Blood gas tension

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 (P_xO₂), carbon dioxide tension (P_xCO₂) and carbon monoxide tension (P_xCO). The subscript x in each symbol represents the source of the gas being measured: "a" meaning arterial, "A" being alveolar, "v" being venous, and "c" being capillary.Blood gas tests (such as arterial blood gas tests) measure these partial pressures.

Oxygen tension

Arterial blood oxygen tension (normal)

P_aO₂ – Partial pressure of oxygen at sea level (160 mmHg in the atmosphere, 21% of standard atmospheric pressure of 760 mmHg) in arterial blood is between 75 mmHg and 100 mmHg.

Venous blood oxygen tension (normal)

P_vO₂ – 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.

Arterial blood carbon dioxide tension

P_aCO₂ – Partial pressure of carbon dioxide at sea level in arterial blood is between 35 mmHg and 45 mmHg.

Venous blood carbon dioxide tension

P_vCO₂ – Partial pressure of carbon dioxide at sea level in venous blood is between 40 mmHg and 50 mmHg.

Carbon monoxide tension

Arterial carbon monoxide tension (normal)

P_aCO – Partial pressure of CO at sea level 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 gas exchange, as the driving force of diffusion across the blood gas barrier and thus blood oxygenation. When used alongside the pH balance of the blood, the P_aCO₂ and HCO⁻
₃ (and lactate) suggest to the health care practitioner which interventions, if any, should be made.

Equations

Oxygen content

${\displaystyle C_{a}{\ce {O2}}=1.36\cdot {\ce {Hgb}}\cdot {\frac {S_{a}{\ce {O2}}}{100}}+0.0031\cdot P_{a}{\ce {O2}}}$

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. S_aO₂ refers to the percent of arterial hemoglobin that is saturated with oxygen. The constant 0.0031 represents the amount of oxygen dissolved in plasma per mm Hg of partial pressure. The dissolved-oxygen term is generally small relative to the term for hemoglobin-bound oxygen, but becomes significant at very high P_aO₂ (as in a hyperbaric chamber) or in severe anemia.

Oxygen saturation

${\displaystyle {\ce {SO2}}=\left({\frac {23,400}{p{\ce {O2}}^{3}+150p{\ce {O2}}}}+1\right)^{-1}}$

This is an estimation and does not account for differences in temperature, pH and concentrations of 2,3 DPG.

See also

• Alveolar air equation
• Fick's laws of diffusion
• Fraction of inspired oxygen