Blood gas tension

Blood gas tension refers to the partial pressure of gases in blood.^[1] There are several significant purposes for measuring gas tension.^[2] The most common gas tensions measured are oxygen tension (P_xO₂), carbon dioxide tension (P_xCO₂) and carbon monoxide tension (P_xCO).^[3] 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.^[3] 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.^[4][5][6]

Venous blood oxygen tension (normal)

P_vO₂ – Oxygen tension in venous blood at sea level is between 30 mmHg and 40 mmHg.^[6][7]

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.^[8]

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.^[9]

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.^[9]

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.^[10] 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.^[10][11]

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.^[12]

Oxygen saturation

${\displaystyle {\ce {SO2}}=\left({\frac {23,400}{{\ce {PO2}}^{3}+150{\ce {PO2}}}}+1\right)^{-1}}$ 

This is an estimation and does not account for differences in temperature, pH and concentrations of 2,3 DPG.^[13]

See also

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

References

1. Severinghaus JW, Astrup P, Murray JF (1998). "Blood gas analysis and critical care medicine". Am J Respir Crit Care Med. 157 (4 Pt 2): S114-22. doi:10.1164/ajrccm.157.4.nhlb1-9. PMID 9563770.
2. Bendjelid K, Schütz N, Stotz M, Gerard I, Suter PM, Romand JA (2005). "Transcutaneous PCO2 monitoring in critically ill adults: clinical evaluation of a new sensor". Crit Care Med. 33 (10): 2203–6. doi:10.1097/01.ccm.0000181734.26070.26. PMID 16215371.
3. Yildizdaş D, Yapicioğlu H, Yilmaz HL, Sertdemir Y (2004). "Correlation of simultaneously obtained capillary, venous, and arterial blood gases of patients in a paediatric intensive care unit". Arch Dis Child. 89 (2): 176–80. doi:10.1136/adc.2002.016261. PMC 1719810. PMID 14736638.
4. Shapiro BA (1995). "Temperature correction of blood gas values". Respir Care Clin N Am. 1 (1): 69–76. PMID 9390851.
5. Malatesha G, Singh NK, Bharija A, Rehani B, Goel A (2007). "Comparison of arterial and venous pH, bicarbonate, PCO2 and PO2 in initial emergency department assessment". Emerg Med J. 24 (8): 569–71. doi:10.1136/emj.2007.046979. PMC 2660085. PMID 17652681.
6. Chu YC, Chen CZ, Lee CH, Chen CW, Chang HY, Hsiue TR (2003). "Prediction of arterial blood gas values from venous blood gas values in patients with acute respiratory failure receiving mechanical ventilation". J Formos Med Assoc. 102 (8): 539–43. PMID 14569318.
7. Walkey AJ, Farber HW, O'Donnell C, Cabral H, Eagan JS, Philippides GJ (2010). "The accuracy of the central venous blood gas for acid-base monitoring". J Intensive Care Med. 25 (2): 104–10. doi:10.1177/0885066609356164. PMID 20018607.
8. Adrogué HJ, Rashad MN, Gorin AB, Yacoub J, Madias NE (1989). "Assessing acid-base status in circulatory failure. Differences between arterial and central venous blood". N Engl J Med. 320 (20): 1312–6. doi:10.1056/NEJM198905183202004. PMID 2535633.
9. Williams AJ (1998). "ABC of oxygen: assessing and interpreting arterial blood gases and acid-base balance". BMJ. 317 (7167): 1213–6. doi:10.1136/bmj.317.7167.1213. PMC 1114160. PMID 9794863.
10. Hansen JE (1989). "Arterial blood gases". Clin Chest Med. 10 (2): 227–37. PMID 2661120.
11. Tobin MJ (1988). "Respiratory monitoring in the intensive care unit". Am Rev Respir Dis. 138 (6): 1625–42. doi:10.1164/ajrccm/138.6.1625. PMID 3144222.
12. "Oxygen Content". Retrieved October 7, 2014.
13. Severinghaus, J. W. (1979). "Simple, accurate equations for human blood O₂ dissociation computations" (PDF). J Appl Physiol. 46 (3): 599–602. doi:10.1152/jappl.1979.46.3.599. PMID 35496.