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Hypocapnia (Respiratory Alkalosis) Causes & Symptoms

Written by Mathew Eng, PharmD | Reviewed by Ana Aleksic, MSc (Pharmacy) | Last updated:
Medically reviewed by
Puya Yazdi, MD | Written by Mathew Eng, PharmD | Reviewed by Ana Aleksic, MSc (Pharmacy) | Last updated:

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Hypocapnia

Every time you breathe out, you’re exhaling carbon dioxide that was made by your body. A delicate balance of carbon dioxide is important to maintain the pH of your blood and your oxygen levels. Find out what can make your carbon dioxide levels drop too low (known as hypocapnia) and why this can be dangerous.

What is Hypocapnia?

Definition

Hypocapnia occurs when the carbon dioxide level (CO2) in your blood becomes too low.

Why exactly is there CO2 in your blood and what does it do?

Your cells are constantly producing CO2 as a byproduct of burning fats and sugars for energy. This CO2 travels through your blood and into your lungs, where it is eventually exhaled whenever you breathe out [1].

When the concentration of CO2 in your blood becomes lower than normal, it is known as hypocapnia (or sometimes hypocarbia). This can be a problem because CO2 also helps regulate your breathing rate and the acid-base balance in your blood [1].

Hypocapnia happens when the CO2 level in your blood becomes lower than normal.

In this article, we’ll break down what causes hypocapnia, what symptoms you can expect, and ways you can avoid it.

Respiratory Alkalosis

Hypocapnia often leads to a condition called respiratory alkalosis; so much so that the two terms are sometimes used interchangeably. Respiratory alkalosis occurs when the CO2 level of the blood drops below normal and the pH of the blood becomes too high [2].

This happens because your body naturally tries to produce more CO2 when your levels are low. The body does this by joining bicarbonate (HCO3-) and hydrogen ions to create more CO2 molecules. The problem is that hydrogen ions determine blood pH; by using up hydrogen ions to create CO2, the pH of the blood increases and becomes basic [3].

Respiratory alkalosis is when you have both hypocapnia (low blood CO2) and alkalosis (high blood pH).

Diagnosis

Hypocapnia is diagnosed by checking the carbon dioxide level in your blood, usually by performing an arterial blood gas (ABG) test [2].

The ABG test measures the pressure exerted by the carbon dioxide in your arteries, which is expressed as pCO2. This test also measures the pressure of oxygen (pO2) and the pH of the blood, which help diagnose respiratory alkalosis [2].

Normally, your pCO2 should be between 35 – 45 mmHg. Levels below this indicate hypocapnia [4].

On the other hand, if your pCO2 levels are above normal, you may have a condition called hypercapnia (the opposite of hypocapnia).

Hypocapnia is diagnosed with a blood test that measures the pressure of carbon dioxide on your arteries. Values below 35-45 mmHg point to hypocapnia.

Hypocapnia Causes

Normally, your body maintains a balance between the steady amount of CO2 that is produced by burning calories and the CO2 you breathe out. Hypocapnia occurs when this balance is disrupted, which usually happens when the amount of CO2 you breathe out increases [2].

In almost all cases, hypocapnia is caused by rapid or deep breathing, also known as hyperventilation. During hyperventilation, more CO2 is exhaled from the lungs, which decreases the amount of CO2 in the blood [2].

Many conditions and factors that can increase your breathing rate and cause you to hyperventilate, including [2]:

  • Stress
  • Anxiety or panic disorder
  • High altitudes
  • Fever
  • Pain
  • Pregnancy
  • Head injury
  • Heart disorders, such as heart failure, low blood pressure, and blood clots
  • Lung disorders, such as asthma and pneumonia

Certain medications are known to cause hyperventilation as well, including [2]:

Hyperventilation commonly causes hypocapnia. Anxiety, fever, lung disorders, and many other factors can cause hyperventilation.

Hypocapnia Symptoms

Common Symptoms

Hypocapnia is well tolerated and the symptoms are usually mild. Some common symptoms include [2]:

  • Tingling sensations (“pins and needles”), usually in the limbs
  • Abnormal heartbeat
  • Muscle cramps

Seizures can occur in severe cases [2].

Hypocapnia causes respiratory alkalosis when the pH of the blood becomes too high. Hyperventilation can also decrease the oxygen supply in the body. These two factors may do additional harm in severe cases [2].

Brain Damage

The brain is especially sensitive to imbalances in blood pH and oxygen supply. Even very brief disruptions in oxygen supply can lead to permanent brain damage [2].

In adults, the effects of severe hypocapnia on the brain include [2]:

  • Worse reaction times
  • Decreased cognition
  • Slower movement
  • Personality changes

Hypocapnia is also a great concern in newborns, especially premature infants. Studies show that severe hypocapnia (pCO2 < 15 mmHg) may impair the blood supply to the brain and deplete antioxidants, which can cause permanent brain damage in babies [2].

Severe hypocapnia can damage the brain and impair cognition.

Heart Problems

Hypocapnia not only decreases the oxygen supply of the body but also increases the body’s oxygen demand. This is because the heart has to work harder during hypocapnia and requires more oxygen. If the muscles in the heart don’t receive enough oxygen, they can become injured [2, 5].

Research also shows that hypocapnia may increase the risk of blood clots and cause the heart to beat abnormally [6, 7].

Hypocapnia burdens the heart and can lead to excessive blood clotting and abnormal heartbeat.

Lung Damage

When your CO2 levels are low, your body’s automatic response is to narrow the airways of your lungs, also known as bronchoconstriction. Your lungs also often stretch out more during hyperventilation. These two factors combined can lead to damage to the lungs [2].

People with asthma are especially vulnerable. Research shows that hypocapnia in asthma patients can reduce lung function, worsen symptoms, and lower quality of life [8].

Hypocapnia can narrow your airways, causing breathing difficulties. It’s especially dangerous in people with asthma.

Pregnancy Complications

Pregnant women naturally have pCO2 levels that are about 10 mmHg lower than nonpregnant women. However, if carbon dioxide levels in pregnant women drop further, there may be negative effects on the baby [2].

Two studies found that a brief drop in CO2 levels seen in mothers during a C-section may negatively affect the baby by decreasing oxygen levels and delaying the start of breathing. Administering CO2 may help prevent these effects [9, 10].

Another study in rabbits found that decreased CO2 levels in the fetus may impair lung development [11].

Pregnant women normally have lower CO2 levels, but abnormally low values during a C-section can harm the baby.

Treatment of Hypocapnia and Hyperventilation

What do you do when hyperventilating? A popular method is for someone who is hyperventilating to breathe into a paper bag. And it’s for a good reason: rebreathing into a paper bag increases your CO2 levels because you are re-inhaling the CO2 you exhaled [1].

However, rebreathing into a paper bag can also be dangerous, mainly because it reduces the amount of oxygen you are breathing in. For this reason, paper bag breathing is no longer recommended for hyperventilation or hypocapnia [12].

Instead, initial treatment of a hyperventilation episode usually involves using breathing techniques to slow down the breathing rate, such as counting between breaths or focusing on the second hand of a clock. The underlying cause of hyperventilation also needs to be addressed. Situational treatments for hyperventilation include [1, 13]:

  • Anti-anxiety medications for anxiety-induced hyperventilation
  • Antibiotics for patients with infections
  • Ventilator support in severe cases
  • Acidic agents for high blood pH
Initial treatment of hyperventilation usually involves breathing techniques to slow the breathing rate. Breathing into a paper bag is not recommended.

Ways to Avoid Hypocapnia

Breathing Techniques

However, it’s not entirely clear how effective breathing exercises are for those who hyperventilate. A lack of research makes it difficult to say one way or another, but anecdotal evidence suggests breathing exercises are helpful for some [14]. With this caveat in mind, let’s discuss some breathing exercises.

Some examples include [14]:

  • Diaphragmatic breathing, which focuses on using the diaphragm (a muscle in the lower part of the chest) to breathe into the belly
  • Buteyko breathing, which focuses on nasal breathing and reducing breathing rate and depth
  • Yogic breathing, which, generally speaking, uses various techniques and mental concentration to slow breathing
Diaphragmatic, Buteyko, and yogic breathing are techniques that may help you take control of your breathing. They may help you avoid hyperventilation and hypocapnia. Though the research on this is far from conclusive.

More About Carbon Dioxide

For more information on carbon dioxide, check out our three-part series:

Irregular CO2 Levels?

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Takeaway

Hypocapnia is when the carbon dioxide level in your blood drops below normal. Respiratory alkalosis, a condition where the pH of the blood becomes too high, is very closely linked to hypocapnia.

The most common cause of hypocapnia is hyperventilation, which causes more carbon dioxide to be exhaled out. Symptoms of hypocapnia are generally mild and may include tingling sensations, muscle cramps, and an abnormal heartbeat.

Breathing exercises are typically used to prevent and treat hyperventilation and hypocapnia, especially if the underlying cause is anxiety. Some examples include diaphragmatic breathing, Buteyko breathing, and yogic breathing.

About the Author

Mathew Eng

PharmD
Mathew received his PharmD from the University of Hawaii and an undergraduate degree in Biology from the University of Washington.
Mathew is a licensed pharmacist with clinical experience in oncology, infectious disease, and diabetes management. He has a passion for personalized patient care and believes that education is essential to living a healthy life. His goal is to motivate individuals to find ways to manage their chronic conditions.

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