Hypokalemia

Background

• Serum potassium <3.5 mEq/L
• Most common electrolyte abnormality encountered in clinical practice
• Severity:
  • Mild: 3.0-3.5 mEq/L
  • Moderate: 2.5-3.0 mEq/L
  • Severe: <2.5 mEq/L (risk of arrhythmia, respiratory failure)
• Every 1 mEq/L decrease in serum K represents ~200-400 mEq total body deficit

Causes

• Decreased intake: malnutrition, anorexia, alcoholism
• GI losses (most common):
  • Vomiting (metabolic alkalosis → renal K wasting)
  • Diarrhea (direct K loss)
  • NG suction, laxative abuse
• Renal losses:
  • Diuretics (loops, thiazides — most common medication cause)
  • Hyperaldosteronism (primary or secondary)
  • Renal tubular acidosis (types 1 and 2)
  • Hypomagnesemia (impairs renal K conservation)
  • Osmotic diuresis (DKA)
• Transcellular shift (K moves into cells):
  • Insulin (therapeutic or endogenous)
  • Beta-2 agonists (albuterol)
  • Alkalosis
  • Catecholamine surge, thyrotoxicosis
  • Hypothermia (shifts K intracellularly)

Clinical Features

• Often asymptomatic with mild hypokalemia
• Muscle weakness (proximal > distal), cramps, myalgia
• Ileus, constipation, nausea/vomiting
• Rhabdomyolysis (severe hypokalemia)
• Cardiac arrhythmias:
  • PACs, PVCs → atrial or ventricular tachycardia → torsades de pointes → VF
  • Potentiates digoxin toxicity

ECG Changes

• Flattened T waves (earliest)
• Prominent U waves (after T wave)
• ST depression
• Prolonged QT interval
• T-U fusion (severe)

Differential Diagnosis

• Medication-induced (diuretics, insulin, albuterol)
• GI losses (vomiting, diarrhea)
• Diabetic ketoacidosis (total body K depleted despite possible normal level)
• Hyperaldosteronism
• Renal tubular acidosis
• Hypomagnesemia
• Bartter/Gitelman syndrome
• Thyrotoxic periodic paralysis

Evaluation

• ECG (look for U waves, flattened T waves, prolonged QT)
• BMP: K level, bicarbonate (alkalosis?), glucose, creatinine
• Magnesium level (hypokalemia refractory to replacement if Mg not corrected)
• Calcium level (concurrent abnormalities)
• Consider: urine K (spot urine K/Cr ratio or 24h K), urine chloride, TSH, cortisol/aldosterone if unexplained
• Digoxin level if on digoxin (hypokalemia increases digoxin sensitivity)

Management

Guiding Principles

• Always check and replace magnesium first — hypokalemia is refractory to correction with concurrent hypomagnesemia
• Oral replacement preferred when possible (better tolerated, less risky)
• IV replacement for severe hypokalemia, ECG changes, or NPO patients

Mild Hypokalemia (3.0-3.5 mEq/L)

• Oral KCl 20-40 mEq PO q2-4h (typical total dose 40-100 mEq/day)
• Increase dietary potassium

Moderate Hypokalemia (2.5-3.0 mEq/L)

• KCl 10-20 mEq/hr IV via peripheral line (max 40 mEq/L concentration peripherally)
  • Higher concentrations require central line
• Max infusion rate: 10-20 mEq/hr (peripheral); up to 40 mEq/hr via central line with cardiac monitoring
• Concurrent oral supplementation

Severe Hypokalemia (<2.5 mEq/L or ECG Changes)

• Continuous cardiac monitoring
• KCl 20-40 mEq/hr IV via central line
• Magnesium sulfate 2g IV (if Mg not checked yet, give empirically)
• Recheck K every 1-2 hours
• May require 200+ mEq total replacement

Special Situations

• DKA: K may be normal or elevated on presentation but total body stores are depleted
  • Replace K before or concurrent with insulin when K <5.3
  • Do NOT start insulin if K <3.3 — replace K to >3.3 first
• Digoxin toxicity: maintain K >4.0 mEq/L
• Refractory hypokalemia: check and replace magnesium; consider amiloride or spironolactone

Disposition

• Admit if K <2.5, symptomatic, ECG changes, arrhythmia, or ongoing losses
• Continuous telemetry for K <3.0 or ECG changes
• Discharge if mild (3.0-3.5), asymptomatic, clear correctable cause, tolerated PO replacement, normal ECG
• Close follow-up with recheck in 24-48 hours

See Also

• Hyperkalemia
• Hypomagnesemia
• Digoxin toxicity
• Diabetic ketoacidosis
• Electrolyte imbalances

References

• Kardalas E, et al. Hypokalemia: a clinical update. Endocr Connect. 2018;7(4):R135-R146. PMID 29540487
• Gennari FJ. Hypokalemia. N Engl J Med. 1998;339(7):451-458. PMID 9700180
• Viera AJ, Wouk N. Potassium disorders: hypokalemia and hyperkalemia. Am Fam Physician. 2015;92(6):487-495. PMID 26371733
• Crop MJ, et al. Role of magnesium in hypokalemia. Crit Care. 2012;16(1):229. PMID 22866973