Bayer HealthCare Rapidlab 800 - Manual (Page 636)

IĆ16 800 Series Operator's Manual
Most potassium is excreted by the kidney, which is the major regulator of
potassium output in the body. Actually, the kidney is better at conserving sodium
and excreting potassium so in cases where potassium intake stops, the kidney
requires time to adjust and stop excreting potassium. Two hormones, insulin and
aldosterone can affect the extracellular level of potassium. Both insulin and
aldosterone influence intercellular uptake of potassium, while aldosterone causes
increased potassium excretion through the kidney.
Because the serum level of potassium is so small, minor changes can have
significant consequences. Therefore, monitoring potassium levels is important
especially in patients who are undergoing surgery, or who are experiencing cardiac
arrhythmias or acute renal failure, and who are being treated with diuretics.
Additionally, regulating serum potassium is significant in cardiac patients who are
receiving digitalis therapy since hypokalemia can increase cardiac sensitivity to
digoxin.
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Potassium Sensor
The potassium sensor is a half-cell that combines with the external reference
sensor to form a complete electrochemical cell. The sensor contains a silver/silver
chloride wire surrounded by an electrolyte solution that has a fixed concentration
of potassium ions. The membrane, which consists of the ionophore valinomycin
immobilized in a plasticized PVC (polyvinyl chloride) matrix, separates the
electrolyte solution from the sample. Valinomycin is a neutral ion carrier that is
highly selective for potassium ions over other clinically encountered cations.
As the sample comes in contact with the membrane of the potassium sensor, a
membrane potential is created by the interaction of potassium ions with the
membrane. The potential developing in the potassium sensor is compared to the
constant potential of the external reference sensor. The final measured potential is
directly proportional to the potassium ion concentration in the sample. The
potential developed by the electrochemical cell varies with the ion activity in each
sample.
Concentration of Chloride
Chloride (Cl – ) is the major extracellular anion in the body. It plays a large role in
maintaining electrical neutrality and normal osmolality, and it participates in the
regulation of acid-base balance. The kidneys are the main regulator of chloride in
the body. Serum levels of chloride usually correspond to increases and decreases of
sodium. Clinically, the serum chloride level alone is rather meaningless. A change
in chloride level does not reveal much about a patient’s condition; it must be
viewed as part of the overall fluid and electrolyte status.