Partial Pressure of Carbon Dioxide (PaCO2)

Why is the Partial Pressure of Carbon Dioxide (PaCO2) Important?

Doctor Auscultating a patient
What is the partial pressure of carbon dioxide (PaCO2) and why is it important in COPD?. Getty Images/Universal Images Group

When you have COPD, your health care provider will most likely want to know what your partial pressure of carbon dioxide, or PaCO2, level is. PaCO2 is measured by arterial blood gases. Partial pressure refers to the pressure exerted by a specific gas in a mixture of other gases.  The partial pressure of carbon dioxide at sea level is ordinarily 38 to 42 mm Hg.

Normal PaC02 levels in the blood range from around 40 in the alveoli to around 45 in capillary blood in the lungs.

If your PaCO2 is higher than 45 mm Hg, then you have too much carbon dioxide in your blood. This is also referred to as hypercapnia. Depending upon how high your levels are, your health care provider may take steps to correct an imbalance.

What Are Arterial Blood Gases (ABGs)?

The partial pressure of carbon dioxide is measured using arterial blood gases (ABGs). Using a needle, a health care professional will draw blood from an artery to assess the effects of cardiopulmonary pathology or disease on ventilation and oxygenation. In addition to measuring PaCO2 levels, ABGs measure partial pressure of oxygen (PO2) and pH.

Normal levels of blood pH range from 7.36 to 7.44.

Normal range of PCO2 is 38-42

In hospitalized patients, the levels of PaCO2 can be elevated due to retention of carbon dioxide thus leading to high oxygenation levels that are in fact misleading. To truly determine oxygenation, an ABG must be drawn.

How Exactly Is an ABG Done?

As blood passes through the lungs, oxygen moves into the blood and carbon dioxide passes out. An ABG is used to assess this blood as it circulates to oxygenate entire body. An ABG is usually performed on the radial artery in the wrist or the femoral artery in the groin.

When done by an experienced healthcare professional, an ABG usually doesn't result in any complications.

However, an ABG can be painful because arteries are located deeper in the body than veins, which are more superficial. After an ABG, swelling and bruising may occur.

Partial Pressure of Carbon Dioxide (PaCO2) in Respiration

Whereas oxygen diffuses from the alveoli (the tiniest of air sacs in the lungs) to capillaries in the lung to be distributed through the body, CO2 diffuses from the capillaries into the alveoli to be exhaled in the next breath.  This process is referred to as gas exchange.

The partial pressure of carbon dioxide (PaCO2) is important in that gases travel from an area of higher concentration to an area of lower concentration by diffusion.

The normal PaCO2 in blood returning from the body and entering the capillaries is around 45 mm Hg, whereas that in the alveoli is ordinarily around 40 mm Hg.  This creates the concentration gradient which results in CO2 diffusing from the capillaries into the alveoli to be exhaled.

Conditions Which Can Impact PaCO2

There are several conditions which may result in an increased level of PaCO2 in the blood, including:

  • Central nervous system depression (such as with drugs, or head injuries)
  • Obstructive lung disease such as COPD and asthma
  • Neuromuscular disease such as ALS
  • Decreased strength of the muscles of respiration such as the intercostal muscles and the diaphragm

Most of the time an increased level of PaCO2 results from hypoventilation - decreased numbers or volume of breaths - whether that occurs due to inability to take deep breaths, central nervous system depression, or lung disease.

Importance and Significance of PaCO2 in COPD

Carbon dioxide is in equilibrium with bicarbonate in the blood, which when elevated, creates an acidic environment in the blood.  When breathing is diminished due to hypoventilation, the increased levels of carbon dioxide in the blood (referred to as hypercapnia) result in what is known as respiratory acidosis.

Lung diseases such as COPD don't usually result in hypoventilation until the disease is very advanced.  Early on, COPD in contrast often results in a low PaCO2 due to an increased breathing effort and rate in an attempt to increase the oxygen saturation in the blood.  In late stage COPD, however, severe muscle weakness in the respiratory muscles can alter this process, and carbon dioxide may then build up in the blood leading to respiratory failure.

A long standing concern has been that giving oxygen to someone with severe COPD would result in apnea (no breathing.)  Understanding PaCO2 can help to explain this.  The level of PaCO2 and PaO2 are detected by chemoreceptors in the aorta and carotid body, which in turn communicates with the part of the brain known as the medulla.  The medulla in turn is responsible for controlling breathing effort. When the level of oxygen is low or the level of carbon dioxide is high, the body is stimulated to increase breathing efforts.   Physicians were warned about giving patients oxygen, which in theory could induce hypercapnia.  Despite concerns, this problem does not appear to be quite as significant as once thought.

Sources:

Abdo, W., and L. Heunks. Oxygen-induced hypercapnia in COPD: myths and facts. Critical Care. 2012. 16(5):323.

U.S. National Library of Medicine. MedlinePlus. Blood gases. Udpated 08/25/14. https://www.nlm.nih.gov/medlineplus/ency/article/003855.htm

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