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Effective decontamination strategies for poisoned horses (Proceedings)

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Aug 01, 2008

Principles of Case Management

Once a determination has been made that an animal has been exposed to a toxicant or is intoxicated, a general approach to case management should adhere to the following principles: (1) stabilize vital signs (this may include administration of an antidote if sufficient information concerning a specific toxin exposure is immediately available), (2) obtain a history and clinically evaluate the patient, (3) prevent continued systemic absorption of the toxin, (4) administer an antidote if indicated and available, (5) enhance elimination of absorbed toxicant, (6) provide symptomatic and supportive care, and (7) closely monitor the patient. It is recommended that the above principles be followed in sequence with modifications made depending on the circumstances of the case. For example, there may not be an antidote for a given plant toxin or a way to significantly enhance its elimination once systemically absorbed.

Gastrointestinal Decontamination

Gastrointestinal decontamination (GID) is a critical component of case management. Appropriate and timely decontamination may prevent the onset of clinical signs or significantly decrease the severity or shorten the course of intoxication. GID consists of three components: (1) gastric evacuation, (2) administration of an adsorbent and (3) catharsis.

Gastric Evacuation

Approaches to gastric evacuation include induction of emesis with emetics or gastric lavage (GL); either one or the other approach is used depending on the species affected and clinical condition of the animal. Since horses do not vomit, induction of emesis is not a viable option in this species. GL may be used as an alternative depending on the condition and temperament of the horse. GL requires the placement of a stomach tube and the repeated instillation of water or normal saline to remove stomach contents. Airway protection is essential whenever GL is performed. As large a nasogastric tube as possible with terminal fenestrations is introduced into the stomach. Tube placement is confirmed by visual inspection, palpation, detection of gurgling noises through the tube or reflux/aspiration of gastric contents. Tepid tap water or normal saline (5 to 10 ml/kg) is introduced into the stomach with minimal pressure application and is withdrawn by aspiration or allowed to return via gravity flow. The instillation of water or saline is repeated until the last several washings are clear; numerous cycles may be required. Activated charcoal (AC) +/- cathartic can be administered via the tube just before its removal. The initial lavage sample should be retained for possible toxicologic analysis.

Administration of an Adsorbent

Realistically, the only adsorbent routinely used in veterinary medicine is AC. AC is produced in a two-step process. The first step involves the pyrolysis of carbonaceous materials such as wood, coconut or peat followed by high temperature treatment with activating agents such as steam or carbon dioxide. The activation step increases the adsorptive capacity of the material by increasing its total surface area as a result of the formation of a maze of internal pores. The rate of adsorption of a toxicant is dependent on the external surface area while the adsorptive capacity is dependent on the internal surface area. Adsorption is believed to be due to hydrogen bonding, ion-ion, dipole and van der Waal's forces.

AC is likely to be an effective adsorbent for most organic toxicants, although the adsorptive capacity of AC for most has not been experimentally determined. Nonpolar, poorly water soluble organic substances are more likely to be adsorbed and adsorption is enhanced with an increase in molecular size of the toxicant. Small, polar, water-soluble organics are less well adsorbed. In vitro studies have demonstrated that adsorption begins almost immediately after instillation of AC but may not reach equilibrium for 10 to 25 minutes. AC has been shown to decrease the systemic absorption of a number of drugs including aspirin, acetaminophen, barbiturates, glutethimide, phenytoin, theophylline, cyclic antidepressants and most inorganic and organic chemicals. However, AC does not adsorb alcohols, strong acids and alkalies, iron and lithium. Even if a toxicant is not well adsorbed by AC, partial adsorption may be sufficient to affect clinical outcome. In general, the efficacy of AC administration is directly related to the rapidity of administration after toxicant ingestion.

AC is available as a powder, an aqueous slurry or combined with cathartics such as sorbitol. If using a powder, 2 to 5 g of AC should be mixed with tepid tap water (~ 1 g AC to 5 ml of water) to form a slurry. The slurry should be administered via a stomach tube using minimal pressure.