason requires immediate medical attention. If you are visiting at high altitude -- especially above 8,000 feet -- seek immediate medical care if you notice any shortness of breath, headache, insomnia, fluid retention or a cough. These may represent high altitude pulmonary edema, a potentially life-threatening cause of hypoxemia.
Reviewed by: M. Denise Daley, M.D.