Extracorporeal Elimination Of Poisons

Extracorporeal Elimination Of Poisons

Introduction:-


Poisons are substances that can cause disturbances to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism. 

Initial management for all poisonings includes ensuring adequate cardiopulmonary function and providing treatment for any symptoms such as seizures, shock, and pain. If the toxin was recently ingested, absorption of the substance may be able to be decreased through gastric decontamination in addition to that some poisons have specific antidotes.

An extracorporeal medical procedure is a medical procedure which is performed outside the body.

Although intoxication is a common problem in adult and pediatric medicine, serious morbidity is unusual. In 2004, only 3% of all toxic exposures reported to the Toxic Exposure Surveillance System of the American Association of Poison Centers were treated in an ICU and in only 0.05% extracorporeal treatment was needed.

Indications of extracorporeal elimination:-

The treatment of intoxication with an extracorporeal technique is justified if elimination of the toxin can be increased by 30% or more using an extracorporeal technique.

Extracorporeal elimination of poisons is used if there are signs of severe toxicity as:

1- Ingested quantity associated with severe toxicity.
2- Ingestion of a toxin with serious delayed effects.
3- Natural removal mechanism impaired.
4- Clinical condition deteriorating.

5- Clinical evidence of severe toxicity: hypotension, coma, metabolic acidosis, respiratory depression, dysrhythmias or cardiac decompensation.

Techniques:-

1- Hemodialysis:

During hemodialysis, toxins and other substances are cleared from the blood by diffusion across a semipermeable membrane down a concentration gradient from blood into dialysate. In order to be removed by hemodialysis, the toxic substance must be water soluble and must have a low molecular weight, low protein binding and a low volume of distribution.

The clearance of a toxic substance depends on membrane surface area and type, as well as on blood and dialysate flow rates. The larger the membrane surface, the greater the amount of toxin removed. Newer high-flux membranes can also remove high-molecular weight substances. Increasing blood and dialysate flow rates can increase the concentration gradient between blood and dialysate, thus optimizing the rates of diffusion and elimination. The major drawback of hemodialysis is the risk of rebound toxicity after cessation of the treatment, due to redistribution of the toxin.

                                                                                      

The principle of hemodialysis is the same as other methods of dialysis; it involves diffusion of solutes across a semipermeable membrane. Hemodialysis utilizes counter current flow, where the dialysate is flowing in the opposite direction to blood flow in the extracorporeal circuit. Counter-current flow maintains the concentration gradient across the membrane at a maximum and increases the efficiency of the dialysis.

Fluid removal (ultrafiltration) is achieved by altering the hydrostatic pressure of the dialysate compartment, causing free water and some dissolved solutes to move across the membrane along a created pressure gradient.

Hemodialysis

2- Hemoperfusion:

Hemoperfusion is a medical process used to remove toxic substances from a patient's blood. The technique involves passing large volumes of blood over an adsorbent substance. The adsorbent substance most commonly used in hemoperfusion is resins and activated carbon.

During hemoperfusion, the blood passes through a cartridge containing a sorbent material able to adsorb the toxin. There are three types of sorbents: charcoal based sorbents, synthetic resins and anion exchange resins. In order to be removed by hemoperfusion, the toxic substance must have binding affinity to the sorbent in the cartridge and a low volume of distribution. Charcoal efficiently removes molecules in the 1000–1500 kDa range, but does not remove protein-bound molecules. Resins are more effective in the removal of protein-bound and lipid-soluble molecules. Despite their efficacy, the use of hemoperfusion cartridges has declined over the last 20 years, due to limitations of their indications and shelf life. Moreover, hemoperfusion is technically more difficult to perform than hemodialysis, and lacks the possibility of correcting acid–base, fluid and electrolyte abnormalities.

3- Hemofiltration:

As in dialysis, in hemofiltration one achieves movement of solutes across a semi-permeable membrane. However, solute movement with hemofiltration is governed by convection rather than by diffusion. With hemofiltration, dialysate is not used. Instead, a positive hydrostatic pressure drives water and solutes across the filter membrane from the blood compartment to the filtrate compartment, from which it is drained. Solutes, both small and large, get dragged through the membrane at a similar rate by the flow of water that has been engineered by the hydrostatic pressure. So convection overcomes the reduced removal rate of larger solutes (due to their slow speed of diffusion) seen in hemodialysis.

4- Molecular adsorbent recirculating system:

MARS is a blood purification system, aimed at removing albumin-bound toxic molecules. It consists of three serial extracorporeal circuits: a blood circuit, an albumin detoxification circuit and a hemodialysis circuit. The patient’s blood passes the blood compartment of a high-flux dialyzer, where albumin flows through the dialysate compartment in a countercurrent fashion. Protein-bound and water soluble substances can enter the albumin circuit by means of diffusion. The albumin circuit contains two filters, an activated charcoal filter which absorbs the toxins and an anion-exchange resin filter to cleanse the albumin. Finally, the albumin passes through the blood compartment of a second dialyzer, where small molecules are filtered down a concentration gradient to bicarbonate dialysate. Although the efficacy of MARS in the removal of protein-bound drugs such as diltiazem, phenytoin and theophylline has been demonstrated in case reports, the use of MARS is limited by its availability, technical applicability and high costs.

References:-


1- Poison at Dorland's Medical Dictionary.
2-Anne-Corne´ lie J.M. de Pont," Extracorporeal treatment of intoxications".
Rountree, L. G., and Turner, B. B. The removal of diffusible substances from the circulating blood by means of dialysis. Tn. Assoc. Am. Phys., 28:51, 1913.
4-Feinfeld DA, Rosenberg JW, Winchester JF. Three controversial issues in extracorporeal toxin removal. Semin Dial 2006; 19:358–362.
5-Palmer BF. Effectiveness of hemodialysis in the extracorporeal therapy of phenobarbital overdose. Am J Kidney Dis 2000; 36:640–643.
6- Tan HK. Molecular adsorbent recirculating system (MARS). Ann Acad Med Singapore 2004; 33:329–335.