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Dehydration, Hypernatremia & Hyponatremia — Comprehensive Clinical Reference

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DEHYDRATION

Definition & Classification

Severity Assessment

Clinical Findings

• Decreased skin turgor, dry mucous membranes
• Sunken anterior fontanelle (infants)
• Cool, mottled skin, poor capillary refill
• Tachycardia, hypotension, oliguria
• In severe cases: hypovolemic shock

Key Investigation

• Serum electrolytes (Na, K, Cl, HCO₃)
• Serum osmolality
• BUN, creatinine
• Urine specific gravity / osmolality
• Blood gas (metabolic acidosis possible)
• Weight (best single indicator of fluid loss)

Treatment — 3 Steps

1. Deficit replacement — estimate from weight loss
2. Maintenance therapy — ongoing physiologic needs
3. Replacement of ongoing losses

• Isotonic dehydration → 0.9% NaCl or Ringer's lactate
• Hypernatremic dehydration → slow rehydration with hypotonic fluids (rate < 0.5 mEq/L/hr Na fall to avoid cerebral edema)
• Hyponatremic dehydration → isotonic or slightly hypertonic fluids

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HYPERNATREMIA

Definition

• Moderate: 146–159 mEq/L (fairly tolerated)
• Severe: > 160 mEq/L (high risk of morbidity)

Pathophysiology

Causes (by Volume Status)

Drug-Induced Nephrogenic DI

Clinical Features

• Muscle weakness, restlessness, lethargy, insomnia
• Severe: coma, seizures, central pontine myelinolysis
• Signs of dehydration may be LESS obvious in hypernatremic dehydration (water shifts from ICF→ECF, partially maintains ECF volume)

Investigations

Differential Diagnosis

• Central (neurogenic) DI
• Nephrogenic DI
• Essential hypernatremia (reset osmostat)
• Hyperaldosteronism (Conn's / Cushing's)
• Hypertonic saline administration
• Osmotic diuresis (hyperglycemia, mannitol)
• Insufficient water intake (elderly, unconscious, infants)

Treatment

• Hypovolemic: correct volume deficit first with isotonic NaCl, then switch to hypotonic fluids
• Euvolemic (DI): free water replacement with hypotonic solutions (D5W, 0.45% NaCl)
• Hypervolemic: diuretics + free water (with caution)

• Chronic (> 48 hrs): correct Na at ≤ 0.5 mEq/L/hour (≤ 10–12 mEq/L/day) to avoid cerebral edema
• Acute (< 48 hrs): can correct more rapidly

• Central DI: DDAVP (desmopressin) — intranasal 10 µg once or twice daily; IV preferred in ICU
• Nephrogenic DI (lithium-induced): Amiloride 5–10 mg/day
• Nephrogenic DI (general): low-sodium diet + thiazide diuretics + NSAIDs

ICU Management

• IV DDAVP for central DI (preferred over intranasal for accuracy)
• Hourly urine output monitoring
• Serial serum Na every 2–4 hours
• Replace free water deficit: Free Water Deficit (L) = TBW × [(Serum Na/140) – 1]
• Half the deficit in first 12–24 hrs, remainder over 24–48 hrs
• Avoid hypotonic fluids at excessive rates
• Treat underlying cause

OPD Prescription

Contraindications

• Do NOT correct too rapidly → risk of cerebral edema (brain re-swells as cells recapture idiogenic osmoles)
• DDAVP contraindicated in: hyponatremia, heart failure (relative), polydipsia-related hyponatremia
• Hypotonic fluids contraindicated in hypovolemic shock (use isotonic first)

ECG in Hypernatremia

• Hyperkalemia (if coexistent): peaked T waves, widened QRS, sine wave pattern
• Direct hypernatremia effects on ECG: generally minimal, but may show prolonged QT in severe cases and tachycardia from dehydration

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HYPONATREMIA

Definition

• Mild: 130–135 mEq/L
• Moderate: 125–129 mEq/L
• Severe: < 125 mEq/L

Pathophysiology

Causes (by Volume Status)

SIADH Criteria (Euvolemic Hyponatremia)

• Serum Na < 135 mEq/L
• Serum osmolality < 275 mOsm/kg
• Urine osmolality > 100 mOsm/kg (often > 300)
• Urine Na > 40 mEq/L
• Normal renal, adrenal, thyroid function
• No diuretic use

Clinical Features

Investigations

Differential Diagnosis

• SIADH (malignancy, pulmonary disease, CNS disease, drugs, surgery, pain, nausea)
• Hypothyroidism
• Adrenal insufficiency (Addison's)
• Heart failure
• Cirrhosis / hepatic failure
• Nephrotic syndrome
• CKD / ESRD
• Diuretic-induced (thiazides > loops)
• Polydipsia (psychogenic/beer potomania)
• Pseudohyponatremia (hyperlipidemia, hyperproteinemia)
• Hyperglycemia (translocational hyponatremia)
• Nephrogenic syndrome of inappropriate antidiuresis (NSIAD)

Treatment Algorithm by Symptoms

• Immediate 3% hypertonic NaCl IV bolus: 150 mL over 20 min (repeat ×2 if needed)
• Target: raise Na by 5 mEq/L acutely to stop symptoms
• Admit to ICU

• 3% NaCl infusion at 0.5–2 mL/kg/hr OR vaptans (if euvolemic)
• Correct to symptom resolution (raise 5–10 mEq/L over 24 hrs)

• Fluid restriction (first-line for euvolemic/SIADH)
• Treat underlying cause
• Vaptans (tolvaptan, conivaptan) for euvolemic/hypervolemic

Rate of Correction

• Maximum safe rate: 10–12 mEq/L in 24 hours; 18 mEq/L in 48 hours
• Exceeding this → Osmotic Demyelination Syndrome (ODS) / Central Pontine Myelinolysis
• High-risk patients for ODS: alcoholism, malnutrition, liver disease, hypokalemia, Na < 105 mEq/L

ICU Management

• Continuous cardiac monitoring
• Hourly urine output
• Serum Na every 2–4 hours during acute correction
• If Na rises too fast: give D5W or DDAVP to slow correction
• No additional active therapy for 24 hours after successful hypertonic saline treatment
• Correct concurrent hypokalemia (worsens hyponatremia and ODS risk)
• Seizures: benzodiazepines + 3% NaCl

OPD Prescription

Contraindications

• Vaptans (tolvaptan/conivaptan) CONTRAINDICATED in:
  • Hypovolemic hyponatremia (worsens hypotension)
  • Serum creatinine > 3.0 mg/dL (ineffective)
  • Hepatic cirrhosis (tolvaptan — EMA restriction)
  • Duration > 30 days (FDA hepatotoxicity warning)
  • Concurrent use with hypertonic saline within 24 hrs (risk of ODS)
• Rapid correction contraindicated — ODS risk
• Hypotonic fluids contraindicated in hypovolemic shock
• Thiazide diuretics are a common cause — must be stopped

ECG in Hyponatremia

Baseline ECG is mandatory to detect co-existing electrolyte abnormalities (K, Mg) that accompany sodium disorders.

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QUICK COMPARISON TABLE

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