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USMLE – Hypokalemia

Pathophysiology
1. Plasma potassium levels of < 3.5 mmol/L due to increased potassium losses, decreased potassium intake, shift of potassium intracellularly
2. K+ wasting diuretics (furosemide, thiazides)
3. Prolonged diarrhea or vomiting
4. Metabolic alkalosis
5. Excessive sweating
6. Hyperaldosteronism

Signs and Symptoms
1. Symptoms occur if plasma K+ is < 3.0 mmol/L
2. Weak and tired legs
3. Fatigue
4. Myalgias
5. Hypoventilation due to respiratory muscle weakness
6. Paralysis
7. Nocturia, polyuria, polydipsia

Characteristic Test Findings
1. Flattening of T waves
2. Presence of “u” waves

Associated Conditions
1. Amphotericin B
2. Steroids
3. Laxative abuse
4. Theophylline
5. Tetracycline
6. Renal cell carcinoma
7. Cushing’s syndrome
8. Treatment of diabetic ketoacidosis hypokalemia causes hypertension

Biochemistry
Hypokalemia causes delayed ventricular repolarization

Treatment
1. Repletion either by mouth or IV (no more than 20 mmol/h)
2. Correction of ongoing losses

Tips for USMLE
1. Any condition with high renin will have low potassium
2. Hypokalemia in setting of digitalis use can precipitate digitalis toxicity (even if levels are normal)
3. Hypokalemia can precipitate hepatic encephalopathy in cirrhotics
4. Look for hypokalemia in any question with prolonged diarrhea or vomiting

Lillian Thompson By Lillian Thompson

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