Principles of electrolyte disturbance

Pathophysiology

Serum electrolytes concentrations are usually maintained within a narrow range by complex renal and endocrine homeostatic mechanisms.

Endocrine and renal systems finely tune absorption, distribution and excretion of electrolytes.

Derangements occur when there are

1. Primary impairments in renal/endocrine homeostatic mechanisms
2. Marked changes in intake, excretion or body compartment shifts which cannot be compensated
3. Drug/Toxic effects that influence the above

Deficits are due to:

• Inadequate intake/absorption (rare except in prolonged fasting)
• Excessive losses (3 main sources - kidney - urine, GIT - vomiting/diarrhoea/fistulas, skin)
• Body compartment shifts (e.g. sodium/glucose, potassium/hydrogen)

Excesses are due to

• Excessive intake/absorption (rare except in inappropriate IV therapy or drug/food intake)
• Inadequate excretion (i.e. via kidney)
• Body compartment shifts (e.g. potassium from intracellular fluid, calcium from bone)

In addition:

For predominantly extracellular electrolytes e.g.  Na, serum concentration is a function of total body electrolyte content and total body water content. 

Therefore, low sodium concentration may be due to either inadequate sodium or excess water

Clinical symptoms

The degree of symptoms are both a function of the severity of derangement and the rapidity of onset

e.g. chronic renal failure patients can tolerate higher serum potassium levels

see Clinically significant electrolyte disturbances

Complications

Serious complications are associated with impairment of:

Respiratory function

• laryngospasm/tetany - hypocalcaemia
• hypoventilation - hypoermagnesemia

Cardiac function

• Arrhythmias - hypokalaemia/hyperkalaemia, hypermagnesaemia
• Hypotension - hypocalcaemia, hypermagnesaemia, hypophophataemia

Brain function

• Cerebral oedema - hyponatraemia
• Seizures/Coma - various

Management

• The urgency of treatment depends on the degree of derangement and the acuity of onset
• Directed first to resuscitation issues - maintaining airway, assist ventilation, treating arrhythmias, managing hypotension
• Directly correcting the derangement or moderating its effects (e.g. calcium in hyperkalaemia)
• Treating the underlying cause

NB rapidly treating electrolyte disturbances can also cause complications! In non-life threatening disturbances, slow correction over 48 hours is recommended.