Urinary Alkalinisation for Poisoning Management

Urinary alkalinisation is a therapeutic technique used to enhance the elimination of certain drugs in poisoning by raising urinary pH (typically to ≥7.5) using intravenous sodium bicarbonate. This increases the ionization of weakly acidic drugs in the renal tubular fluid, which reduces their lipid solubility and back-diffusion (reabsorption) across the tubular epithelium, thereby trapping the drug in the urine ("ion trapping") and increasing its clearance.

Principle: Which Drugs Respond to Alkalinisation?

• Urinary alkalinisation is effective only for drugs that are weak acids with a pKa in a range where ionization is significantly altered by raising urine pH (roughly pKa 3.0–7.5).
• The classic, textbook indications for alkaline diuresis are salicylate (aspirin) poisoning and phenobarbitone (a long-acting barbiturate) poisoning.
• It has no meaningful role in poisoning with weak bases or drugs with a high volume of distribution, since these are not significantly affected by urinary pH changes and are largely tissue-bound.

Analysis of Each Option

• Option 1: Lithium

  Lithium is an inorganic cation (Li⁺), not a weak acid, and it is not affected by urinary pH manipulation at all — its handling by the kidney mirrors sodium, and it is not "ionized" or "de-ionized" by alkalinisation. Enhanced elimination of lithium in severe toxicity is achieved with saline diuresis and hemodialysis, not urinary alkalinisation. This option is incorrect.

• Option 2: Alprazolam

  Alprazolam is a benzodiazepine with a very large volume of distribution and extensive hepatic metabolism; it is not renally eliminated in a pH-dependent unchanged form. Urinary alkalinisation does not enhance its clearance, and treatment of benzodiazepine overdose is supportive (with flumazenil in select cases), not alkaline diuresis. This option is incorrect.

• Option 3: Phenytoin

  Although phenytoin is a weak acid, it is almost completely (>90%) protein-bound and extensively hepatically metabolized, with very little unchanged drug excreted renally. As a result, urinary alkalinisation does not produce a clinically useful increase in its elimination. Phenytoin toxicity is managed supportively. This option is incorrect.

• Option 4: Phenobarbitone (correct answer)

  Phenobarbitone is the classic example of a poison whose excretion is significantly enhanced by urinary alkalinisation. As a long-acting barbiturate and a weak acid (pKa ~7.3), a significant fraction is excreted unchanged by the kidneys. Alkalinising the urine ionizes the drug within the renal tubule, preventing its reabsorption and substantially increasing (multi-fold) its rate of urinary excretion. Along with salicylates, phenobarbitone poisoning is a standard clinical indication for alkaline diuresis (multiple-dose activated charcoal is also used adjunctively).

Conclusion

Among the options, only phenobarbitone meets the pharmacokinetic criteria (weak acid, pKa in the therapeutic range, significant renal excretion of unchanged drug) required for urinary alkalinisation to meaningfully enhance elimination. Lithium is managed with saline diuresis/dialysis, while alprazolam and phenytoin are eliminated mainly by hepatic metabolism and are not amenable to alkaline diuresis. Hence, Phenobarbitone is the correct answer.