Blood carries oxygen to cells and tissues to support their metabolic activities. According to the book "Common Surgical Diseases," published in 2008, low blood oxygen levels -- also known as hypoxemia -- occur when the level of oxygen in arterial vessels is lower than 80 millimeters of mercury, written as mm Hg. If the oxygen level in the blood is too low, organs like the brain and heart can become hypoxic, meaning they are not receiving enough oxygen to function normally.
The most common effects of low blood and tissue oxygen levels are related to the respiratory system. As such, shortness of breath is generally one of the first symptoms. Anxiety or restlessness, fatigue and headaches are also common symptoms of mild hypoxemia. In an effort to increase the amount of oxygen in the blood, the respiration rate may increase to more than 24 breaths per minute. Heart rate is also frequently elevated to above 100 beats per minute to help circulate oxygen to meet tissue demands.
If hypoxemia becomes more severe, brain function may become impaired, creating symptoms such as decreased attention span, confusion and disorientation. Breathing may become irregular, with cycles of deeper and shallower breathing called Cheyne-Stokes pattern breathing. Endurance for physical activity decreases further, and motor function, particularly for fine movements, can also become impaired. Cyanosis, a bluish discoloration of the skin and mucous membranes, becomes visible. As hypoxemia worsens, brachycardia -- a heart rate of less than 60 beats per minute -- and a drop in blood pressure may occur. Ultimately, coma and death can result from severe, untreated hypoxemia.
A low blood oxygen level lasting for several days or longer is called chronic hypoxemia, and signs and symptoms will vary depending on the severity and duration. Fatigue, lethargy and irritability are common symptoms, as is impaired judgment. Respiratory patterns may be irregular, and arrhythmias -- abnormal heartbeats -- are also often present. Polycythemia, an increase in the number of red blood cells, develops more slowly, accompanied by a ruddy complexion. Clubbing, a bulbous appearance to the fingertips and nails, may also occur. Pulmonary hypertension -- high blood pressure in the lungs -- and pulmonary edema, leading to right heart enlargement or failure, can result from long-term untreated hypoxemia.
When to Seek Medical Attention
Unexplained shortness of breath should be discussed with a doctor, particularly if it occurs at rest or if it's associated with abrupt awakenings at night, a potential sign of sleep apnea. A visit to high altitude -- especially above 8,000 feet -- can cause high altitude pulmonary edema, a potentially life-threatening condition marked by shortness of breath, headache, insomnia, fluid retention and a cough, possibly with a frothy pink sputum. Seek immediate medical attention for any of these symptoms at high altitude. Sudden or severe shortness of breath, whether or not it affects the ability to function, should also be evaluated immediately.