Abbreviations: ECS, extracellular space; ICS, intracellular space.
shrinkage of osmosensitive neurons and thereby to osmotic-mediated ADH-stimulation (25). ADH reduces the excretion of free water and enhances the development of hyponatremia.
Excessive thirst and drinking of hypotonic fluids in the presence of high ADH levels may lead to hyponatremia. Only when a fluid uptake is not possible anymore might hypernatremia develop, indicating severe hypertonic dehydration with poor clinical prognosis.
An increase in glucose of 500 mg/dL moves about 1L of water from intracellular to the extracellular space. Therefore one can calculate that an increase of only 100 mg/dL glucose leads to a decrease of the plasma sodium by about 1.7mmol/L.
Osmotic diuresis leads to loss of electrolytes, mainly sodium and potassium as well as free water. The consequence is an increase in sodium and osmolality in serum, if fluid intake is not increased simultaneously.
The treatment of hyperglycemic patients with insulin lowers osmolality in plasma and consequently water is shifted to the intracellular space. This leads to a swelling of cells on one hand, and to an increase of sodium concentration in serum on the other hand (26). This is a reason why patients with initial normal sodium concentrations in plasma rarely develop hypernatremia. The extent of the expected hypernatremia can be estimated via calculation of the corrected sodium concentration. This is an indication of the sodium concentration, which would appear after insulin therapy (27). The calculation is: [corrected Na+] = [measured Na+] + [delta glucose (mg/dL)/42].
Example: glucose 600 mg/dL, measured plasma Na+: 130mmol/L, delta glucose: 600-100=500; corrected sodium concentration: 130mmol/L + 12=142 mmol/L.
Volume substitution with ringer or isotonic saline solution will adjust sodium losses. To avoid cerebral pontine myelinolysis, plasma sodium concentration should not be increased by more than 12 mmol/L per day. Hypertonic sodium solution (e.g., 3% saline solution) should be avoided even in severe hyponatremia (28). In cases of severe hypernatremia (>150 mmol/L) or hyperosmolality (>320 mOsm/L) volume substitution should be performed with half isotonic saline solution or hyposmolar ringer solution.
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