⚖️ Chapter 46: Acid–Base and Electrolyte Disturbances

Nelson's Pediatric Symptom-Based Diagnosis | Anion Gap · Metabolic Acidosis · Respiratory Acidosis/Alkalosis · Hyponatremia · Hypernatremia · Hypokalemia · Hyperkalemia · RTA · Winter's Formula · Delta Ratio · Corrected Sodium

🔍 Acid–Base and Electrolyte Disturbances: Key Concepts

📊 Anion Gap (AG)
AG = Na - (Cl + HCO3). Normal 8-12. Elevated AG metabolic acidosis: MUDPILES (methanol, uremia, DKA, propylene glycol, iron/INH, lactic acidosis, ethylene glycol, salicylates).
🩸 Metabolic Acidosis (low HCO3)
Increased AG: DKA, lactic acidosis, toxins, renal failure. Normal AG (hyperchloremic): diarrhea, RTA (types 1,2,4), ureteral diversion. Winter's formula: expected pCO2 = 1.5(HCO3)+8 ±2.
🌬️ Respiratory Disorders
Respiratory acidosis (high pCO2): hypoventilation (asthma, pneumonia, CNS depression). Respiratory alkalosis (low pCO2): hyperventilation (anxiety, sepsis, salicylates early).
💧 Hyponatremia (Na <135)
Hypovolemic: diuretics, vomiting, diarrhea, adrenal insufficiency. Euvolemic: SIADH, hypothyroidism. Hypervolemic: CHF, cirrhosis, nephrotic syndrome. Treat based on volume status. Rapid correction risks osmotic demyelination.
🔥 Hypernatremia (Na >145)
Water deficit: diarrhea, DKA, diabetes insipidus, insensible losses. Correct slowly (0.5 mEq/L/hr) to avoid cerebral edema.
⚡ Potassium Disorders
Hypokalemia: vomiting, diarrhea, diuretics, alkalosis, hyperaldosteronism. Hyperkalemia: renal failure, adrenal insufficiency, hemolysis, K-sparing diuretics. ECG changes: peaked T waves, wide QRS, sine wave (hyperkalemia); U waves, flat T (hypokalemia).

💡 Key takeaway: Always interpret electrolytes with the clinical context. Winter's formula checks respiratory compensation in metabolic acidosis. Delta ratio helps diagnose mixed disorders. Correct sodium for hyperglycemia: corrected Na = measured Na + 0.016*(glucose - 100).

🩺 Clinical Approach to Acid–Base and Electrolyte Disturbances

🔹 Step 1: Assess primary acid-base disorder
• pH <7.35 = acidosis; pH >7.45 = alkalosis.
• pCO2: >45 = respiratory acidosis; <35 = respiratory alkalosis.
• HCO3: <22 = metabolic acidosis; >28 = metabolic alkalosis.
🔹 Step 2: Calculate anion gap (AG = Na - (Cl + HCO3))
• Normal AG 8-12. Elevated AG → think MUDPILES (methanol, uremia, DKA, propylene glycol, iron/INH, lactic acidosis, ethylene glycol, salicylates).
• Normal AG (hyperchloremic) metabolic acidosis → diarrhea, RTA, ureteral diversion, acetazolamide.
🔹 Step 3: Check compensation using Winter's formula (for metabolic acidosis)
• Expected pCO2 = 1.5 x HCO3 + 8 ± 2.
• If measured pCO2 > expected → mixed respiratory acidosis.
• If measured pCO2 < expected → mixed respiratory alkalosis.
🔹 Step 4: Evaluate sodium disorders
• Hyponatremia: assess volume status (hypovolemic, euvolemic, hypervolemic). Calculate serum osmolality. Correct slowly (max 8-10 mEq/L per 24h).
• Hypernatremia: water deficit. Correct slowly (0.5 mEq/L/hr) to avoid cerebral edema. Check urine osmolality.
🔹 Step 5: Potassium disorders – check ECG and treat accordingly
• Hyperkalemia: calcium gluconate (cardioprotection), albuterol, insulin+glucose, kayexalate, dialysis.
• Hypokalemia: oral/IV potassium replacement. Check magnesium (hypomagnesemia causes refractory hypokalemia).