Hypoxia (Low Oxygen) During Pregnancy

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The word hypoxia refers to a low supply of molecular oxygen (O2 ,a gas whose molecules each consist of two atoms of oxygen) in the environment in which you are breathing. This happens when you are on a mountain or flying in an aircraft where the pressure of O2 in the air —called the O2 partial pressure— is much lower than what it is at sea level. Hypoxia also refers to a low concentration of O2 in a body tissue. Frequently, the word hypoxia in tissues of the body is used interchangeably with the word hypoxemia, meaning a lower-than-normal concentration of O2 in arterial blood (blood in systemic arteries, meaning arteries that nourish body tissues, as opposed to the pulmonary arteries that carry oxygen-depleted blood to the lungs). In talking about oxygen in the blood, hypoxemia is technically the correct term, but don’t be thrown off if you hear someone talk about hypoxia in this very same context.

Hypoxemia is defined based on either the O2 partial pressure in arterial blood (abbreviated PaO2) or the percentage of hemoglobin (Hb) that is carrying O2 (called oxygen saturation, “O2 sat”, or SpO2). As we have discussed in previous posts, hemoglobin is a protein that carries O2 within red blood cells. For a person breathing sea level air (air in a place where the elevation is zero), which has a total pressure of 760 mmHg and an O2 partial pressure of 160 mmHg, PaO2 normally ranges from 75 or 80 mmHg up, to 100 or 105 mmHg, depending on the person and situation. This means that you will be considered hypoxemic, if your PaO2 is below 75 or 80 mmHg, depending on which lab tests your arterial blood. SpO2 (oxygen saturation in the blood) is normally above 95 percent (usually it’s at least 97 percent at sea level). People below 90 percent are always called hypoxic, but sometimes values from 90-94 percent are called hypoxic, or mildly hypoxic.

As noted above, the terms hypoxia and hypoxemia are often used interchangeably. This is because hypoxemia makes your body tissues hypoxic and also because the blood is the easiest tissue in which to measure O2 concentration and pressure. Hypoxemia can result from environmental hypoxia, such as being at high altitude. This includes flying in an aircraft, in which cabins are typically pressurized to the equivalent of being outside at an altitude of 1,800–2,400 meters (6,000-8,000 feet), depending on the aircraft type.

Lung problems and congenital heart defects also cause hypoxia in the body. An example of a lung problem that is particularly relevant to pregnancy is pulmonary embolism (PE), a life threatening condition that can result as a complication of venous thrombosis (clotting in a deep vein) which can happen due to increased clotting tendency and slowed venous blood flow during pregnancy.

Another category of hypoxia and hypoxia is that caused by poisons. These include carbon monoxide (CO), which causes hypoxemia and hypoxia throughout body tissues. They also include cyanide, which causes hypoxia within body cells. There is also a hypoxia condition particular to pregnancy, called intrauterine hypoxia –hypoxia in the womb. This can develop from a variety of problems in the mother, the placenta, or the fetus. The risk for intrauterine hypoxia is elevated with certain pregnancy complications, particularly preeclampsia, being pregnant for the first time, and smoking.

A pregnant woman suffering from hypoxia may be cyanotic, meaning that her skin and mucous membranes will show a bluish discoloration. Much more specific information can be obtained through pulse oximetry, which is taken with a device that is clipped to your finger that measures your O2 saturation. PaO2 is measured from a sample of your arterial blood, if your doctor orders an arterial blood gas (ABG) test.

If intrauterine hypoxia develops, this leads to intrauterine growth restriction, meaning that the fetus is underweight and also small-for-gestational-age. This is associated with a variety of problems that include delayed brain development and cognitive problems during childhood.

Hypoxemia/hypoxia is treated with 100 percent O2 given through a nasal cannula (a tube in your nose) or a mask. This is extremely effective and carries no risk for the fetus; in fact, it helps the fetus by reducing the risks of hypoxia. 100 percent O2 can be administered whether your hypoxia is due to a medical condition or injury or to being at altitude. Although the treatment is called “100 percent” O2, it’s only 100 percent if the oxygen is delivered through a very special kind of masks or through other special devices. Otherwise, the oxygen mixes with room air, so you receive more than the usual amount of oxygen, but not 100 percent.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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