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You know that oxygen is a gas that we must breathe all the time to stay alive, but what is the story of oxygen in connection with pregnancy and with the COVID-19 pandemic? In both cases, the answer has to do with the lungs, with the cells that carry oxygen in your blood, and with the trillions of cells in your body that use the oxygen in order to harness energy from numerous other chemicals. Let’s begin the discussion with some background on oxygen and air and then ease our way into pregnancy and COVID-19 as they relate to oxygen.
In most medical situations, patients either breathe what doctors call “room air”, or they are on what’s called supplemental oxygen (also called O2, because each molecule consists of two oxygen atoms), meaning that the breathing gas contains more O2 than room air. Room air is basically just air, the same air that you breathe outside, of which oxygen makes up just about 21 percent. Most of the rest consists of a different gas called nitrogen. You could live breathing air that contains no nitrogen at all, but air provides us with something else apart from oxygen. It provides pressure; it pushes on and inside the body. Normally, nitrogen provides most of that pressure, but oxygen from tanks can be delivered to a patient through tubing, either to enrich the air so that it contains more than 21 percent O2, or to replace room air entirely so that the patient is breathing 100 percent O2.
As humans –in fact, as mammals– we get almost all of our body’s needed oxygen through the lungs. From the lungs, the oxygen is transported by blood to all of the body tissues. Although this happens with some of the oxygen simply dissolved into the water component of blood the way that you may dissolve sugar into your coffee each morning, most oxygen in blood is carried within red blood cells (RBCs). Each RBC is basically package of a substance called hemoglobin (Hb). Up to four molecules of O2 can attach to each Hb molecule like four paperclips sticking to a magnet. After attaching to the Hb within a RBC as it passes through the lungs, a molecule of oxygen remains attached until the RBC reaches a capillary (a small blood vessel) within a tissue where oxygen is needed. This can be a tissue where the supply of oxygen is low, such as heart muscle, or, if you are pregnant, it can be the placenta, where special capillaries allow oxygen molecules to transfer from hemoglobin in the mother’s RBCs to hemoglobin in the RBCs of the fetal blood.
Without mixing of maternal and fetal blood, the fetal blood is able to draw oxygen from the mother’s blood, because fetal hemoglobin (known as HbF) acts as a slightly stronger magnet. Thus, even if the supply of oxygen in the mother’s blood gets a little low, the fetus has a way to get maternal oxygen that has entered through mother’s lungs. Meanwhile, as oxygen moves from the air through the lungs, through the blood, into body cells and to the fetus (or to an embryo during early pregnancy), another gas, carbon dioxide (CO2) moves in the other direction. CO2 exits body cells, exists the fetus and moves through the placenta, and enters the mother’s blood, where it can be carried in a few different ways until it reaches the lungs. Once in the lungs, CO2 is exhaled from the body, but while in the blood CO2 has a very strong influence on the acidity of the blood. Consequently, the rate of breathing and the volume of air that moves in and out of the lungs with each breath have an enormous impact on body chemistry. If everything is working right, the body keeps the breathing and acidity all in tune with your needs, including getting you and your fetus plenty of oxygen. But things are easily thrown out of whack when problems in the lungs prevent oxygen from entering the blood sufficiently, such that a patient must be connected to a mechanical ventilator.
Now, with COVID-19, there is a virus called SARS-CoV-2, which generally enters into the upper part of the respiratory system, where it can cause cold-like symptoms, or in many people even no symptoms at all. In a fraction of infected people, however, the virus can get down into the lower part of the respiratory system, into the air sacs, balloon-like structures where O2 and CO2 move between air and blood. In the air sacs, the virus can attach to different types of lung cells, which it can infect and use as factories to make little baby viruses. In addition to spreading the infection within the lungs, this damages the infected lung cells. When one type of lung cell is infected, the amount of oxygen that can get into the blood decreases. When another type of lung cell is infected, the lungs are hindered in their ability to make a soapy substance that allows the air sacs to inflate easily when you inhale. Known as surfactant, the soapy substance is sometimes sprayed into the lungs of newborns that are born too prematurely to make enough of their own, but the same treatment has not been proven (at least not yet) to benefit adults suffering from a lung problem similar to the problem premature infants often have with their lungs. Known as ARDS, for acute (or adult) respiratory distress syndrome, this is a dreaded lung complication of COVID-19.
With an increasing number of air sacs collapsing and unable to allow oxygen to move from the air to the blood, a person with the severe form of COVID-19 struggles to breath. The percentage of hemoglobin that is carrying oxygen –known as the oxygen saturation– decreases. If this happens during pregnancy, the fetus could draw oxygen from the mother initially, because of the special fetal hemoglobin that we discussed above, plus doctors could give the woman supplemental oxygen, through a tube into the nose or through a mask. But if the mother’s oxygen levels in the blood were to continue to drop, despite high levels of oxygen coming in through a mask, both she and the fetus would be in trouble. On top of this, without the soapy surfactant, air sacs would not only collapse; they’d also rub against one another. This causes inflammation, a kind of irritation that sends a bunch of chemicals through the body, causing still more life-threatening problems.
Fortunately, most cases of COVID-19 do not progress to the severe stage, especially in younger women, including those who are pregnant. But when the disease does lead to severe complications like ARDS, that’s when a ventilator becomes necessary, and we’ll visit this topic in a upcoming post.
