Sodium, potassium and chloride are the three electrolytes most commonly considered, while bicarbonate is measured much less frequently.  These ions are all freely diffusible throughout the entire ECF (plasma, interstitial fluid, lymph).

The electrolytes are all normally present to vast excess in the diet, and in the healthy individuals this excess is simply excreted via the kidney and/or gut. Dietary electrolyte deficiencies are quite difficult to organize on an experimental basis and are seldom encountered clinically; the normal excretory capacity is such that even quite alarmingly excessive intakes do not lead to clinical problems unless water is restricted.

This means that clinical fluid/electrolyte problems are associated not with dietary factors but with abnormal fluid losses, and it is the amount and composition of the fluid being lost which will determine the direction of any electrolyte abnormalities seen.

Serum sodium and potassium levels can be determined using flame photometer or spectrophotometer. The latter also used for measuring serum chloride level.

Sodium

• Sodium is the electrolyte, which is the most intimately associated with water balance and most disturbances tend to be primarily fluid problems. Symptoms are generally related to changes in volume of body fluid compartments, particularly intracellular (ICF) and/or extracellular fluids (ECF) volumes in the central nervous system.
• Hypernatraemia occur when loss of a low-sodium fluid occurs, as in vomiting, excessive panting, and diabetes insipidus. It is also seen when restricted water intake prevents normal sodium excretion.
• Hyponatraemia occurs when loss of a high-sodium fluid occurs – the most common instance of this is in renal failure when the kidney cannot concentrate the urine, and the fast urine flow through the tubules also prevents effective Na/K exchange in the loop of Henle and leads to the production of a high-sodium urine.

Potassium

• Potassium is primarily an intracellular ion and concentrations in the ECF are low.
• Hyperkalaemia occurs when loss of a low-potassium fluid is occurring. The cause of significant hyperkalaemia is nearly always a failure of the kidney to excrete potassium. However, not all renal failure cases do have high plasma potassium concentrations.  Fairly acute cases (e.g. nephrotoxicity) may indeed be hyperkalaemic, but chronic renal failure cases soon begin to compensate and may even be hypokalaemic as a result of vomiting.
• Hypokalaemia occurs most commonly due to persistent loss of a high-potassium fluid. Diarrhea is considered to be the classic instance, but note that persistent vomiting even without diarrhoea will have a similar or even more marked effect. Hypokalaemia will also be seen in patients on long-term fluid therapy being given potassium-free fluids such as dextrose saline or isotonic saline. In addition, note that prolonged use of potassium-losing diuretics (e.g. frusemide) will lead to hypokalaemia and it is essential that plasma potassium be monitored in patients on this type of treatment.

Chloride

• Chloride tends to be the least regarded of the electrolytes but can often give quite useful information.  As an anion, its concentration is affected by concentrations of the other main anion, bicarbonate. This means that in acidotic patients with low bicarbonate concentrations  chloride  is generally high, while in alkalotic patients with high bicarbonate concentrations  chloride  is generally low (in an effort to maintain the anion/cation balance). In the absence of significant acid/base disturbances plasma chloride concentration generally parallels that of sodium. Specific symptoms of chloride abnormalities (as distinct from sodium or acid/base disturbances) are not generally recognized.
• Hyperchloraemia occurs in acidosis, and is also found in nearly all conditions where hypernatraemia occurs. Note that the assessment of the severity of dehydration by measuring chloride concentration (on the assumption that as water is lost chloride concentration will increase) is not valid.
• Hypochloraemia occurs in alkalosis and in addition is often found in conditions which are associated with hyponatraemia. Hypochloraemia without hyponatraemia can occur when significant volumes of a high chloride/low sodium fluid are being lost. This generally means hydrochloric acid, i.e. gastric secretions, and so persistent vomiting just after eating is one possible cause.

Prof. Mahmoud Rushdi