Hypocapnia (Respiratory Alkalosis) Causes & Symptoms

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 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 the blood becomes lower than normal.

In this article, we’ll break down what causes hypocapnia, what symptoms are typically associated, and when to see a doctor.

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 (HCO_3-) 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 people 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. Your healthcare provider will also do a physical exam [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].

Lab results are commonly shown as a set of values known as a “reference range”, which is sometimes referred to as a “normal range”. A reference range includes the upper and lower limits of a lab test based on a group of otherwise healthy people.

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 the arteries. Values below 35-45 mmHg point to hypocapnia.

Some lab-to-lab variability occurs due to differences in equipment, techniques, and chemicals used. Don’t panic if your result is slightly out of range — as long as it’s in the normal range based on the laboratory that did the testing, your value is normal.

However, it’s important to remember that a normal test doesn’t mean a particular medical condition is absent. Your doctor will interpret your results in conjunction with your medical history and other test results.

Have in mind that a single test isn’t enough to make a diagnosis. Your doctor will interpret this test, taking into account your medical history and other tests. A result that is slightly low/high may not be of medical significance, as this test often varies from day to day and from person to person.

Hypocapnia Causes

Causes shown here are commonly associated with hypocapnia. Work with your doctor or other health care professional for an accurate diagnosis.

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 most 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
Severe anemia
Fever
Pain
Pregnancy
Head injury or other trauma
Heart disorders, such as heart failure, low blood pressure, and blood clots
Lung disorders, such as asthma and pneumonia
Liver disease
Tumors

Overdose of certain medications is also known to cause hyperventilation as well, including [2]:

Aspirin (when taken in excessive amounts)
Methylxanthines, a class of compounds that include theophylline, caffeine, and theobromine
Beta-agonists, a group of drugs used to treat asthma, COPD, and heart failure
Progesterone, a hormone used to improve fertility in women and treat menopausal symptoms

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

Hypocapnia Symptoms

Common Symptoms

Hypocapnia is well tolerated and the symptoms are usually mild. Symptoms may vary depending on the underlying cause. 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].

When to See a Doctor

Urgently see your doctor or go to the nearest hospital if you have any of the symptoms described above or if you are experiencing respiratory problems, such as difficulty breathing or hyperventilation.

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 the 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 might harm the baby.

Treatment of Hypocapnia and Hyperventilation

The treatment of hypocapnia/respiratory alkalosis depends on the underlying cause.

Symptoms of hyperventilation can be similar to the symptoms of other serious health conditions, which is why it’s paramount to seek medical help if you experience them.

Some options your doctor might bring up for hyperventilation- and anxiety-related causes are covered below.

These strategies should only be used if a doctor has confirmed the exact cause of hyperventilation.

Anxiety-Related Causes

Breathing into a Paper Bag: Helpful or Not?

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.

Initial treatment of hyperventilation usually involves breathing techniques to slow the breathing rate. Breathing into a paper bag is not recommended.

Breathing Techniques

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 [13].

With this caveat in mind, let’s discuss some breathing exercises.

Suggested techniques may include [13]:

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

Anecdotally, diaphragmatic, Buteyko, and yogic breathing are techniques that may help better control breathing, but the research on them is far from conclusive.

Other Potential Causes

Most importantly, the underlying cause of hyperventilation needs to be addressed. Situational treatments for hyperventilation include [1, 14]:

Anti-anxiety medications for anxiety-induced hyperventilation
Antibiotics for patients with infections
Ventilator support in severe cases
Acidic agents for high blood pH

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 may include tingling sensations, muscle cramps, and an abnormal heartbeat.

The treatment of hypocapnia/respiratory alkalosis depends on the underlying cause. People who are experiencing hyperventilation should urgently see a doctor.

If the cause is anxiety, various breathing exercises might help slow breathing and reduce hyperventilation.

Learn More About Carbon Dioxide

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