Siemens RAPIDLAB 248 - Page 130

Page G-6 248 Operator’s Manual
pCO
2
Carbon dioxide (CO
2
) is produced during normal cell metabolism and is
released into the blood stream where it is transported to the kidneys
and lungs for excretion. CO
2
is transported through the blood as
bicarbonate (HCO
3
–
), dissolved CO
2
, and carbonic acid (H
2
CO
3
).
The levels of HCO
3
-
, H
2
CO
3
, and dissolved CO
2
play a major role in
maintaining the pH in blood. pH is proportional to the acid-base
relationship.
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 (pCO
2
)
in the blood is combined with the measured pH, the values can be
incorporated into the Henderson-Hasselbalch equation to determine the
HCO
3
–
in addition to the ctO
2
. Since the pCO
2
value is proportional to
the content of dissolved CO
2
/HCO
3
–
, the value for pCO
2
can be used
with pH not only to calculate HCO
3
–
, but also to aid in the
differentiation of acid-base abnormalities.
The measurement of pCO
2
is essential in determining ventilatory
status. Because the lungs are primarily responsible for controlling pCO
2
levels, changes in pCO
2
reflect respiratory status. For example, an
increase in CO
2
indicates decreased ventilation as CO
2
is retained, and
a decrease in CO
2
indicates increased ventilation (hyperventilation) as
CO
2
is expired from the lungs.
Together, pH and pCO
2
provide a more definitive diagnostic tool in
assessing respiratory function. An increase in the pCO
2
value and a
decrease in pH indicates respiratory acidosis - a condition where CO
2
is
retained by the lungs. A decrease in the pCO
2
value and an increase in
pH indicates respiratory alkalosis - a condition where the lungs are
expiring too much CO
2
relative to the amount produced.
pO
2
Oxygen (O
2
) is essential for cell and tissue metabolism in the body. The
cardiopulmonary system is responsible for transporting oxygen to the
cells. Oxygen transport involves four major steps: convection and
diffusion from the air into the pulmonary circulation, combination of O
2
from the lungs with haemoglobin in red blood cells, transportation of
the O
2
through the arteries to the cell, and finally the release into the
tissues and utilization of O
2
at cellular level.
Since it is not possible to measure intra-cellular oxygen tension,
arterial pO
2
has become a standard for clinical evaluation of arterial
oxygenation status. pO
2
(A) measurement, which indicates the oxygen
tension in arterial blood, reflects the pressure or driving force for
moving oxygen from one location to the next due to pressure
differential. Although not a measurement of the O
2
content, this
provides a measurement tool to evaluate the pulmonary gas exchange
efficiency from an arterial blood sample.