Electrolyte disorders (Na, K, Ca, Mg)

Sodium Disorders - Salty Situations

• Hyponatremia (Serum Na < 135 mEq/L): Primarily a water balance issue.
  • Symptoms: Nausea, headache, confusion, seizures, coma.
  • Management depends on volume status & acuity.
• Hypernatremia (Serum Na > 145 mEq/L): Indicates a free water deficit.
  • Causes: ↓water intake (impaired thirst), ↑water loss (diabetes insipidus, fever, diarrhea).
  • Calculate free water deficit: $TBW \times ( (Serum\ Na / 140) - 1 )$
  • Treatment: Replace free water deficit slowly with D5W or hypotonic saline.

⭐ Correction Rate is Key: Rapid correction of chronic hyponatremia (>48h) by > 8-10 mEq/L in 24h risks Osmotic Demyelination Syndrome (ODS). Similarly, rapid correction of hypernatremia risks cerebral edema.

Potassium Disorders - K-Drama

• Hypokalemia (< 3.5 mEq/L)

  • Causes: Diuretics (loop/thiazide), diarrhea, vomiting, hyperaldosteronism, RTA types 1 & 2.
  • ECG: Flattened/inverted T waves, U waves, ST depression.
  • Sx: Muscle weakness, paralysis, arrhythmias.
  • Tx: Oral or IV $K^+$ repletion (IV if severe/arrhythmias). Max infusion rate 10-20 mEq/hr.

• Hyperkalemia (> 5.2 mEq/L)

  • Causes: Renal failure, ACE inhibitors, ARBs, spironolactone, crush injury, tumor lysis syndrome.
  • ECG: Peaked T waves, wide QRS, sine wave pattern.
  • Sx: Muscle weakness, paralysis, cardiac arrest.

⭐ In hyperkalemia with ECG changes, the first step is always IV Calcium Gluconate to stabilize the cardiac membrane, not to lower serum $K^+$.

📌 C BIG K Drop (Tx): Calcium, Bicarb/β-agonist, Insulin, Glucose, Kayexalate, Diuretics/Dialysis.

Calcium Disorders - A Bone to Pick

• Hypocalcemia (<8.5 mg/dL):

  • Causes: Hypoparathyroidism (post-surgical), pancreatitis, ↓Mg, Vit D deficiency.
  • Signs: Tetany (Chvostek, Trousseau signs), perioral numbness, prolonged QT interval.
  • Tx: IV Calcium Gluconate for severe symptoms.

• Hypercalcemia (>10.5 mg/dL):

  • 📌 Mnemonic: "Stones, bones, groans, psychiatric overtones."
  • Signs: Constipation, bone pain, confusion, polyuria. Shortened QT interval.
  • Tx: Aggressive IV fluids, calcitonin, bisphosphonates.

⭐ Malignancy (via PTHrP) is the most common cause of hypercalcemia in hospitalized patients, whereas primary hyperparathyroidism is the #1 outpatient cause.

Magnesium Disorders - The Underdog Ion

• Hypomagnesemia (< 1.8 mg/dL)
  • Causes: Chronic alcoholism, PPIs, diuretics, diarrhea.
  • Features: Neuromuscular hyperexcitability (tetany, seizures), EKG shows prolonged QT interval → Torsades de Pointes.
  • 
• Hypermagnesemia (> 2.6 mg/dL)
  • Causes: Renal failure, iatrogenic overdose (e.g., pre-eclampsia treatment).
  • Features: ↓ Deep tendon reflexes (DTRs), bradycardia, respiratory depression.
  • Treatment: IV Calcium Gluconate (antagonism).

⭐ Refractory Hypokalemia & Hypocalcemia: Always check magnesium levels! Hypomagnesemia impairs PTH secretion and promotes renal K+ loss, making K+/Ca++ correction difficult.

High-Yield Points - ⚡ Biggest Takeaways

• Correcting chronic hyponatremia too fast causes osmotic demyelination; rapid hypernatremia correction risks cerebral edema.
• Hyperkalemia (peaked T-waves) needs immediate IV calcium gluconate for cardiac protection.
• Hypokalemia (flat T-waves, U-waves) presents with muscle weakness and ileus.
• Post-thyroidectomy hypocalcemia causes perioral numbness and Chvostek/Trousseau signs.
• Symptomatic hypercalcemia (“stones, bones, groans”) requires aggressive IV fluids first.
• Hypomagnesemia can precipitate refractory hypokalemia and torsades de pointes.