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Metabolic Alkalosis: Vomiting of Pregnancy Can Trigger this Condition

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When doctors say that somebody has metabolic alkalosis, they mean that the pH of the person’s body tissues is higher than 7.45. The normal pH of most body tissues ranges from 7.35 to 7.45, but various situations can cause alkalosis. In contrast to what you may have heard from people pushing the so-called alkaline diet to raise your pH, the main control over pH throughout the body comes from the kidneys and the lungs. The kidneys control how much of an electrolyte called bicarbonate (HCO3) stays in the body, or leaves the body. The lungs control how much carbon dioxide (CO2) leaves the body and how much stays in the body. The amount of CO2 in the body, in turn, controls the acidity of body tissues. The more slowly one breathes, the more CO2 remains in the blood and the more acidic the blood becomes. In contrast with many other body tissues, the pH in the stomach is very low, because special cells in the stomach, called parietal cells, produce acid to help in the digestion of food. Eating more alkaline foods may help with stomach conditions sometimes, but it won’t have an impact in the acidity of the blood, because the lungs and kidneys are working to keep the pH within that narrow range and also because chemicals in the blood, called buffers, prevent the pH from drifting significantly.

Problems in the body, on the other hand, can lead to pH problems, including metabolic alkalosis. This can happen because HCO3 is retained or because another electrolyte, called chloride (Cl), may be eliminated through the gastrointestinal (GI) tract, or kidneys. Elimination of acid through the GI tract also causes metabolic alkalosis, as does excretion of too much acid through the kidneys, due to a range of electrolyte and fluid disturbances. Certain medications, rare genetic conditions, excess of certain hormones, and consumption of too much calcium (such as in calcium supplements or milk) also can cause metabolic alkalosis. These and other causes can lead to metabolic alkalosis in pregnant women, but the main reason for metabolic alkalosis in pregnancy is vomiting. Causing a loss of both acid and Cl, vomiting can occur as part of the common nausea and vomiting of pregnancy (NVP), or more intensely as part of a condition called hyperemesis gravidarum (HG). HG is a complication of pregnancy featuring severe nausea and vomiting, as well as weight loss and electrolyte disturbances, including metabolic alkalosis, typically also with dehydration. NVP strikes mostly in the first trimester, usually beginning between 4 and 6 weeks gestation and peaking from week 8 to week 12, whereas HG tends to strike more in the middle of pregnancy.

While NVP can cause you to lose enough acid and chloride to develop metabolic alkalosis, HG is more likely to cause metabolic alkalosis. This is because HG involves more intense, more frequent vomiting compared with NVD. On the other hand, NVD strikes 50-90 percent of pregnancies, while HG affects only 0.5 to 3 percent of pregnancies, and only a fraction of vomiting cases lead to metabolic alkalosis. Apart from vomiting, various other factors can cause metabolic alkalosis, or exacerbate alkalosis that was triggered by vomiting. These factors include hypokalemia (low potassium), dehydration, decreased filtering of blood through kidneys, decreased chloride levels, certain medications, and certain genetic conditions. They also include an excess of aldosterone (a hormone that helps increase blood pressure by keeping water and sodium in the body) and ingestion of licorice, which also causes high blood pressure.

As for the diagnosis of metabolic alkalosis, it is based on laboratory tests, but doctors can get initial clues from factors in the history of your condition, such as that you have been vomiting a lot or that you are suffering from muscle twitching, spasms, or tremors, numbing sensations, lightheadedness, and confusion. Laboratory assessment of your acid-base status requires blood samples, including a standard blood sample taken from a vein and also blood taken from an artery, typically the radial artery in your arm, for a test called arterial blood gases (ABG). In an ABG, the pH of the blood is tested, along with concentrations of electrolytes and the pressure of carbon dioxide in the blood. Doctors pay close attention to the HCO3 concentration, which is elevated in metabolic alkalosis. They also pay close attention to the concentration of potassium concentration (K+), which is typical too low in cases of alkalosis.

Metabolic alkalosis can cause various problems throughout the body. Because of the low potassium that accompanies metabolic alkalosis, the heartbeat can be affected in dangerous ways. The nervous system, the muscles, and liver all can also be affected, producing symptoms that include muscle twitching, spasms, or tremors, numbing sensations, lightheadedness, and confusion. If your respiratory system is healthy, it will compensate partly for metabolic alkalosis by slowing the breathing rate, which will give the baby a good amount of protection. If the alkalosis is severe and present at the time of delivery, however, the newborn may have alkalosis, which will show up as slowed breathing, decreased muscle tone, and a range of metabolic and organ disruptions that will require careful management in the neonatal intensive care unit.

The strategy for treating metabolic alkalosis is to treat the underlying causes. Additionally, if needed, doctors will work to reverse the chemistry that has been pushing the acid base balance toward higher than normal pH. Outside of pregnancy, this can be achieved with a type of medication called a carbonic anhydrase inhibitor, the main one being acetazolamide. However, there are concerns about acetazolamide use during pregnancy, due to laboratory animal studies showing possible risks to the fetus. There are very few human data to back up the laboratory animal studies, so if alkalosis is severe, there are cases when doctors may administer the medication during pregnancy. Acetazolamide is not thought to be harmful in nursing women. Reversal of alkalosis also can be achieved with intravenous infusion of hydrochloric acid, the same type of acid that the stomach produces and that you lose when you vomit. The infusion must be carried out very slowly and carefully to avoid overshooting and causing the opposite condition, acidosis.

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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