Dysbalance natria (pediatrics)

Sodium imbalance is one of the most common ionic changes. Sodium jis the major cation in ECF and is crucial for the osmolality of this body compartment. In ICF, the concentration of sodium is not the same exerywhere, it ranges from 3 to 35 mmol/l, but it is always lower than in ECF. The difference in Na concectrations between ICF and ECF (sodium concentration gradient) is the result of active metabolic activity of cells, especiallly so-called sodium pumps = Na-K-ATPase. By the action of this mechanism located in the cell membrane, sodium ions are constantly expelled from the ICF into the ECF, potassium ions move in the opposite direction. In situations of energy depletion, the activity of Na-K-ATPase is paralyzed and the movement of both ions changes to the opposite.

Dietary intake of sodium sis quite individual, 4 – 15 g NaCl (ie. 70 – 250 mmol/24 hours). Most of the intake of sodium is excreted by kidneys, but significant extrarenal losses can occur through the skin during excessive sweating (especially in children with a high cincentration of Na in sweat - patients with cystic fibrosis). With good kidney function and no increase in the proportion of extrarenal losses, it is possible to calculate with a balanced sodium balance, ie. after intake, its urinary excretion corresponds. This aspect alloows a practical evaluation of the sodium balance, where the amount of Na excreted in the urine can be converted to NaCl intake.

NaCl intake in g/24 h. = urine excreted Na in mmol/24 h : 17

Example: if the amount of Na excreted was 148 mmol/24h, it means that the child receied 148: 17 = 8.7 g NaCl.

Changes in sodium concentration are always accompanied by redistribution of body water in individual compartments. ECF hyponatremia leads to a decrease in its osmolality with sucsequent transfer of water to ICF. This redistribution is accompanied by an increase in ECF osmolality. ECF hypernatremia causes exactly the opposite change. Proper kidney function is crucial for all of the above data. Dosium ions freely penetrate the glomerular membrane and their concentration in the glomerular filtrate is identical to the S-Na value. 50-70 % of sodium is resorbed in the proximal renal tubule and <1% of the total filtered sodium is finally excreted in the final urine. The value determining which part of the filtered sodium is excreted in the urine is called the fractional excretion of Na (FE Na). Determination of FE Na is often used to evaluate tubular function of the kidneys and it is also one of the auxiliary parameters to distinguish the initial cause of acute renal failure (difference between renal and prerenal ARF).

FE-Na = U-Na / S-Na : U-Kr / S-Kr (normal FE-Na is < 1%)