Renal, Fluids, Electrolytes & Acid-Base · Topic 13 of 22

Hypokalemia

Renal & electrolytes · potassium

Hypokalemia

Replace potassium orally when you can, IV within safe rate limits when you must — and fix magnesium, or the potassium will not hold.

Key numbers

Potassium below 3.5 mEq/LK⁺ < 3.5 mEq/L

IV KCl ≤ 10 mEq/h peripherally≤ 10 mEq/h peripheral

01 · Defined byPotassium below 3.5 mEq/L

Hypokalemia is serum potassium below 3.5 mEq/L: mild 3.0–3.4 rarely causes symptoms; moderate 2.5–3.0 causes weakness and can progress to paralysis and respiratory failure; severe below 2.5 causes ECG changes or severe symptoms.

02 · The ruleIV KCl ≤ 10 mEq/h peripherally

Give IV potassium chloride ≤ 10 mEq/h peripherally (≤ 40 mEq/L) and ≤ 20 mEq/h centrally with continuous ECG; faster rates (to 40 mEq/h) only for hypokalemic arrhythmia, with hourly potassium.

03 · DoCheck magnesium; replete it first if low

Check magnesium in every hypokalemic patient: hypomagnesemia causes renal potassium wasting, so potassium stays refractory until magnesium is repleted — give magnesium first or with the potassium, and suspect depletion even with a normal level.

Causes & risk factors

Why it happens

Causes: diuretics, GI losses, cellular shiftsLoop and thiazide diuretics are the commonest cause; others are diarrhea or laxatives, vomiting or gastric suction (renal loss via alkalosis and aldosterone), mineralocorticoid excess, and shifts from insulin, beta-2 agonists or nutrition support.

Keep K⁺ above 4.0 after MIPatients with heart disease or on digoxin are at risk even with mild hypokalemia; keep potassium above 4.0 mEq/L after MI and correct potassium and magnesium in every patient with torsades.

Presentation

What you'll see

Flat T waves and prominent U waves must knowECG changes usually appear below 3 mEq/L: ST sagging, flattened T waves and growing U waves (a T–U fusion can mimic long QT), with PACs, PVCs, tachyarrhythmias, AV block and eventually VF.

Weakness, cramps, ileus, paralysisMuscle effects include weakness, cramps, fasciculations, paralytic ileus, hypoventilation and rhabdomyolysis; persistent hypokalemia impairs urine concentration, causing polyuria.

Diagnosis & monitoring

Work-up

Each 1 mEq/L drop ≈ 200–400 mEq deficitA 1 mEq/L fall in serum potassium reflects about a 200–400 mEq total-body deficit; even severe deficits rarely need more than 100–120 mEq in 24 hours unless losses continue.

Do right now

In this order

First moves

Flat T waves and prominent U waves

→

Oral KCl unless ECG changes or severe

→

IV KCl ≤ 10 mEq/h peripherally≤ 10 mEq/h peripheral

→

Check magnesium; replete it first if low

Why each step

Flat T waves and prominent U wavesECG changes usually appear below 3 mEq/L: ST sagging, flattened T waves and growing U waves (a T–U fusion can mimic long QT), with PACs, PVCs, tachyarrhythmias, AV block and eventually VF.

Oral KCl unless ECG changes or severeReplace orally with potassium chloride 20–80 mEq/day in divided doses; go IV for ECG changes, severe symptoms, inability to take oral drugs, or hospitalized patients on digoxin or with heart disease and ongoing losses.

IV KCl ≤ 10 mEq/h peripherallyGive IV potassium chloride ≤ 10 mEq/h peripherally (≤ 40 mEq/L) and ≤ 20 mEq/h centrally with continuous ECG; faster rates (to 40 mEq/h) only for hypokalemic arrhythmia, with hourly potassium.

Check magnesium; replete it first if lowCheck magnesium in every hypokalemic patient: hypomagnesemia causes renal potassium wasting, so potassium stays refractory until magnesium is repleted — give magnesium first or with the potassium, and suspect depletion even with a normal level.

Red flags & don'ts

Avoid harm

Never dilute IV potassium in dextrose avoidMix IV potassium in saline, not dextrose: dextrose stimulates insulin release, which drives potassium into cells and can transiently worsen hypokalemia.

The rules

Know these

DKA: hold insulin until K⁺ ≥ 3.5In DKA or HHS, total-body potassium is depleted even when serum potassium is normal or high; below 3.5 mEq/L, give potassium first and delay insulin until it reaches 3.5.

Oral or IV potassium?

Commonly confused

Use when	No ECG changes or severe symptoms; patient can take it by mouth
Dose or rate	20–80 mEq/day in divided doses
Monitoring	Periodic potassium
Pitfall	GI irritation — divide doses

Commonly confused

Tell apart

Urine potassium separates renal from GI lossWhen the cause is unclear, exclude shifts, then measure 24-hour urine potassium and serum magnesium: urine potassium under 15 mEq suggests GI loss or poor intake; above 15 points to renal wasting.

Renal loss plus hypertension: think aldosteronismRenal potassium wasting with hypertension suggests primary aldosteronism (or Liddle syndrome); with normal blood pressure think diuretics, vomiting, hypomagnesemia, or rarely Bartter or Gitelman syndrome.

More detail

Reference

Show 4 reference facts

For diuretic-induced hypokalemia, a potassium-sparing diuretic (spironolactone or triamterene) can be added instead of, or with, potassium chloride; routine supplementation is not needed for every patient on diuretics.

Other drug causes: high-dose beta-2 agonists, theophylline, piperacillin and amphotericin B (which also causes hypomagnesemia); thyrotoxic periodic paralysis causes sudden hypokalemic weakness.

Liquid potassium chloride acts within 1–2 hours but tastes bitter and is poorly tolerated above 25–50 mEq per dose; wax-matrix or microencapsulated tablets cause less GI irritation and bleeding.

In DKA, once potassium is between 3.5 and 5.0 mEq/L, add about 20–30 mEq of potassium per liter of IV fluid to keep it at 4–5 mEq/L while insulin runs.

Pearl

Potassium that will not come up usually means magnesium is low — replace both.

Sources · Merck Manual Professional — Hypokalemia (updated Dec 2025) · Merck Manual Professional — Hypomagnesemia · Merck Manual Professional — Diabetic Ketoacidosis (acute complications of diabetes mellitus) · Diabetic ketoacidosis: considerations and residual controversies in management after the 2024 ADA, EASD, JBDS, AACE and DTS joint consensus. Endocr Metab Immune Disord Drug Targets 2026 · Merck Manual Professional — Complications of Acute Coronary Syndromes · Drew BJ et al. Prevention of torsade de pointes in hospital settings: AHA/ACCF scientific statement. Circulation 2010;121:1047–1060 — PMC · Kardalas E et al. Hypokalemia: a clinical update. Endocr Connect 2018;7:R135–R146 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).