Blood Gasses

• Total carbon dioxide is clinically measured by: Acidification of serum or plasma sample and measurement of carbon dioxide released Alkalinization and measurement of total bicarbonates

• Acidifying the sample converts the various forms of CO2 in plasma to gaseous CO2 by dilution with an acid buffer. • Alkalinizing the sample converts all CO2 and carbonic acid to HCO3− • In indirect electrode-based methods (ISE) the amount of released gaseous CO2 after acidification is determined by a PCO2 electrode • The enzymatic reactions (after alkalinization) lead to decrease in absorbance of NADH at 340nm in proportion to total carbon dioxide content.

• Same sample types used for Na+ or K+ may be assayed for carbon dioxide concentration, samples must be analyzed promptly • Samples must be centrifuged in unopen tubes (air contains less CO2 than does plasma, and CO2 will escape from the specimen into the air, of up to 4 to 5 mmol/L in 1 hour)

• Total carbon dioxide in adults is 22 to 28 mmol/L, instrument dependent • Alterations in bicarbonates is a sign of acid-base imbalance