What are the initial steps in investigating and managing hypokalaemia (low potassium levels)?

Investigating Hypokalaemia: A Systematic Approach

Initial Assessment and Severity Classification

Begin by obtaining a repeat serum potassium measurement to exclude pseudohypokalaemia from hemolysis during phlebotomy, then classify severity: mild (3.0-3.5 mEq/L), moderate (2.5-2.9 mEq/L), or severe (<2.5 mEq/L). 1, 2

Immediate ECG Evaluation

• Obtain a 12-lead ECG to identify cardiac manifestations including ST depression, T wave flattening, prominent U waves, or arrhythmias 1, 2
• Severe hypokalaemia can precipitate ventricular arrhythmias including ventricular tachycardia, torsades de pointes, and ventricular fibrillation 1
• Patients with ECG changes, neuromuscular symptoms, or potassium ≤2.5 mEq/L require urgent treatment and cardiac monitoring 1, 2, 3

Determine the Underlying Cause

Medication Review

• Identify potassium-wasting medications: thiazide diuretics, loop diuretics (furosemide, bumetanide, torsemide), corticosteroids, beta-agonists, and insulin 1, 2, 4
• Review for medications causing transcellular shifts: insulin excess, beta-agonist therapy, or catecholamine administration 1, 5
• Assess for digoxin use, as hypokalaemia dramatically increases toxicity risk and can cause life-threatening arrhythmias 1

Clinical History Priorities

• Gastrointestinal losses: chronic vomiting, diarrhea, high-output stomas/fistulas, or laxative abuse 1, 2, 6
• Renal losses: polyuria, hyperaldosteronism, renal tubular acidosis, or Bartter/Gitelman syndrome 2, 3
• Inadequate intake: malnutrition, eating disorders, or severely restricted diets 5, 3
• Transcellular shifts: thyrotoxicosis, alkalosis, or recent insulin administration 2, 4

Essential Laboratory Investigations

First-Line Tests

• Serum magnesium level: Hypomagnesemia is the most common reason for refractory hypokalaemia and must be corrected before potassium levels will normalize 1, 3
• Renal function (creatinine, eGFR): Essential for determining replacement strategy and monitoring safety 1
• Serum sodium and chloride: Identify concurrent electrolyte disturbances and metabolic alkalosis 1
• Blood glucose: Rule out diabetic ketoacidosis or hyperglycemia causing transcellular shifts 1, 2

Second-Line Investigations (When Cause Unclear)

• 24-hour urine potassium or spot urine K+/creatinine ratio: Distinguish renal from extrarenal losses 2, 3
• Arterial blood gas: Assess for metabolic alkalosis or acidosis 2
• Plasma renin and aldosterone levels: If primary or secondary hyperaldosteronism suspected 2, 3

Management Strategy Based on Severity

Severe Hypokalaemia (<2.5 mEq/L or Symptomatic)

• Requires immediate aggressive IV potassium supplementation in a monitored setting with continuous cardiac monitoring 1, 2, 3
• Establish large-bore IV access for rapid administration 1
• Correct concurrent hypomagnesemia first (target >0.6 mmol/L using organic magnesium salts like aspartate, citrate, or lactate rather than oxide) 1, 3
• Recheck potassium levels within 1-2 hours after IV correction to ensure adequate response and avoid overcorrection 1
• Rates exceeding 20 mEq/hour should only be used in extreme circumstances with continuous cardiac monitoring 1

Moderate Hypokalaemia (2.5-2.9 mEq/L)

• Oral potassium chloride 20-60 mEq/day divided into 2-3 doses is preferred if gastrointestinal tract is functioning 1, 2, 3
• Target serum potassium 4.0-5.0 mEq/L, particularly in patients with heart disease or those on digoxin 1
• Recheck potassium and renal function within 3-7 days after starting supplementation 1

Mild Hypokalaemia (3.0-3.5 mEq/L)

• Dietary advice to increase potassium-rich foods (bananas, oranges, potatoes, tomatoes, legumes, yogurt) may be sufficient 1
• Consider oral potassium supplementation 20-40 mEq/day if dietary measures inadequate or patient has cardiac disease 1
• For diuretic-induced hypokalaemia, adding potassium-sparing diuretics (spironolactone 25-100 mg daily, amiloride 5-10 mg daily, or triamterene 50-100 mg daily) is more effective than chronic oral supplements 1

Critical Medication Adjustments

Medications to Question or Hold

• Digoxin orders should be questioned in severe hypokalaemia as this dramatically increases arrhythmia risk 1
• Thiazide and loop diuretics should be reduced or temporarily held until hypokalaemia corrects 1
• Most antiarrhythmic agents should be avoided (except amiodarone and dofetilide) as they exert cardiodepressant and proarrhythmic effects 1

Special Considerations for RAAS Inhibitors

• In patients taking ACE inhibitors or ARBs alone or with aldosterone antagonists, routine potassium supplementation may be unnecessary and potentially harmful 1
• These medications reduce renal potassium losses, making supplementation deleterious 1
• Reduce or discontinue potassium supplements when initiating aldosterone receptor antagonists to avoid hyperkalemia 1

Monitoring Protocol

Initial Phase (First Week)

• Check potassium and renal function within 2-3 days and again at 7 days after initiating treatment 1
• For IV potassium administration, recheck levels within 1-2 hours 1
• When adding potassium-sparing diuretics, monitor every 5-7 days until values stabilize 1

Maintenance Phase

• Monthly monitoring for first 3 months, then every 3-6 months thereafter 1
• More frequent monitoring required in patients with renal impairment (eGFR <45 mL/min), heart failure, diabetes, or concurrent medications affecting potassium 1

Common Pitfalls to Avoid

• Never supplement potassium without checking and correcting magnesium first - this is the most common reason for treatment failure 1, 3
• Administering digoxin before correcting hypokalaemia significantly increases risk of life-threatening arrhythmias 1
• Failing to correct sodium/water depletion first in gastrointestinal losses, as hypoaldosteronism from volume depletion paradoxically increases renal potassium losses 1
• Not recognizing that serum potassium is an inaccurate marker of total-body deficit - mild hypokalaemia may reflect significant total-body depletion 3
• Waiting too long to recheck potassium after IV administration can lead to undetected hyperkalemia 1
• Combining potassium supplements with potassium-sparing diuretics or aldosterone antagonists without close monitoring risks severe hyperkalemia 1