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| Parameters that reflect the carbon dioxide content of blood |
Apr 2008 |
Chris Higgins |
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Health demands that despite quite significant variation in
its rate of production, the amount of carbon dioxide in blood is
maintained within well-defined narrow limits. The homeostatic
mechanisms that maintain the normal content of carbon dioxide in
blood are disturbed in a range of respiratory and metabolic
diseases and in these circumstances measurement is clinically
useful. Three parameters generated during arterial blood gas
analysis reflect the carbon dioxide content of blood. They are:
partial pressure of carbon dioxide (pCO2),
bicarbonate concentration (HCO3-) and total
concentration of carbon dioxide (ctCO2). The
main purpose of this article is to define these parameters and the
relationship between them. Of the three, only
blood pCO2 is actually measured during
blood gas analysis, the other two being derived by calculation from
measured pCO2 and pH. Consideration will
be given to the validity of this calculation. Routine chemical
profiling of blood plasma/serum often includes measurement of total
CO2 (sometimes referred to as ‘plasma/serum
bicarbonate’). This measured parameter and its relationship with
calculated total CO2 and bicarbonate generated during
blood gas analysis will also be discussed. The article begins with
a brief overview of the mechanisms by which carbon dioxide is
transported in blood and the significance of carbon dioxide for
maintenance of normal blood pH (acid-base balance). |
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