Siemens Rapidlab 1200, Operator'S Manual, Page: 56 / 448

1-36 Rapidlab 1200 Operator’s Guide: System Overview and Intended Use
02087462 Rev. V
The levels of HCO
3
–
, H
2
CO
3
, and dissolved CO
2
play a major role in maintaining the pH
in blood. This relationship is best described through the Henderson-Hasselbalch equation:
acid
base
pK pH log
+ =
Substituting HCO
3
–
as the base and dissolved CO
2
and H
2
CO
3
as the acid, the equation
reads as follows:
3 2 2
3
log
CO H CO
HCO
pK pH
+
+ =
−
Taking the equation further, pH is seen as being proportional to the acid-base relationship:
3 2
3
CO H
HCO
pH
−
α
Although other acids and bases are present in the blood, the H
2
CO
3
/HCO
3
–
relationship is
sensitive and dynamic and typically reflects other acid-base changes.
When the measurement of the partial pressure of carbon dioxide ( p CO
2
) in the blood is
combined with the measured pH, the values can be incorporated into the
Henderson-Hasselbalch equation to determine HCO
3
–
and c tCO
2
. Because the p CO
2
value is proportional to the content of dissolved CO
2
/HCO
3
–
, the value for p CO
2
can be
used along with pH not only to calculate HCO
3
–
but also to aid in the differentiation of
acid-base abnormalities.
This analyte reflects the overall respiratory status. Thus high p CO
2
indicates respiratory
suppression or failure, whereas low p CO
2
indicates hyperventilation (which in turn may
stem from hypoxia, anxiety, fever, cerebral disease, cirrhosis, or excessive mechanical
ventilation). Extreme abnormalities of
p CO
2
reflect a potentially life-threatening
pathophysiologic state that must be corrected promptly.
6,7
Together, pH and p CO
2
provide a more definitive diagnostic tool in assessing respiratory
function. An increase in the p CO
2
value and a decrease in pH indicates respiratory
acidosis, a condition in which CO
2
is retained by the lungs. A decrease in the p CO
2
value
and an increase in pH indicates respiratory alkalosis, a condition in which the lungs are
expiring too much CO
2
relative to the amount produced.