Toxicology – Enhanced Elimination Techniques III

Peritoneal Dialysis

Mechanism of Action

Dialysis fluid is placed into the peritoneal cavity, and the peritoneal membrane acts as a semipermeable barrier.

Toxicants diffuse from the bloodstream into the dialysate, which is subsequently drained and replaced.

Substances are more readily removed when they are:

• Water soluble
• Minimally protein bound
• Low molecular weight
• Distributed mainly within the bloodstream

Potential Indications

Peritoneal dialysis has historically been used for selected severe poisonings, particularly when hemodialysis is unavailable or technically impossible.

Examples have included:

• Methanol
• Ethylene glycol
• Salicylates
• Theophylline

It has also been considered in infants or neonates when other extracorporeal techniques cannot be performed.

Current Role

Peritoneal dialysis removes most toxicants much more slowly than hemodialysis. Modern intermittent hemodialysis is therefore preferred for most poisonings requiring extracorporeal removal.

Complications

• Peritonitis
• Bowel or abdominal-organ injury
• Fluid and electrolyte abnormalities
• Hypotension
• Volume overload
• Catheter-related complications

Previous abdominal surgery or adhesions can make treatment technically difficult.

Key Point

Peritoneal dialysis now has a very limited role in poisoning and is generally considered only when more effective extracorporeal techniques are unavailable.

Urinary Alkalinization

Mechanism of Action

Urinary alkalinization increases urinary elimination of certain weak acids.

Increasing urine pH causes these substances to become more ionized within the renal tubule. The ionized molecules cannot readily diffuse back across cell membranes, producing “ion trapping” and increasing urinary excretion.

Major Indication

The most important toxicologic indication is:

• Salicylate poisoning

Urinary alkalinization increases renal salicylate elimination and also helps maintain alkalemia, which reduces movement of salicylate into tissues such as the brain.

Other substances whose elimination can theoretically or measurably increase include:

• Phenobarbital
• Chlorpropamide
• 2,4-D herbicides

However, clinical benefit is best established for salicylate toxicity.

Urine alkalinization is also used during high-dose methotrexate therapy to reduce renal precipitation and nephrotoxicity, although this is primarily an oncologic rather than poisoning indication.

Monitoring / Complications

Important problems include:

• Hypokalemia
• Metabolic alkalosis
• Volume overload
• Hypernatremia
• Hypomagnesemia

Adequate potassium is particularly important because hypokalemia makes successful urine alkalinization more difficult.

Patients with severe salicylate poisoning may require hemodialysis rather than alkalinization alone.

Key Point

Think of urinary alkalinization primarily as an enhanced elimination technique for salicylates.

Urinary Acidification

Mechanism of Action

Historically, urine was acidified in an attempt to increase elimination of weakly basic drugs through ion trapping.

Current Role

There are no routine toxicologic indications for urinary acidification.

Potential harms outweigh the limited improvement in drug elimination.

Complications can include:

• Systemic metabolic acidosis
• Worsening kidney injury
• Increased precipitation of myoglobin or hemoglobin within renal tubules
• Electrolyte disturbances

For this reason, urinary acidification is considered an obsolete enhanced-elimination technique.

Key Points

• Peritoneal dialysis: rarely used because hemodialysis generally clears toxicants more efficiently.
• Urinary alkalinization: particularly important for salicylate poisoning through ion trapping of weak acids.
• Urinary acidification: no longer recommended because its risks outweigh its benefits.
• Severe poisoning should not be managed according to pharmacokinetic principles alone; the patient’s clinical condition determines whether more definitive treatment such as hemodialysis is required.