Hyperkalemia vs Hypokalemia for NCLEX: Signs, ECG and Treatment

Quick answer: Hyperkalemia is a serum potassium above 5.0 mEq/L; hypokalemia is below 3.5 mEq/L. Both cause muscle weakness and dangerous dysrhythmias, but the ECGs differ: high potassium causes peaked T waves, a widening QRS and eventually a sine wave, while low potassium causes flattened T waves, ST depression and U waves. For hyperkalemia with ECG changes, give IV calcium first. Replace low potassium orally when possible, never by IV push.

Potassium items hand you a lab value, an ECG strip or a medication list and ask for the action that protects the heart. The two imbalances share symptoms, so focus on what separates them: the ECG, the cause and the treatment order.

Hyperkalemia vs hypokalemia: how do they compare?

Normal serum potassium is 3.5-5.0 mEq/L (some labs list 5.1 or 5.2 as the upper limit), and for potassium mEq/L equals mmol/L. Other core ranges are on the NCLEX lab values cheat sheet.

Weakness alone will not separate them: U waves and furosemide point low; peaked T waves and kidney failure point high.

What causes high and low potassium?

Potassium changes through intake, shifts between cells and blood, and losses. Most of it sits inside cells, so small shifts move the serum level fast. The fluid and electrolytes review applies this logic to the other electrolytes.

Causes of hypokalemia

• Losses: loop and thiazide diuretics, vomiting, NG suction, diarrhea, laxative misuse, excess mineralocorticoid effect (hyperaldosteronism, Cushing’s syndrome) and low magnesium.
• Shift into cells: insulin, albuterol and alkalosis.
• Low intake: eating disorders, alcohol use disorder, prolonged NPO status.

Causes of hyperkalemia

• Poor excretion: kidney injury or failure, missed dialysis and Addison’s disease.
• Medications: ACE inhibitors, ARBs, potassium-sparing diuretics, trimethoprim, NSAIDs, potassium supplements and salt substitutes (potassium chloride).
• Shift out of cells or cell breakdown: acidosis, untreated DKA, crush injuries, burns, rhabdomyolysis and tumor lysis.
• False high: a hemolyzed or delayed sample. Redraw it, but do not delay treatment if the ECG shows changes.

The kidneys excrete about 80-90% of potassium intake, so kidney function sits behind many hyperkalemia questions; see the renal and GU review.

How do the ECG changes differ?

Hyperkalemia on the ECG

The classic progression is peaked T waves; then a prolonged PR with flattened or absent P waves; then a widening QRS; then a sine wave that can become ventricular fibrillation or asystole. Bradycardia and heart block also occur. The UK Kidney Association notes that the changes do not always appear in order and that peaked T waves appear in only about a third of people with moderate to severe hyperkalemia, so a normal-looking ECG does not rule it out.

Hypokalemia on the ECG

Low potassium flattens or inverts the T wave, depresses the ST segment and adds a U wave; as T and U merge, the QT can look prolonged. The rhythm risk is ventricular: PVCs, ventricular tachycardia, torsades de pointes and ventricular fibrillation. Practice these strips with the cardiac dysrhythmias and ECG practice questions.

What does the nurse do first for hyperkalemia?

With a high potassium and ECG changes, treatment runs protect, shift, remove, then monitor and prevent recurrence (the UK Kidney Association sequence). First, the nurse starts cardiac monitoring, obtains a 12-lead ECG, confirms IV access and stops potassium sources such as potassium-containing IV fluids.

Sodium polystyrene sulfonate carries an intestinal necrosis warning and should not be given with sorbitol. Patiromer can lower magnesium. Sodium zirconium cyclosilicate contains sodium and can cause edema. U.S. labels for all three say they are not for emergency treatment because of delayed onset. The 2026 UK Kidney Association guideline does use sodium zirconium cyclosilicate alongside calcium and insulin for severe hyperkalemia, but on the exam a binder is never the first action.

Sodium bicarbonate is not routine. Hyperkalemia from digoxin toxicity calls for digoxin immune Fab. Recheck potassium repeatedly (the UK guideline suggests 1, 2, 4, 6 and 24 hours), because insulin and albuterol only shift potassium and the level can rebound.

How is hypokalemia treated safely?

Oral potassium is preferred when the client can swallow and has no severe symptoms; give it with water and food to limit GI irritation. IV replacement is for severe hypokalemia (commonly below 2.5 mEq/L), ECG changes or a client who cannot take oral drugs.

• Never by IV push. The FDA label for potassium chloride concentrate warns that direct injection may be instantaneously fatal. Always dilute it; premixed bags are preferred.
• Use a pump and stay within policy. Typical limits are about 10 mEq per hour peripherally and up to about 20 mEq per hour centrally with continuous cardiac monitoring. Labeling allows up to 40 mEq per hour only in urgent cases (potassium below 2.0 mEq/L with ECG changes or paralysis) under continuous cardiac monitoring. Limits vary by facility.
• Confirm urine output. Labels list kidney failure as a contraindication or a reason for extreme caution; the usual adult benchmark is at least 0.5 mL/kg per hour, about 30 mL per hour.
• Watch the IV site for pain, phlebitis and infiltration; never speed up a late infusion.
• Check magnesium. Over half of significant hypokalemia comes with low magnesium, and potassium often will not correct until it is replaced.
• Prefer a saline carrier. Dextrose triggers insulin release, which pushes potassium into cells.

The digoxin label lists hypokalemia and low magnesium as risk factors for toxicity, so a client on digoxin plus a loop diuretic needs close potassium monitoring (see the therapeutic drug levels guide).

Potassium before insulin in DKA: 3.3 or 3.5?

In DKA, total body potassium is depleted even when the serum level looks normal or high, because acidosis and insulin deficiency push potassium out of cells. Insulin moves it back in, so the serum level falls once insulin starts.

• Exam-taught: 3.3 mEq/L. The 2009 ADA consensus delays insulin until potassium is above 3.3 mEq/L, and most nursing texts and NCLEX-style items still use this number.
• Current guideline: 3.5 mmol/L. The 2024 ADA/EASD consensus report delays insulin until potassium is above 3.5 mmol/L, replacing it at 10 mmol per hour meanwhile. It adds potassium to IV fluids below 5.0, targets 4-5, and rechecks 2 hours after insulin starts, then every 4 hours.

The principle is the same: fluids first, check potassium, replace a low level, then start insulin. On the exam, use 3.3 unless the item says otherwise; at the bedside, follow your facility’s protocol. See DKA vs HHS for the full sequence.

Sample NCLEX-style potassium questions

Frequently Asked Questions

What is the difference between hyperkalemia and hypokalemia?

Hyperkalemia is a serum potassium above 5.0 mEq/L; hypokalemia is below 3.5 mEq/L. Hyperkalemia usually comes from kidney failure, potassium-sparing drugs or cell breakdown and shows peaked T waves. Hypokalemia usually comes from diuretics or GI losses and shows flattened T waves and U waves. Both cause weakness and can cause fatal dysrhythmias.

Why is calcium gluconate given first for hyperkalemia if it does not lower potassium?

Calcium stabilizes the heart muscle membrane against high potassium, lowering the immediate risk of a fatal dysrhythmia within minutes. Insulin, albuterol, binders and dialysis lower the potassium but take longer. Calcium lasts only 30 to 60 minutes, so the other treatments must start right away.

What is the maximum rate for IV potassium?

Limits vary by facility. Many policies cap peripheral infusions at about 10 mEq per hour and allow up to about 20 mEq per hour through a central line with continuous cardiac monitoring. Labeling allows up to 40 mEq per hour only in monitored urgent cases. IV potassium is always diluted and pump-controlled, never given by IV push, because a rapid rise can stop the heart.

Should I use 3.3 or 3.5 as the potassium cutoff before insulin in DKA?

Most nursing texts and exam items use 3.3 mEq/L, from the 2009 ADA consensus. The 2024 ADA/EASD consensus report raised it to 3.5 mmol/L, the same number in mEq/L. For the exam, use 3.3 unless the item gives another value; in practice, follow your facility’s protocol. Either way, replace low potassium before starting insulin.

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