LessonsPhysiological Adaptation

Potassium, sodium and calcium

Predict the findings of each major electrolyte disturbance, name the clients who develop them, and say which are treated urgently.

Potassium: excitability, up and down

Potassium sets the resting membrane potential, so both directions show up in muscle and on the monitor. The gut is what tells them apart.

  • High (above 5.0): peaked T waves, then a widening QRS; muscle weakness; cramping and diarrhea. Causes: kidney failure, potassium-sparing diuretics, ACE inhibitors, acidosis, crush injury.
  • Low (below 3.5): flattened T waves with a U wave; muscle weakness; constipation to the point of ileus. Causes: loop and thiazide diuretics, vomiting, nasogastric suction, alkalosis.
Intravenous potassium is ALWAYS diluted and always given by pump, never as a push, and the rate is capped. It is one of the few drug errors that kills within minutes.

Sodium: it is really about water

Sodium disturbances present neurologically, because the brain sits in whatever the serum is doing. Low sodium swells brain cells: confusion, headache, then seizures. High sodium shrinks them: thirst, agitation, then the same end point.

The speed of correction matters as much as the direction. Correcting a low sodium too quickly causes osmotic demyelination; correcting a high sodium too quickly causes cerebral edema. Either way the answer to "how fast" is slowly, with frequent neurological checks.

  • Low: SIADH, heart failure, excessive plain water, diuretics, adrenal insufficiency.
  • High: dehydration, diabetes insipidus, tube feeds without enough water, an unconscious client who cannot ask for a drink.

Calcium: the opposite of what you would guess

Low calcium makes nerves MORE excitable, not less: tingling around the mouth and fingers, muscle twitching, Chvostek and Trousseau signs, and eventually tetany and laryngospasm. High calcium makes everything sluggish: weakness, constipation, confusion, and kidney stones.

The clients to expect it in are the ones the exam uses: after thyroid or parathyroid surgery, in chronic kidney disease, after multiple transfusions, and — for high calcium — in malignancy, particularly myeloma and bone metastases.

Tingling around the mouth after a thyroidectomy is an airway warning. It precedes laryngospasm, and it is one of the findings that outranks almost anything else on the unit.

Worth remembering

  • High potassium peaks the T wave and speeds the gut; low potassium flattens it and stops the gut.
  • Intravenous potassium is always diluted, always by pump, never pushed.
  • Correct sodium slowly in both directions; the speed causes the harm.
  • Low calcium makes nerves MORE excitable — perioral tingling after thyroid surgery is an airway warning.
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