Toxicology – Enhanced Elimination Techniques I

Enhanced elimination techniques are used in selected severe poisonings to increase removal of a toxicant from the body or temporarily support the patient while normal elimination occurs. Choice depends on the toxicant’s pharmacokinetics and the patient’s clinical condition.

Cardiopulmonary Bypass (CPB)

Mechanism of Action

Cardiopulmonary bypass primarily provides temporary circulatory and respiratory support rather than directly removing the poison.

By maintaining organ perfusion during otherwise refractory cardiovascular collapse, it can provide time for:

• Hepatic metabolism
• Renal elimination
• Redistribution of the toxicant
• Recovery from reversible cardiotoxicity

Modern extracorporeal support such as VA-ECMO has largely assumed this role in many severe poisonings.

Indications

May be considered for otherwise refractory cardiovascular collapse caused by a potentially reversible poisoning, particularly when conventional resuscitation has failed.

It has historically been reported in severe cardiotoxic drug poisonings such as:

• Flecainide
• Lidocaine and other local anesthetics

Limitations / Complications

• Requires specialized personnel and equipment
• Major vascular access is required
• Bleeding and anticoagulation complications can occur
• It does not necessarily provide substantial direct toxicant clearance

Key Point

Think of extracorporeal circulatory support as a way to “buy time” for recovery and endogenous drug elimination, rather than as a conventional dialysis technique.

Exchange Transfusion

Mechanism of Action

The patient’s blood is progressively removed and replaced with donor blood or blood components.

This can remove toxicants that are largely confined to the intravascular compartment and replace damaged blood cells.

Possible Indications

Rarely considered for:

• Severe methemoglobinemia refractory to standard therapy
• Selected severe poisoning in neonates or infants when other extracorporeal techniques are unsuitable
• Severe toxin-induced hemolysis in exceptional circumstances

Limitations / Complications

• Transfusion reactions
• Hypothermia
• Hypotension
• Hypocalcemia
• Coagulopathy
• Thrombocytopenia
• Infection and other transfusion-related complications

Key Point

Exchange transfusion is now an uncommon, specialized rescue technique and is most useful when the relevant toxicant or toxic effect is concentrated within the blood.

Hemodialysis

Mechanism of Action

Blood passes along a semipermeable membrane, allowing toxic substances to diffuse into the dialysate.

A toxicant is generally easier to dialyze when it has:

• Low molecular weight
• Low protein binding
• Small volume of distribution
• High water solubility

Modern high-flux dialysis can remove some substances that older dialysis systems handled poorly.

Important Dialyzable Poisons

Hemodialysis has an established role in selected severe poisonings involving:

• Methanol
• Ethylene glycol
• Lithium
• Salicylates
• Theophylline

It can also simultaneously correct:

• Severe metabolic acidosis
• Electrolyte abnormalities
• Fluid disturbances

Dialysis may occasionally be used for other toxicants, but increased clearance alone does not mean that dialysis improves clinical outcomes enough to justify the procedure.

Complications

• Hypotension
• Fluid shifts
• Electrolyte abnormalities
• Vascular-access complications
• Bleeding related to anticoagulation

Important Considerations

Some antidotes, including fomepizole, can themselves be removed during dialysis, so antidote regimens may require adjustment under specialist guidance.

After dialysis, the serum toxicant concentration may occasionally rebound because drug stored in tissues redistributes back into the bloodstream. Additional treatment may therefore be required.

Key Point

Hemodialysis is most useful when the poison remains substantially in the bloodstream and can readily cross the dialysis membrane.

Hemoperfusion

Mechanism of Action

Instead of relying primarily on diffusion across a membrane, hemoperfusion passes blood through a cartridge containing an adsorbent material, historically activated charcoal or resin.

The toxicant binds to the cartridge and is removed from circulation.

Potentially Suitable Toxicants

Hemoperfusion works best when a substance:

• Has a relatively small volume of distribution
• Has low endogenous clearance
• Can be effectively adsorbed by the cartridge

Historically important examples include:

• Theophylline
• Carbamazepine
• Phenobarbital

Limitations / Complications

• Thrombocytopenia
• Bleeding and anticoagulation complications
• Vascular-access complications
• Hypotension
• Possible rebound after treatment

Unlike hemodialysis, hemoperfusion does not effectively correct metabolic acidosis, electrolyte abnormalities, or fluid disturbances.

Current Role

Hemoperfusion is used much less frequently today because modern hemodialysis is more readily available and effective for many dialyzable poisonings.

Key Points

• CPB/VA-ECMO: supports circulation while the body clears the poison.
• Exchange transfusion: replaces circulating blood and is rarely used.
• Hemodialysis: removes small, water-soluble, relatively unbound toxicants and simultaneously corrects acid-base/electrolyte problems.
• Hemoperfusion: directly adsorbs circulating toxicants but has a more limited modern role.
• Decisions about extracorporeal treatment depend on the patient’s clinical severity as well as toxicant characteristics, rather than serum concentration alone.
• Early consultation with toxicology and nephrology/critical-care specialists is important when extracorporeal treatment may be required.

Cardiopulmonary bypass primarily provides temporary circulatory and respiratory support rather than directly removing the poison. By maintaining organ perfusion during otherwise refractory cardiovascular collapse, it can provide time for:  Hepatic metabolism Renal elimination Redistribution of the toxicant Recovery from reversible cardiotoxicity  Modern extracorporeal support such as VA-ECMO has largely assumed this role in many severe poisonings. Indications

May be considered for otherwise refractory cardiovascular collapse caused by a potentially reversible poisoning, particularly when conventional resuscitation has failed. It has historically been reported in severe cardiotoxic drug poisonings such as:  Flecainide Lidocaine and other local anesthetics  Limitations / Complications  Requires specialized personnel and equipment Major vascular access is required Bleeding and anticoagulation complications can occur It does not necessarily provide substantial direct toxicant clearance  Key Point

Think of extracorporeal circulatory support as a way to “buy time” for recovery and endogenous drug elimination, rather than as a conventional dialysis technique.

Exchange Transfusion Mechanism of Action

The patient’s blood is progressively removed and replaced with donor blood or blood components. This can remove toxicants that are largely confined to the intravascular compartment and replace damaged blood cells. Possible Indications

Rarely considered for:  Severe methemoglobinemia refractory to standard therapy Selected severe poisoning in neonates or infants when other extracorporeal techniques are unsuitable Severe toxin-induced hemolysis in exceptional circumstances  Limitations / Complications  Transfusion reactions Hypothermia Hypotension Hypocalcemia Coagulopathy Thrombocytopenia Infection and other transfusion-related complications  Key Point

Exchange transfusion is now an uncommon, specialized rescue technique and is most useful when the relevant toxicant or toxic effect is concentrated within the blood.

Hemodialysis Mechanism of Action

Blood passes along a semipermeable membrane, allowing toxic substances to diffuse into the dialysate. A toxicant is generally easier to dialyze when it has:  Low molecular weight Low protein binding Small volume of distribution High water solubility  Modern high-flux dialysis can remove some substances that older dialysis systems handled poorly. Important Dialyzable Poisons

Hemodialysis has an established role in selected severe poisonings involving:  Methanol Ethylene glycol Lithium Salicylates Theophylline  It can also simultaneously correct:  Severe metabolic acidosis Electrolyte abnormalities Fluid disturbances  Dialysis may occasionally be used for other toxicants, but increased clearance alone does not mean that dialysis improves clinical outcomes enough to justify the procedure. Complications  Hypotension Fluid shifts Electrolyte abnormalities Vascular-access complications Bleeding related to anticoagulation  Important Considerations

Some antidotes, including fomepizole, can themselves be removed during dialysis, so antidote regimens may require adjustment under specialist guidance. After dialysis, the serum toxicant concentration may occasionally rebound because drug stored in tissues redistributes back into the bloodstream. Additional treatment may therefore be required. Key Point

Hemodialysis is most useful when the poison remains substantially in the bloodstream and can readily cross the dialysis membrane.

Hemoperfusion Mechanism of Action

Instead of relying primarily on diffusion across a membrane, hemoperfusion passes blood through a cartridge containing an adsorbent material, historically activated charcoal or resin. The toxicant binds to the cartridge and is removed from circulation. Potentially Suitable Toxicants

Hemoperfusion works best when a substance:  Has a relatively small volume of distribution Has low endogenous clearance Can be effectively adsorbed by the cartridge  Historically important examples include:  Theophylline Carbamazepine Phenobarbital  Limitations / Complications  Thrombocytopenia Bleeding and anticoagulation complications Vascular-access complications Hypotension Possible rebound after treatment  Unlike hemodialysis, hemoperfusion does not effectively correct metabolic acidosis, electrolyte abnormalities, or fluid disturbances. Current Role

Hemoperfusion is used much less frequently today because modern hemodialysis is more readily available and effective for many dialyzable poisonings. Key Points  CPB/VA-ECMO: supports circulation while the body clears the poison. Exchange transfusion: replaces circulating blood and is rarely used. Hemodialysis: removes small, water-soluble, relatively unbound toxicants and simultaneously corrects acid-base/electrolyte problems. Hemoperfusion: directly adsorbs circulating toxicants but has a more limited modern role. Decisions about extracorporeal treatment depend on the patient’s clinical severity as well as toxicant characteristics, rather than serum concentration alone. Early consultation with toxicology and nephrology/critical-care specialists is important when extracorporeal treatment may be required.