When is forced alkaline diuresis indicated?

Indications for Forced Alkaline Diuresis

Forced alkaline diuresis is primarily indicated for moderately severe salicylate poisoning that doesn't meet criteria for hemodialysis, and for severe poisoning with chlorphenoxy herbicides (2,4-dichlorophenoxyacetic acid and mecoprop). 1

Primary Indications

• Salicylate poisoning: First-line treatment for moderately severe cases that don't require hemodialysis 1, 2
• Phenobarbital poisoning: Although effective, multiple-dose activated charcoal is superior as first-line treatment 1
• Chlorphenoxy herbicides: Effective for 2,4-dichlorophenoxyacetic acid and mecoprop poisoning when combined with high urine flow (approximately 600 mL/h) 1

Secondary Indications

• Barbiturate poisoning: Particularly effective for long-acting barbiturates like phenobarbital (20-25% excreted unchanged in urine) compared to short-acting agents like pentobarbital (<5% excreted unchanged) 3
• Rhabdomyolysis: Indicated when myoglobinuria develops, such as following nerve agent poisoning that causes myonecrosis 3
• Fluoride poisoning: Evidence from volunteer studies suggests increased elimination, though clinical confirmation is limited 1
• Methotrexate toxicity: Limited supporting evidence, but clinically employed 1

Mechanism of Action

Urine alkalinization works through ion trapping of weak acids in the renal tubules:

• Increases the ionized fraction of weak acids in the urine 1
• Prevents reabsorption across the renal tubular epithelium 1
• Most effective for compounds with pKa values between 3.0 and 7.5 3

Implementation Protocol

1. Administration: Intravenous sodium bicarbonate to achieve urine pH ≥7.5 1
2. Hydration: Aggressive hydration to promote urine flow and excretion 3
3. Monitoring: Regular assessment of:
   • Urine pH (target ≥7.5) 1
   • Serum electrolytes, especially potassium 1
   • Fluid balance 2
   • Acid-base status 4

Contraindications and Cautions

• Renal failure or oliguria: Contraindicated due to inability to achieve adequate urine flow 3
• Fluid overload: Risk of pulmonary edema, especially in patients with cardiac dysfunction 2
• Electrolyte disturbances: Particularly hypokalemia, which requires correction 1
• Alkalotic tetany: May occur with excessive alkalinization 1

Special Considerations

• Tumor Lysis Syndrome: Historically used but now controversial due to risk of calcium phosphate precipitation; not recommended when rasburicase is used 3
• Barbiturate poisoning: More effective for long-acting agents (phenobarbital) than short-acting ones due to differences in renal clearance 3
• Salicylate poisoning: Urine alkalinization alone may be safer and equally effective as forced alkaline diuresis 2, 5

Complications

• Hypokalemia: Most common complication, requires potassium supplementation 1
• Fluid retention: Particularly problematic in salicylate poisoning which already causes sodium and fluid retention 2
• Metabolic alkalosis: Blood pH values approaching 7.70 have been recorded 1
• Calcium phosphate precipitation: Risk increases with alkalinization, especially in tumor lysis syndrome 3

Monitoring During Treatment

• Vital signs: Heart rate, blood pressure, respiratory rate 3
• Urine output: Maintain at least 100 mL/hour in adults 3
• Urine pH: Target ≥7.5 1
• Serum electrolytes: Particularly potassium, calcium, and phosphate 3
• Acid-base status: Monitor for excessive alkalosis 4

Remember that while forced alkaline diuresis was historically used for many toxins, current evidence supports its use primarily for salicylate and chlorphenoxy herbicide poisoning, with more limited roles in other specific poisonings 1.