Electrolyte & Fluid Replacement

The kidney’s whole job is keeping electrolytes and fluid volume in a narrow range, because the heart and nerves run on them. Potassium in particular sets the electrical excitability of every cardiac cell — stray too far in either direction and the heart’s rhythm destabilizes. Many of the drugs on the other Renal and Cardiovascular pages disturb these balances: loop and thiazide diuretics waste potassium and magnesium, and that lost potassium is exactly what makes digoxin toxic.

So replacement isn’t a benign "top-up." Correcting these values — especially by IV — is among the most error-prone, high-consequence things a nurse does, which is why concentrated electrolytes are ISMP high-alert medications.

Both too little and too much potassium are dangerous. Hypokalemia (often from diuretics or GI losses) causes weakness, cramps, and arrhythmias and potentiates digoxin toxicity. Hyperkalemia causes the peaked T-waves and, ultimately, cardiac arrest. Replacing potassium safely has one absolute rule: IV potassium is NEVER given as an IV push or bolus — a rapid rise in serum potassium can stop the heart.

Instead, IV potassium must be diluted and infused slowly on a pump — generally ≤ 10 mEq/hr peripherally (higher rates require central access and continuous ECG monitoring), and it stings the vein. A second key fact: you often can’t fix low potassium without fixing low magnesium first — magnesium is required for the cell to hold onto potassium, so the two are replaced together.

Magnesium deficiency (diuretics, alcohol use, GI loss) causes tremor, arrhythmias, and refractory hypokalemia; IV magnesium is given slowly with monitoring for loss of deep tendon reflexes (the first sign of dangerous hypermagnesemia). Calcium (gluconate/chloride) treats hypocalcemia’s tetany and is the membrane-stabilizing antidote in hyperkalemia and magnesium toxicity — it protects the heart while other measures lower the level.

Finally, IV fluids are chosen by tonicity. Isotonic fluids (0.9% saline, lactated Ringer’s) stay in the vascular space to restore volume (dehydration, blood loss). Hypotonic fluids (0.45% saline) shift water into cells for cellular dehydration. Hypertonic fluids (3% saline, D10) pull water out of cells — powerful and reserved for specific problems like symptomatic hyponatremia. Matching the fluid to the deficit is the core skill.