If you are pregnant, or are preparing to become pregnant, you probably know that your body is in for some changes. Changes in the shape and size of the body and its parts are obvious from the outside. We have known about these changes since before we were human, but you may be wondering about changes inside, changes in your physiology. As noted in the first installment of this series, many physiological changes that develop as pregnancy progresses and various body systems are affected. Today, let’s explore the respiratory system, which is vital for supply oxygen and removing carbon dioxide, both for you and for your embryo or fetus. Changes during pregnancy in the lungs are related to changes in the shape of the body and enlargement of the uterus, to hormonal changes, and changing metabolic needs and a reduction on the concentration of hemoglobin and red blood cells.
As you know, you have two lungs, together occupying most of the volume within the cavity inside your chest, the intrathoracic cavity. Your lungs expand as the intrathoracic cavity enlarges, due to expansion of the chest wall and to the diagram moving downward, into your abdomen. This happens in cycles, each time you breathe, but as your womb grows, there is a decreasing amount of space in your abdomen, so the architecture around the lungs changes. These anatomic changes, combined with other factors, such as hormonal changes leads some aspects of breathing to change a lot, while others only a little. One factor that changes a lot is the amount of air remaining in your lungs after you finish exhaling, which means that it is more difficult to force extra air out at the end of the normal exhalation in comparison with what you can force out when you are not pregnant. Another respiratory factor that changes substantially during pregnancy is what doctors call the minute ventilation (VE), which basically means the volume of air that you exchange between your lungs and the outside air per minute. In pregnancy, the VE increases by 20 to 50 percent. This happens because of an increase in the tidal volume VT, the volume of are that you exchange between your lungs and the outside air during each breath. You calculate the VE by multiplying the VT by the respiratory rate, the amount of breaths per minute.
The consequence on the VE increasing is that you off-gas carbon dioxide —remove it from the body— more quickly than normally when you are moving less air back and forth between your lungs and the outside air. This causes the concentration of carbon dioxide in your blood to decrease, which in turn causes the pH of your blood to increase, meaning that your blood becomes less acidic, more alkaline. This is called respiratory alkalosis. It’s a mild respiratory alkalosis, because the pH rises only slightly, but it’s enough of a change for your kidneys to notice it, and over time, the kidneys compensate by working to make the blood more acidic.
Because the lungs normally receive all of the blood that passes through the right side of the heart, the lungs also can be the location of an embolism —a clot that has develops, because of an embolus, a piece of material that originated somewhere else and traveled in the blood stream to the new site. In the lungs, an embolism is called a pulmonary embolism (PE) and usually it happens, because of an embolus coming from a blood clot in a deep vein, usually in the leg or pelvis. During pregnancy, especially during the final weeks and for a few weeks after delivery, you are at particular risk of developing such a deep venous thrombosis, putting you at risk of developing a PE, which manifests with breathing difficulty and requires immediate diagnosis and treatment.
A still more rare, but very severe, complication of pregnancy that affects the lungs is called an amniotic fluid embolism (AFE), which is called an embolism, only because it starts with a piece of debris —amniotic fluid— that can reach the lungs. However, breathing problems that develop in AFE are due to an immune reaction, similar to anaphylaxis, rather than to physical obstruction of the flow of blood through the lungs as happens in a PE.
Whereas a PE results from an embolus getting stuck in blood vessel that is too narrow for it to pass, the lungs actually serves as a kind of filter for removing such traveling clots when they are small. For this reason, women who have connections between the right and left side of the heart —the most common such connection is called a patent foramen ovale, present in 25-30 percent of all people— are at risk for an embolus generated by a deep venous clot to bypass the lungs, move into the left side of the heart and cause an embolism in a site other than the lungs, for instance in the brain.