Siemens Rapidlab 1200, Руководство оператора, Страница: 62 / 448

1-42 Rapidlab 1200 Operator’s Guide: System Overview and Intended Use
02087462 Rev. V
Concentration of Sodium
Sodium (Na
+
) is the most abundant cation in the extracellular space in the body. Na
+
is the
major determinant of extracellular osmotic regulation and plays a central role in
determining body fluid volume. The kidneys are the primary regulator of sodium and
consequently water volume; only minimal amounts of sodium are lost through the skin
and other insensible sites. Two regulatory hormones, aldosterone and the antidiuretic
hormone (ADH), affect kidney function and hence sodium balance. Aldosterone
stimulates the kidneys to reabsorb sodium; ADH stimulates the kidneys to reabsorb water.
Maintaining sodium homeostasis is essential in order to regulate body fluids, maintain
electrical potential in muscle cells, and control cellular membrane permeability.
Abnormal concentrations of Na
+
stem from deficit or overload of total body water or of
sodium itself. These abnormal concentrations arise from diverse clinical conditions, such
as congestive heart failure, liver disease (cirrhosis), renal disease, neuropsychiatric
disorders (causing abnormal fluid intake), intravenous fluid therapy, excessive fluid loss
(vomiting, diarrhea, heat stroke), drug therapy (diuretics), diabetes mellitus (causing
osmotic diuresis), and imbalances of hormones (ADH, mineralcorticoid, glucocorticoid)
that regulate sodium and water excretion. An extremely abnormal plasma sodium
concentration may itself directly cause altered mental status, stupor, coma, seizures, brain
swelling, brain dehydration leading to cerebral hemorrhage or, ultimately, death. Thus
extreme abnormalities of sodium reflect a potentially life-threatening pathophysiologic
state that must be corrected promptly.
6,12
Sodium Sensor
The sodium 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 sodium and
chloride ions. The membrane, a specially formulated glass capillary that is highly selective
for sodium ions over other clinically encountered cations, separates the electrolyte
solution from the sample.
As the sample comes in contact with the membrane of the sensor, a potential develops due
to the exchange of sodium ions in the membrane. The potential developing across the
membrane is compared to the constant potential of the external reference sensor. The final
measured potential is proportional to the sodium ion concentration in the sample. The
potential developed by the electrochemical cell varies with the ion activity in each sample.
Concentration of Potassium
Potassium (K
+
) is the major intracellular cation. K
+
plays an important role in maintaining
cell membrane potential in neuromuscular tissue. The normal level within cells is
150 mmol/L, while the normal extracellular potassium level is only 4 mmol/L. A depletion
of extracellular potassium causes an increase in the transmembrane electrical potential
gradient, which impedes the impulse formation and propagation involved in muscle
contraction.