Hypoxia occurs when there is a low amount of oxygen in the body. People commonly encounter low-oxygen conditions when they travel to high altitudes and experience “thin air,” where the lower air pressure results in less oxygen available to breathe.
Some diseases can also restrict the amount of oxygen that enters the lungs, such as chronic obstructive pulmonary disease (COPD), pulmonary fibrosis (scarring in the lungs), sleep apnea, pneumonia, and more. These diseases can make it difficult to breathe, and they can also trigger changes in the blood vessels of the lungs, leading to pulmonary hypertension (PH).
“Pulmonary” is a term referring to the lungs, while “hypertension” means high blood pressure. “Pulmonary hypertension,” therefore, refers to high blood pressure in the blood vessel connecting the heart to the lungs. This special blood vessel runs from the heart, connects to tiny air sacs in the lungs, and passes back into the heart again. This is how oxygen gets from the air into our bloodstream. A common measurement of blood pressure is around 120 over 80 mmHg in healthy people. This measurement reflects the blood pressure circulating in the body.
Pulmonary artery blood pressure measures the blood pressure in the pulmonary artery, which carries blood from the heart to the lungs. Because blood does not have to travel far from the heart to the lungs, normal pressure in the pulmonary artery is relatively low — less than 20 mmHg. Pulmonary artery pressure above 25 mmHg is associated with PH. High pulmonary vascular resistance, or the resistance against blood flow through the pulmonary blood vessels, is another sign of PH.
Hypoxia-induced PH in humans was first reported in 1947, when scientists found that people who were temporarily exposed to low levels of oxygen showed rapid increases in their pulmonary artery blood pressure. This response is beneficial in the short term because it gets more blood into the lungs — and enough oxygen in the bloodstream. However, long-term exposure to low oxygen (chronic hypoxia) can have severe adverse effects.
People who travel to high-altitude areas — places in the mountains, for example — can experience symptoms like cough, headaches, dizziness, and shortness of breath. High-altitude traveling is also associated with higher pulmonary artery blood pressure. These symptoms go away when people return to lower altitudes or when they are given oxygen.
Hypoxia-induced PH is the second most common cause of PH after heart disease. The World Health Organization classifies PH into five groups, with hypoxia-induced PH being group 3.
Scientists are still learning exactly how hypoxia leads to PH. Generally, research shows that over time, hypoxic conditions cause the pulmonary arteries to narrow. As the vessels narrow, less blood can flow through. This restricts blood flow to the lungs and causes the heart to pump harder.
Pulmonary arteries are lined with specialized cells called pulmonary arterial smooth muscle cells. These cells can relax to increase blood flow or contract to decrease blood flow through the arteries. When oxygen levels are low in a particular area of the lungs, smooth muscle cells in that area contract to reduce blood flow to that area. This helps divert blood to healthy areas of the lungs that are richer in oxygen.
These changes can occur rapidly, and blood pressure improves when the damaged area of the lung receives proper oxygen again. During prolonged exposures to low oxygen, however, the pulmonary arteries permanently narrow, which makes it more difficult for blood to flow to the lungs — and makes the heart pump harder to compensate.
The heart has four chambers that pump blood to different parts of the heart, lungs, and the rest of the body. Blood is sent to the lungs through the pulmonary artery, which exits through the right ventricle of the heart. As the blood pressure increases in the pulmonary artery, the right ventricle works harder to pump. Over time, the right ventricle becomes enlarged (right ventricular hypertrophy). Eventually, the overworked right ventricle cannot pump blood well, leading to right heart failure and death.
Hypoxia-inducible factors (HIFs) play a major role in the changes in pulmonary blood vessels in low-oxygen conditions. HIFs are transcription factors, which means that they are responsible for turning on certain genes in our body. In normal oxygen conditions, HIFs are not active. Low-oxygen conditions signal HIFs to activate and change different gene expressions. Although some HIFs usually help control immune responses, studies of mice show that other HIFs contribute to changes in the pulmonary blood vessels that lead to PH in hypoxic conditions.
Other processes that can contribute to hypoxia-induced PH include:
Not everyone who lives at a high altitude experiences hypoxia-induced PH. Some populations that have lived at high altitudes for thousands of years show genetic adaptations to their environments that help them avoid hypoxia-induced PH. For example, Tibetan people have genetic changes in how HIFs function that might make them less likely to experience PH.
One systematic review compared pulmonary artery pressure measurements for people living at high altitudes to those living at low altitudes. Although high-altitude populations had higher pulmonary artery pressures, researchers suggested that less than 1 percent of this population experiences severe PH.
The symptoms of hypoxia-induced PH are similar to other types of PH. They include:
Diagnosing hypoxia-induced PH depends on your symptoms, medical history, pulmonary artery pressure measurement, heart function, lung function, and exposure to low-oxygen conditions. Many of these require specialized tests, such as lung scans, echocardiography, and lung function tests.
There are no specific treatments for hypoxia-induced PH. Treatments depend on the underlying cause, so talking with your health care team is important in finding the right treatment for you.
Hypoxia-induced PH caused by traveling to high altitudes can often be reversed by returning to lower elevations. For most people with hypoxia-induced PH (especially caused by COPD), long-term oxygen treatment can help relieve symptoms and prevent the underlying disease from getting worse.
Unfortunately, there are not a lot of studies on medications that can benefit people with hypoxia-induced PH. Although not routinely recommended by current guidelines, some medications to manage pulmonary arterial hypertension may also be used in some cases to treat hypoxia-induced PH. These medications include:
Treating underlying conditions such as lung disease or sleep apnea can improve hypoxia-induced PH. Quitting smoking is essential in preventing lung disease or keeping existing lung disease from getting worse.
Medications to manage lung disease can also improve your ability to breathe and get enough oxygen. Inhaled bronchodilators (inhalers) and steroid pills are common treatments for COPD. Sleep apnea can be treated with a device to help you breathe at night, like a continuous positive airway pressure (CPAP) machine or supplemental oxygen.
Preconditioning or training for short periods in hypoxic conditions can help improve resistance to hypoxia. One study of rats showed that a nanocurcumin supplement coupled with physical training helped protect against hypoxia at high altitudes.
Exercise can also be helpful for people with hypoxia-induced PH caused by other diseases. Pulmonary rehabilitation programs focus on teaching you how to increase your fitness to improve symptoms and breathe more efficiently.
On myPHteam, the social network for people with pulmonary hypertension and their loved ones, more than 47,000 members come together to ask questions, give advice, and share their stories with others who understand life with pulmonary hypertension.
Have you experienced hypoxia-induced pulmonary hypertension? Share your experience in the comments below, or share your story on myPHteam.