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Forced Alkaline diuresis

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Here is a comprehensive overview of Forced Alkaline Diuresis (FAD):

Forced Alkaline Diuresis

Definition & Principle

Forced alkaline diuresis is a technique that combines high-volume intravenous fluid administration (to increase urine output) with urinary alkalinization (using sodium bicarbonate) to enhance renal elimination of certain toxins and drugs. The key mechanism exploits ion trapping:
  • Weak acids (e.g., salicylate, phenobarbital) exist in equilibrium between ionized and non-ionized forms
  • In alkaline urine (pH >7.5), weak acids become ionized → cannot diffuse back across tubular epithelium → are excreted in urine
  • This significantly increases renal clearance

Mechanism of Ion Trapping

Drug TypepKaEffect of Alkaline Urine
Weak acids (salicylates, barbiturates)3–8Ionized → trapped in tubular lumen → excreted
Weak bases—Alkaline urine favors non-ionized form → decreased excretion (opposite effect)
The goal is to raise urine pH to 7.5–8.5 while maintaining systemic pH <7.6.

Indications

  1. Salicylate poisoning — the primary and most well-established indication
    • Alkaline diuresis ionizes salicylate in the tubular lumen, trapping it and enhancing urinary excretion
    • NaHCO₃ is given cautiously; target urine pH ~7.5–8
    • Avoid systemic pH >7.6 (especially with coexisting respiratory alkalosis)
    • Acetazolamide is not recommended (causes systemic acidosis which paradoxically drives salicylate into the CNS)
    • Hemodialysis is preferred in severe poisoning, especially with coexistent renal failure — Brenner and Rector's The Kidney
  2. Rhabdomyolysis — to prevent myoglobin-induced acute kidney injury (AKI)
    • Aggressive IV fluids + consider forced alkaline diuresis
    • Alkalinization reduces myoglobin precipitation in tubules and limits lipid peroxidation
    • Bicarbonate therapy is controversial and requires close monitoring of arterial pH and serum calcium every 2 hours, ideally in ICU
    • Beneficial effect is debated; volume resuscitation remains the cornerstone — Harrison's Principles of Internal Medicine 22E; Campbell Walsh Wein Urology
  3. Phenobarbital / barbiturate overdose — alkalinization increases ionization and renal clearance
  4. Lithium toxicity
    • Osmotic or forced alkaline diuresis is used for more severe symptoms (Li⁺ levels 2–3 mmol/L)
    • Above 3 mmol/L → peritoneal or hemodialysis preferred — The Maudsley Prescribing Guidelines, 15th ed.
  5. Uric acid nephropathy / Tumor lysis syndrome — alkaline diuresis to increase uric acid solubility in tubules (though rasburicase has largely replaced this) — Harrison's Principles of Internal Medicine 22E

Protocol

ComponentDetails
IV fluid0.9% NaCl or dextrose-saline + NaHCO₃
Bicarbonate1–2 mmol/kg bolus, then infusion to maintain urine pH 7.5–8.5
Urine output target3–6 mL/kg/hr
MonitoringUrine pH (hourly), arterial blood gas, serum electrolytes (K⁺, Ca²⁺, Na⁺)

Complications & Monitoring

  • Hypokalemia — most common and important complication; alkalosis drives K⁺ into cells; must replace potassium to maintain urine alkalinization (hypokalemia causes paradoxical aciduria, defeating the purpose)
  • Hypocalcemia — alkalosis reduces ionized calcium; monitor especially in rhabdomyolysis
  • Metabolic alkalosis — systemic pH >7.6 is dangerous; avoid
  • Fluid overload / pulmonary edema — risk with large volumes
  • Hypernatremia — from NaHCO₃ load

Contraindications / Cautions

  • Renal failure (reduced ability to excrete fluid load and adjust pH)
  • Pulmonary edema or cardiac failure
  • Acetazolamide should NOT be used as a substitute — it alkalinizes urine but causes systemic acidosis, which worsens toxin distribution into tissues

When to Escalate to Hemodialysis

Forced alkaline diuresis is a temporizing or adjunctive measure. Hemodialysis is indicated when:
  • Severe salicylate poisoning (especially with renal failure)
  • Lithium levels >3 mmol/L
  • Failure to respond to diuresis
  • Hemodynamic compromise
— Brenner and Rector's The Kidney; The Maudsley Prescribing Guidelines
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