Bayer HealthCare Rapidlab 800 - Manual (Page 629)

Operating Principles IĆ9
pH is clinically significant as a means of determining acid-base disturbances.
Acid-base disorders can result in several pathologic conditions. An acid-base
disorder resulting initially from ventilatory dysfunction is called a primary
respiratory acidosis or alkalosis, while a disorder due to renal or gastrointestinal
inadequacy is referred to as metabolic acidosis or alkalosis. Using acceptable
therapeutic ranges, a pH less than 7.3 indicates acidosis, and a pH greater than 7.5
indicates alkalosis.
14
pH Sensor
The pH sensor, which is based on ISE technology, is a half-cell that forms a
complete electrochemical cell when combined with the external reference sensor. It
contains a silver/silver chloride wire surrounded by a buffer solution. A glass
membrane that is highly sensitive and specific for hydrogen ions separates the
sample from the solution.
Figure IĆ5.āăpH Sensor
Silver/Silver Chloride
Wire
Buffer Solution
Sample Path
As the sample comes in contact with the membrane of the pH sensor, a membrane
potential develops due to the exchange of hydrogen ions in the membrane. The
silver/silver chloride inner conductor transmits the potential to a voltmeter where it
is compared to the constant potential of the reference sensor. The final measured
potential reflects the hydrogen ion concentration of the sample and is used to
report the pH value of the sample.
Carbon Dioxide Tension (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
). CO
2
exists in a dynamic state in the blood as seen in the
following equation:
CO
2
+ H
2
O
{ H
2
CO
3
{ HCO
3
– + H +