Carbon Dioxide Retention in Patients with Chronic Obstructive Pulmonary Disease
Dr Claude Farah, Respiratory and Sleep Physician, Macquarie University Hospital Clinic, NSW

What is carbon dioxide?
Carbon dioxide (CO2) is a gas in the air that makes up about 0.03% of the earth's atmosphere. CO2 is also a waste gas and a by-product of the body's metabolism (biochemical processes occurring in cells and which are required to sustain life). During metabolism, oxygen is used and CO2 is produced. CO2 is chiefly removed from the body through the lungs when you breathe out. A high CO2 level is usually an indication that the lungs are not able to keep up with the bodys' needs. In a healthy person, a high CO2 level can occur suddenly during an acute illness. Some people adapt over time to a new baseline where the CO2 level in the body is higher than in healthy lungs. Some people with stable Chronic Obstructive Pulmonary Disease (COPD) can manage with a higher CO2 level than people with healthy lungs.

Effects of high CO2 levels (hypercapnia)
The presence of a high CO2 level in the blood is known as hypercapnia and can cause headaches, lethargy, drowsiness, confusion and, if severe, can lead to coma and death. People with hypercapnia may be flushed and warm to touch, and they may also show a classic flapping tremor of the hands. When asked to hold their arms out in front and bend their wrists back, they are unable to maintain the position of the hands, and as a result, the hands will flap

How does oxygen cause hypercapnia?
Too much supplemental oxygen can cause or worsen hypercapnia by a number of different mechanisms including:

1. Dramatically changing the relationship between air exchange within the lungs and blood flow within the lungs, resulting in reduced removal of CO2.
2. Reducing the drive to breathe. People with healthy lungs rely on high CO2 levels to stimulate the brain to breathe. People with COPD and high CO2 levels may become less sensitive to a high CO2 level. The person then relies more on low oxygen levels to stimulate their breathing. Giving too much oxygen removes this stimulus to breathe, and thus reduces the removal of CO2 from the lungs.
3. Both oxygen and CO2 bind to the haemoglobin in the red blood cells. Giving too much oxygen can push the CO2 from the haemoglobin into the bloodstream, causing CO2 levels to rise in the blood.

Uncontrolled oxygen therapy, or receiving too much oxygen, can make people who usually have higher CO2 levels retain more until it reaches dangerous levels.

Some people with low oxygen levels are prescribed supplemental oxygen at home as a long-term treatment for their lung condition. In this instance, the treating physician usually determines the oxygen flow within an acceptable range. It is important that oxygen therapy is used to maintain blood levels within this acceptable range and not in an effort to reduce perceived breathlessness. Breathlessness in COPD is rarely due to low oxygen levels alone. If someone experiences worsening breathlessness they need to see their doctor and address the possible causes. Patients should not increase the oxygen flow simply to treat the symptom of breathlessness.

How to decrease the risk of hypercapnia with oxygen therapy
The critical oxygen level is an oxygen saturation of approximately 90% (this is measured by a finger pulse oximeter), equivalent to a blood oxygen level of 55-60 mmHg (this is measured from a blood sample taken from an artery, commonly in the wrist). This blood test is known as an arterial blood gas or ABG. Therefore, controlled oxygen therapy, to maintain oxygen saturation at around 90% (88-92% is an acceptable range) will minimise the risk of hypercapnia. It is important to avoid too much oxygen and minimise the risk of worsening CO2 levels in this situation. Some patients who are very sensitive to the adverse effects of too much oxygen may choose to wear a medical alert bracelet to alert paramedics about their lung condition in the event of an emergency.