Siemens Rapidlab 1200, Руководство оператора, Страница: 64 / 448

1-44 Rapidlab 1200 Operator’s Guide: System Overview and Intended Use
02087462 Rev. V
This assay is used most commonly to distinguish high-anion-gap acidoses (ketoacidosis,
lactic acidosis, uremia, poisoning from several specific agents) from hyperchloremic
acidoses (loss of alkali as in severe diarrhea, hypoaldosteronism, potassium-sparing
diuretics, renal tubular acidosis). In the absence of acidosis, changes in plasma chloride
concentration tend to parallel those of sodium. Thus chloride is high in dehydration and
low in overhydrated states.
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Chloride Sensor
The chloride sensor is a half-cell that combines with the external reference sensor to form
a complete electrochemical cell capable of measuring chloride concentration in a sample.
The sensor contains a silver/silver chloride wire surrounded by an electrolyte solution that
has a fixed concentration of chloride ions. The membrane is a derivitized quaternary
ammonium compound that is immobilized in a polymer matrix. It acts as an ion exchanger
with a high selectivity for chloride ions over other ions present in the sample, and
separates the electrolyte solution from the sample.
As the sample comes in contact with the membrane of the chloride sensor, chloride ion
exchange occurs at the membrane and creates a membrane potential. The potential that
develops in the chloride sensor is compared to the constant potential of the external
reference sensor. The final measured potential is directly proportional to the chloride ion
concentration in the sample. The potential developed by the electrochemical cell varies
with the ion activity in the sample.
Concentration of Ionized Calcium
Ionized calcium (Ca
++
) is the physiologically active form of calcium, which comprises
approximately 45% of the total calcium in plasma. Ca
++
is essential for the contractility of
smooth vascular muscle and plays a vital part in cardiovascular function. Ca
++
is also
important in muscle function, nerve function, and bone formation, and it is a cofactor in
many cellular hormone and enzyme reactions.
The action of the parathyroid hormone (PTH) — 1,25 dihydroxyvitamin D (1,25D) — and
calcitonin closely controls the concentration of calcium in extracellular fluid, and
regulates the transport of calcium across the gastrointestinal tract, kidney, and bone.
Calcium is one of the most tightly controlled analytes in the body with fluctuations of less
than 5% occurring about the mean during a 24-hour period.
14
Ca
++
abnormalities typically stem from parathyroid disease, vitamin D imbalance, renal
disease, pancreatitis, drug effects, abnormalities of magnesium or phosphorus,
malignancy, or sarcoidosis. An extreme abnormality of Ca
++
may cause neuromuscular
symptoms, tetany, altered mental status, seizures, heart failure, or arrhythmias. Thus
extreme abnormalities of ionized calcium reflect a potentially life-threatening
pathophysiologic state that must be corrected promptly.
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