15.2 Electrolyte Disturbances: Sodium, Potassium & Calcium Management

15.2 Electrolyte Disturbances: Sodium, Potassium & Calcium Management

Electrolyte disturbances in pediatric patients represent acute medical emergencies requiring nuanced understanding of cellular physiology, fluid compartments, and pharmacotherapy. Rapid shifts in serum osmolality and transmembrane electrical gradients directly threaten central nervous system and cardiac integrity. Clinical pharmacists must master the differential diagnosis of volume status, calculate exact deficit kinetics, and implement tiered emergency pharmacologic protocols for life-threatening dysnatremias, hyperkalemia, and calcium imbalances.

---

Sodium Disturbances: Hyponatremia

Hyponatremia is defined as a serum sodium concentration $< 135\text{ mEq/L}$, with severe hyponatremia defined as $< 120\text{ to } 125\text{ mEq/L}$. True hypotonic hyponatremia decreases extracellular fluid osmolality, establishing a transcellular osmotic gradient that drives water into brain cells, causing acute cerebral edema, raised intracranial pressure, uncal herniation, and death. Because pediatric brains have a higher brain-to-cranial vault volume ratio and immature $Na^+/K^+$ ATPase extrusion pumps, children are at substantially higher risk of cerebral herniation than adults at identical serum sodium levels.

Diagnostic Classification by Volume Status

Diagnostic Algorithm for Pediatric Hypotonic Hyponatremia (Serum Osm < 280 mOsm/kg):

                              [Assess Extracellular Volume Status]
                                                │
         ┌──────────────────────────────────────┼──────────────────────────────────────┐
         ▼                                      ▼                                      ▼
[HYPOVOLEMIC HYPONATREMIA]             [EUVOLEMIC HYPONATREMIA]               [HYPERVOLEMIC HYPONATREMIA]
(Dry mucosa, tachycardia,              (Normal turgor, no edema,              (Peripheral edema, ascites,
 orthostasis, flat veins)               normal blood pressure)                 pulmonary rales, S3 gallop)
         │                                      │                                      │
  Check Urine Sodium                     Check Urine Sodium                     Check Urine Sodium
  ┌──────┴──────┐                        ┌──────┴──────┐                        ┌──────┴──────┐
  ▼             ▼                        ▼             ▼                        ▼             ▼
< 20 mEq/L    > 20 mEq/L               > 20-40 mEq/L   < 20 mEq/L              < 20 mEq/L    > 20 mEq/L
Extrarenal    Renal Loss               • SIADH        Water intoxication      • CHF          Acute/Chronic
Losses        • Diuretics              • CNS/Lung     (Formula dilution)      • Nephrotic    Renal Failure
• Diarrhea    • Cerebral Salt Wasting    infection                            • Cirrhosis
• Vomiting    • Adrenal Crisis (CAH)   • Carbamazepine

1. Hypovolemic Hyponatremia: Total body sodium deficit exceeds water deficit.
   • Etiology: Gastrointestinal losses (diarrhea, vomiting), burns, third-spacing, adrenal insufficiency (congenital adrenal hyperplasia [CAH] with 21-hydroxylase deficiency), or Cerebral Salt Wasting (CSW) seen after neurosurgery or traumatic brain injury.
   • Laboratory Profile: Low urine sodium ($< 20\text{ mEq/L}$) in extrarenal losses; elevated urine sodium ($> 20\text{ to } 40\text{ mEq/L}$) in renal salt wasting, diuretic therapy, or CSW.
   • Treatment: Volume restoration with isotonic crystalloids ($0.9%\text{ NaCl}$). CSW requires vigorous volume and sodium replacement, often with oral salt supplementation or fludrocortisone.
2. Euvolemic Hyponatremia: Normal total body sodium with expanded total body water.
   • Etiology: Syndrome of Inappropriate Antidiuretic Hormone (SIADH) is the primary cause. Secondary to CNS disorders (meningitis, encephalitis, intracranial hemorrhage, tumors), pulmonary pathologies (pneumonia, bronchiolitis, mechanical ventilation), post-operative stress, and medications (carbamazepine, oxcarbazepine, vincristine, cyclophosphamide, SSRIs).
   • Diagnostic Hallmarks of SIADH: Serum hypoosmolality ($< 275\text{ mOsm/kg}$), concentrated urine (urine osmolality $> 100\text{--}300\text{ mOsm/kg}$, almost always exceeding serum osmolality), elevated urine sodium ($> 20\text{ to } 40\text{ mEq/L}$) despite normal dietary salt intake, clinical euvolemia, and normal adrenal/thyroid function.
   • Treatment: Fluid restriction to $50%\text{ to } 75%$ of maintenance is the first-line therapy. In refractory cases, oral urea or loop diuretics with oral sodium chloride tablets may be added. Vasopressin receptor antagonists (vaptans) are rarely used in pediatrics due to unpredictably rapid overcorrection risks.
3. Hypervolemic Hyponatremia: Total body water expansion exceeds total body sodium expansion.
   • Etiology: Congestive heart failure, nephrotic syndrome, acute/chronic renal failure, and hepatic cirrhosis.
   • Laboratory Profile: Urine sodium $< 20\text{ mEq/L}$ (reflecting poor effective arterial blood volume) unless renal failure is the primary cause.
   • Treatment: Sodium and fluid restriction combined with loop diuretics (furosemide $1\text{ to } 2\text{ mg/kg}$ IV).

Acute Symptomatic Hyponatremia: Emergency Management

When hyponatremia is complicated by neurological emergencies—such as generalized tonic-clonic seizures, altered mental status, stupor, or signs of impending brainstem herniation—clinicians must intervene immediately without waiting for diagnostic workups.

• Pharmacologic Agent: $3%\text{ Hypertonic Saline}$ ($513\text{ mEq/L } Na^+$ and $Cl^-$, or $\sim 0.5\text{ mEq/mL}$).
• Dose: $3\text{ to } 5\text{ mL/kg}$ IV infused over 10 to 20 minutes (maximum single bolus $100\text{ to } 150\text{ mL}$).
• Repetition: If active seizures or coma persist, the $3\text{ to } 5\text{ mL/kg}$ bolus may be repeated once or twice until seizure cessation.
• Target Response: The goal of acute hypertonic saline is not to normalize serum sodium, but to raise the serum sodium by $3\text{ to } 5\text{ mEq/L}$. This small acute elevation expands extracellular osmolality sufficiently to decrease brain water content by $5%\text{ to } 10%$, rapidly halting cerebral herniation and terminating seizures.

The 24-Hour Correction Ceiling: Preventing Osmotic Demyelination Syndrome

Transcellular Osmotic Dynamics in Hyponatremia Correction:

[Chronic Hyponatremic State] ───( Rapid Hypertonic Overcorrection )───► [Dehydration of Brain Cells]
Brain cells lost organic osmoles                                        Oligodendrocytes shrink rapidly;
(taurine, myo-inositol) to adapt                                         myelin sheaths detach from axons
to low extracellular tonicity.                                                       │
                                                                                     ▼
                                                                          OSMOTIC DEMYELINATION
                                                                          SYNDROME (ODS / CPM)
                                                                          • Spastic quadriparesis
                                                                          • Pseudobulbar palsy
                                                                          • "Locked-in" syndrome

[!CAUTION] Osmotic Demyelination Syndrome (ODS): In chronic hyponatremia ($> 48\text{ hours}$ duration), astrocytes extrude organic intracellular osmoles (myo-inositol, taurine, glutamate) to defend cell volume. If serum sodium is corrected too rapidly, the extracellular space becomes acutely hypertonic relative to the depleted brain cells. Water rapidly leaves brain cells, causing dehydration of oligodendrocytes, detachment of myelin sheaths, and catastrophic osmotic demyelination syndrome (formerly central pontine myelinolysis). Symptoms manifest 2 to 6 days post-correction as dysarthria, dysphagia, spastic quadriparesis, encephalopathy, and permanent "locked-in" syndrome.

• Maximum 24-Hour Correction Ceiling: Total serum sodium increase must NOT exceed $8\text{ to } 10\text{ mEq/L}$ in any 24-hour period (or $\le 0.5\text{ mEq/L/hour}$).
• High-Risk Ceiling: In patients with severe malnutrition, advanced liver disease, or baseline serum sodium $< 115\text{ mEq/L}$, limit correction to $6\text{ to } 8\text{ mEq/L}$ in 24 hours.
• Emergency Rescue for Inadvertent Overcorrection: If serum sodium increases $> 10\text{ mEq/L}$ within 24 hours, immediately stop hypertonic infusions, start an infusion of $D5W$ ($5\text{ to } 10\text{ mL/kg/h}$), and administer Desmopressin (DDAVP) ($1\text{ to } 2\text{ mcg}$ IV or SC every 6–8 hours) to re-lower serum sodium and abort demyelination.

---

Sodium Disturbances: Hypernatremia

Hypernatremia is defined as a serum sodium concentration $> 145\text{ mEq/L}$, with severe hypernatremia $> 160\text{ mEq/L}$. In pediatrics, hypernatremia usually results from unreplaced free water losses (dehydration from viral gastroenteritis, osmotic diuresis, central or nephrogenic diabetes insipidus) or salt poisoning (improperly concentrated powdered infant formula, administration of hypertonic sodium bicarbonate).

Pathophysiology & Idiogenic Osmoles

During sustained hypertonicity, brain cells synthesize and accumulate idiogenic osmoles (organic intracellular solutes such as taurine, glycine, sorbitol, and myo-inositol) over 24 to 48 hours to preserve intracellular hydration and resist brain shrinkage. If free water is replaced too rapidly, extracellular osmolality plummets while idiogenic osmoles remain trapped intracellularly. Water rushes into brain cells along the steep osmotic gradient, provoking lethal cerebral edema, brain herniation, seizures, and death.

Controlled 48-Hour Rehydration Protocol

1. Restore Perfusion First: If the child presents in hypovolemic shock, immediately administer an isotonic crystalloid bolus ($20\text{ mL/kg}$ of $0.9%\text{ NaCl}$) over 15 to 30 minutes to restore intravascular volume before addressing the hypertonicity.
2. Calculate Free Water Deficit: $\text{Free Water Deficit (L)} = \text{Total Body Water (TBW)} \times \left( \frac{\text{Serum } Na^+}{140} - 1 \right)$ (Where $\text{TBW} = 0.6 \times \text{weight (kg)}$ in infants, or $0.5 \times \text{weight (kg)}$ in older children).
3. Safe Rate of Correction:
   • Infuse deficit plus maintenance fluids over 48 to 72 hours (at least 48 hours; extend to 72 hours if baseline $Na^+ > 165\text{--}170\text{ mEq/L}$).
   • Target Rate: Decrease serum sodium at a rate $\le 0.5\text{ mEq/L/hour}$, and never exceed $10\text{ to } 12\text{ mEq/L}$ total reduction in any 24-hour window.
   • Solution: Typically $0.2%\text{ to } 0.45%\text{ NaCl}$ in $5%\text{ dextrose}$. Monitor serum sodium every 2 to 4 hours to titrate infusion velocity.

---

Potassium Homeostasis & Disturbances

Potassium is the predominant intracellular cation ($98%$ intracellular; normal serum concentration: $3.5\text{ to } 5.0\text{ mEq/L}$ in children, up to $5.5\text{ to } 6.0\text{ mEq/L}$ in preterm neonates). The resting membrane potential ($E_m$) of cardiac myocytes and excitable tissues is fundamentally determined by the intracellular-to-extracellular potassium ratio via the Nernst equation.

Hypokalemia ($< 3.5\text{ mEq/L}$)

• Etiology: Gastrointestinal losses (diarrhea, vomiting/nasogastric suction), renal wasting (loop and thiazide diuretics, amphotericin B, cisplatin, aminoglycosides), transcellular shifts (insulin administration, beta-2 adrenergic agonists, metabolic alkalosis), and hyperaldosteronism.
• Electrocardiographic (ECG) Findings: Flattening or inversion of T waves, ST-segment depression, prominent U waves (appearing after the T wave), prolonged PR and QTc intervals, and ventricular arrhythmias (torsades de pointes).
• Pharmacologic Replacement:
  • Oral Route (Preferred): Potassium chloride liquid or powder ($1\text{ to } 2\text{ mEq/kg/day}$ divided q6–8h; maximum single dose $40\text{ mEq}$).
  • Intravenous Replacement: Required for severe symptoms, cardiac arrhythmias, or non-functioning GI tracts.
    • Peripheral IV Max Concentration: $40\text{ mEq/L}$ (higher concentrations cause severe chemical phlebitis and pain).
    • Central Line Max Concentration: $60\text{ to } 80\text{ mEq/L}$ (rarely up to $100\text{ mEq/L}$ in fluid-restricted intensive care settings).
    • Maximum Infusion Rate: Standard max rate is $0.25\text{ to } 0.5\text{ mEq/kg/hour}$ (not to exceed $10\text{ to } 20\text{ mEq/hour}$ in older children). Rates $> 0.5\text{ mEq/kg/h}$ require mandatory continuous ECG telemetry.
  • The Magnesium Connection: Hypomagnesemia ($< 1.7\text{ mg/dL}$) causes refractory hypokalemia. Magnesium is an essential cofactor for the $Na^+/K^+$ ATPase; magnesium deficiency impairs intracellular potassium uptake and disinhibits renal outer medullary potassium (ROMK) channels, producing relentless urinary potassium wasting. Serum magnesium must be monitored and repleted (Magnesium sulfate $25\text{ to } 50\text{ mg/kg}$ IV over 30–60 min) to correct hypokalemia.

Hyperkalemia ($> 5.5\text{ mEq/L}$; Critical $\ge 6.5\text{ mEq/L}$)

Hyperkalemia is among the most lethal electrolyte emergencies encountered in pediatric critical care. As extracellular potassium rises, resting membrane potential depolarizes toward threshold, initially increasing myocardial excitability but subsequently causing sodium channel inactivation, conduction slowing, ventricular fibrillation, and asystole.

• Etiology: Acute kidney injury, chronic renal failure, tumor lysis syndrome, crush injury/rhabdomyolysis, severe metabolic acidosis, adrenal crisis (salt-wasting CAH), and medications (potassium-sparing diuretics, ACE inhibitors, ARBs, calcineurin inhibitors [tacrolimus, cyclosporine], trimethoprim, nonsteroidal anti-inflammatory drugs [NSAIDs]).
• Pseudohyperkalemia: Very common in pediatric practice! Caused by in vitro hemolysis during difficult heel sticks, traumatic venipunctures, prolonged tourniquet time, or extreme thrombocytosis/leukocytosis. Always verify unhemolyzed venous or arterial blood gas samples if clinical presentation and ECG do not correlate.
• ECG Progression:
  1. Tall, peaked, symmetric T waves with a narrow base (earliest sign).
  2. Prolongation of the PR interval and flattening/loss of P waves.
  3. Widening of the QRS complex and ST-segment depression.
  4. Merging of the widened QRS with the T wave forming a sine-wave pattern (heralds imminent ventricular fibrillation or asystolic cardiac arrest).

Three-Step Pharmacotherapy Sequence for Acute Severe Hyperkalemia:

[Step 1: MEMBRANE STABILIZATION] ──► Immediately antagonizes cardiotoxicity; does NOT lower serum K+
• Calcium Gluconate 10%: 100 mg/kg IV over 5-10 min (Peripheral IV preferred)
• Calcium Chloride 10%: 20 mg/kg IV over 5-10 min (CENTRAL LINE ONLY)
  Onset: 1-3 minutes; Duration: 30-60 minutes
                │
                ▼
[Step 2: INTRACELLULAR SHIFTING] ──► Rapidly drives potassium into cells; temporary bridge
• Regular Insulin (0.1 units/kg IV) + Dextrose (0.5-1 g/kg = D10W 5-10 mL/kg) over 30 min
• Inhaled Albuterol: 2.5-5 mg nebulized over 15 min
• Sodium Bicarbonate: 1-2 mEq/kg IV over 10-20 min (if concurrent metabolic acidosis)
  Onset: 15-30 minutes; Duration: 2-6 hours
                │
                ▼
[Step 3: TOTAL-BODY ELIMINATION] ──► Permanently clears potassium from the body
• Loop Diuretics: Furosemide 1-2 mg/kg IV (if functional kidneys)
• Potassium Binders: Lokelma (SZC) 2.5-5 g PO; Patiromer; avoid SPS in neonates
• Hemodialysis / Continuous Renal Replacement Therapy (CRRT): Definitive removal
  Onset: 1-4 hours; Sustained removal

Comprehensive Hyperkalemia Pharmacotherapy Table

---

Calcium Disturbances

Total serum calcium consists of three fractions: protein-bound (primarily to albumin, $\sim 40%\text{ to } 45%$), complexed to anions (citrate, phosphate, bicarbonate, $\sim 10%$), and ionized calcium ($Ca^{2+}$, $\sim 50%$), which is the sole biologically active fraction regulating neuromuscular excitability and coagulation.

The Pitfall of Total and "Corrected" Calcium

Traditional clinical formulas attempt to estimate ionized calcium based on total calcium and serum albumin: $\text{Corrected Calcium (mg/dL)} = \text{Measured Total Calcium (mg/dL)} + 0.8 \times [4.0 - \text{Serum Albumin (g/dL)}]$

[!IMPORTANT] BCPPS Board Rule: Corrected calcium formulas have been conclusively proven to be highly inaccurate and unreliable in critically ill pediatric patients and neonates. Acid-base disturbances profoundly alter calcium-albumin binding: alkalosis increases negative charges on albumin, driving binding of free calcium and acutely lowering ionized calcium (precipitating tetany despite a normal total calcium). Acidosis decreases binding, elevating ionized calcium. In pediatric intensive care and neonatal units, ionized calcium must always be measured directly (reference range: $1.15\text{ to } 1.35\text{ mmol/L}$ or $4.6\text{ to } 5.4\text{ mg/dL}$).

Hypocalcemia (Ionized $Ca^{2+} < 1.1\text{ mmol/L}$)

• Clinical Manifestations: Neuromuscular irritability, jitteriness and tremors (neonates), laryngospasm, bronchospasm, tetany, carpopedal spasm, seizures, and prolonged QTc intervals on ECG. Signs include Chvostek sign (facial twitching on tapping facial nerve) and Trousseau sign (carpal spasm on inflating blood pressure cuff).
• Emergency Treatment:
  • Calcium Gluconate 10%: $100\text{ mg/kg}$ ($1\text{ mL/kg}$, max $1\text{--}2\text{ g}$) IV infused slowly over 10 to 30 minutes under continuous cardiac telemetry.
  • Administration Warning: Rapid bolus administration can trigger severe bradycardia, sinus arrest, and hypotension. Stop infusion immediately if heart rate decreases by $> 20\text{ bpm}$. Extravasation of calcium solutions causes severe chemical cellulitis and tissue calcinosis.

Hypercalcemia (Total $Ca > 10.8\text{ mg/dL}$; Ionized $Ca^{2+} > 1.4\text{ mmol/L}$)

• Etiology: Malignancy, primary hyperparathyroidism, subcutaneous fat necrosis of the newborn (post-therapeutic hypothermia), vitamin D toxicity, Williams syndrome, and prolonged immobilization.
• Treatment:
  1. Volume Expansion: $0.9%\text{ NaCl}$ at $1.5\text{ to } 2\times$ maintenance to promote renal calcium excretion.
  2. Loop Diuretic: Furosemide $1\text{ mg/kg}$ IV once intravascular volume is fully repleted (prevents proximal and loop calcium reabsorption).
  3. Calcitonin (Salmon): $4\text{ to } 8\text{ units/kg}$ SC or IM every 12 hours (rapid onset, but subject to tachyphylaxis within 48 hours).
  4. Bisphosphonates: Zoledronic acid ($0.02\text{ to } 0.05\text{ mg/kg}$ IV over 30 min) or Pamidronate ($0.5\text{ to } 1.0\text{ mg/kg}$ IV over 4 hours) for sustained inhibition of osteoclastic bone resorption.

---

Practice Pearls & BCPPS Exam Traps

• Exam Trap 1: Never administer Calcium Chloride via a peripheral IV catheter unless in active cardiac arrest with no other access. Its high osmolarity and ionized calcium concentration trigger catastrophic extravasation necrosis requiring plastic surgery debridement.
• Exam Trap 2: Do not correct chronic hyponatremia faster than $8\text{ to } 10\text{ mEq/L}$ per 24 hours. If an exam question describes a child whose sodium rose by $16\text{ mEq/L}$ in 12 hours who then develops quadriparesis and dysarthria, identify Osmotic Demyelination Syndrome as the mechanism.
• Board Rule: Always verify serum magnesium when evaluating refractory hypokalemia or refractory hypocalcemia; neither can be successfully normalized until hypomagnesemia is fully repleted.