Siemens Rapidlab 1200, Руководство оператора, Страница: 65 / 448

Rapidlab 1200 Operator’s Guide: System Overview and Intended Use 1-45
02087462 Rev. V
When measuring ionized calcium, pH should also be measured. Because
hydrogen ions compete with calcium for calcium binding sites, a change in
sample pH can have a direct effect on calcium levels. For example, a change in pH
of 0.1 can cause a change in calcium of 0.2 mg/dL, which exceeds the span of the
normal range. Its effects, if not taken into account, are clearly significant.
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Calcium Sensor
The calcium sensor is a half-cell that combines with the external reference sensor
to form a complete electrochemical cell capable of measuring calcium levels in a
blood sample. The sensor contains a silver and silver chloride wire surrounded by
an electrolyte solution that has a fixed concentration of calcium ions. A
membrane, consisting of an ionophore imbedded in a polyvinyl chloride
membrane, separates the electrolyte solution from the sample. The ionophore is a
compound that is highly selective for calcium ions over other ions.
When the sample comes in contact with the membrane of the measuring sensor, a
membrane potential develops as calcium ions interact with the membrane. This
membrane potential is compared to the constant potential of the external reference
sensor. The final measured potential is proportional to the calcium ion
concentration in the sample. The potential developed by the electrochemical cell
varies with the ion activity in each sample.
Metabolites
The Rapidlab 1260 and 1265 systems analyze blood samples for glucose and
lactate in addition to pH, blood gases, and electrolytes.
Concentration of Glucose
Glucose is the fundamental molecule in carbohydrate metabolism. Carbohydrates,
which provide a major food supply and energy source for the body, are broken
down into simple sugars such as glucose. Glucose is then absorbed through the
intestine, passes through the liver, and eventually enters the vascular system
where it reaches the cell level to be used as fuel.
A number of factors influence the level of blood glucose. Dietary intake has a
direct effect on glucose concentration. Blood levels of glucose will fluctuate
depending on nutritional condition and the time of day when a sample is taken.
Insulin, a hormone produced by specialized cells in the pancreas, plays an
important role in regulating the blood level of glucose. By promoting
glycogenesis (conversion of glucose to glycogen) and by increasing the
permeability of cells to glucose, insulin can decrease blood glucose levels.