Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension; the most common gas tensions measured are oxygen tension (PxO2), the carbon dioxide tension (PxCO2) and carbon monoxide tension (PxCO). The subscript x in each symbol represents the source of the gas being measured; "a" meaning arterial, "A" being alveolar, "v" being venous, "c" being capillary. Blood gas tests (such as arterial blood gas tests) measure these partial pressures.
Contents
Oxygen tension
PaO2 – Partial pressure of oxygen at sea level (765 mmHg) in arterial blood is between 75 mmHg and 100 mmHg.
PvO2 – Oxygen tension in venous blood at sea level is between 30 mmHg and 40 mmHg.
Carbon dioxide tension
Carbon dioxide is a by-product of food metabolism and in high amounts has toxic effects including: dyspnea, acidosis and altered consciousness.
PaCO2 – Partial pressure of carbon dioxide at sea level (765 mmHg) in arterial blood is between 35 mmHg and 45 mmHg.
PvCO2 – Partial pressure of carbon dioxide at sea level in venous blood is between 40 mmHg and 50 mmHg.
Carbon monoxide tension
PaCO – Partial pressure of CO at sea level (765 mmHg) in arterial blood is approximately 0.02. It can be slightly higher in smokers and people living in dense urban areas.
Significance
The partial pressure of gas in blood is significant because it is directly related to ventilation and oxygenation. When used alongside the pH balance of the blood, the PaCO2 and HCO3 (and Lactate) suggest to the health care practitioner which interventions, if any, should be made. A survey of healthy individuals was done to measure the "normal" values of blood gas pressures and how it varies by age, sex, weight and height. It was also found these values will depend on barometric pressure, and thus altitude. Online calculators exist that will compute the predicted normal values of blood gas tensions and pH based on a patient's age, height, sex, and weight as well as the barometric pressure.
Oxygen content
The constant, 1.36, is the amount of oxygen (ml at 1 atmosphere) bound per gram of hemoglobin. The exact value of this constant varies from 1.34 to 1.39, depending on the reference and the way it is derived. The constant 0.0031 represents the amount of oxygen dissolved in plasma. The dissolved-oxygen term is generally small relative to the term for hemoglobin-bound oxygen, but becomes significant at very high PaO2 (as in a hyperbaric chamber) or in severe anemia.
Oxygen saturation
This is an estimation and does not account for differences in temperature, pH and concentrations of 2,3 DPG.