COPD, or chronic obstructive pulmonary disease, commonly consists of chronic obstructive bronchitis and emphysema. The Merck Manuals Online Medical Library reports that about 12 million people in the United States have COPD and in 2003, COPD caused 122,000 deaths. The most common cause of COPD is smoking tobacco products. The measurement of arterial blood gases, called the ABG test, is an indicator of pulmonary function and can be used in conjunction with other breathing tests and chest X-rays to diagnose COPD.
Oxygen is an arterial blood gas and is required by all cells of the body for metabolism. Oxygen is breathed in through the lungs, where it dissolves into blood. Once in the blood, oxygen is bound to the protein, hemoglobin inside red blood cells. The red blood cells move through vessels and transport oxygen to all tissues of the body. A decrease in airflow in the lungs, caused by COPD may reduce the level of oxygen in the blood.
During metabolism, carbon dioxide is released as a waste product and must be removed from the body. Carbon dioxide is absorbed into the blood stream from tissues and primarily transported in the blood as bicarbonate. The amount of carbon dioxide dissolved in the blood correlates to blood acidity, also known as pH. Carbon dioxide is released from the blood into the alveolar air sacs in the lung and expelled from the airways. The process of moving oxygen into the body and carbon dioxide out is called gas exchange. A person with COPD may exhibit increased levels of carbon dioxide, bicarbonate and blood pH.
An article on MedLine Plus reports that normal respiratory function is indicated by a partial pressure of oxygen that is between 75 and 100 millimeters of mercury, or mmHg, a partial pressure of carbon dioxide between 35 and 35 mmHg, a pH between 7.35 and 7.45, and a bicarbonate concentration of 22-26 milliequivalents per liter. A person with COPD may have an increased pH and partial pressure of carbon dioxide because not enough carbon dioxide is being exchanged in the lungs. People with COPD may also display low levels of dissolved oxygen and oxygen saturation. Oxygen saturation is a measurement of how much oxygen is bound to hemoglobin and is expressed as a percentage. Oxygen saturation less than 94 percent indicates a decrease in respiratory function.