Initial Management of the Poisoned Patient: Perspectives of a Humble Veterinary Toxicologist on Decontamination and Antidotal Treatments

Robert H. Poppenga, DVM, PhD
Chief, Toxicology Laboratory at the University of Pennsylvania
School of Veterinary Medicine

Objectives:

•  Provide a framework for prioritizing therapeutic interventions in an intoxicated patient.

•  Discuss principles of gastrointestinal decontamination (GID).

•  Emesis

•  Adsorbents

•  Cathartics

•  Whole bowel irrigation

•  Facilitating removal of absorbed toxicants.

•  Brief introduction to antidotes.

A number of factors should be considered in deciding the best approach to case management. These factors include substance ingested, amount ingested, co-ingestants, time post ingestion, prior intervention, species/age of animal and concurrent health problems (i.e., liver or kidney dysfunction). In asymptomatic, recently exposed animals, it is important to attempt to determine the severity of exposure to the toxicant. Such an assessment will assist the clinician in choosing the appropriate sequence of management steps to follow (see Figure1). For example, if a dog has recently ingested an amount of an anticoagulant rodenticide that is well below reported toxic doses (less than or equal to 1/10^th of an LD₅₀), then close monitoring at home for several days may be sufficient. Ingestion of higher doses may warrant administration of an adsorbent such as activated charcoal (AC) with or without a cathartic followed by close monitoring in the hospital. In many situations, even where there is a strong suspicion of intoxication, no specific toxicant can be identified and no exposure determined. Alternatively, a known toxicant may have been ingested, but there is no information available concerning its toxicity to the particular animal species exposed. In such situations, extrapolation of toxicity data from other species such as laboratory rodents may be all that is possible. Ultimately, the advice to "treat the patient and not the toxicant" is sound.

Once a determination is 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 toxicant exposure is immediately available), (2) obtain a history and clinically evaluate the patient, (3) prevent continued systemic absorption of the toxicant, (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.^1,2 Obviously, each situation is unique and one or more of the steps may be eliminated. For example, there may not be an antidote for a given toxicant or a way to significantly enhance its elimination once systemically absorbed.

Specific approaches to stabilization of vital signs are discussed more thoroughly in other presentations. Briefly, attention should be paid to maintaining a patent airway and providing adequate ventilation, maintaining cardiovascular function with attention to appropriate fluid and electrolyte administration, maintaining acid-base balance, controlling central nervous system signs such as seizures, and maintaining body temperature. In some situations, it may be critical to administer an antidote quickly. For example, in suspected cholinesterase-inhibiting insecticide intoxications, administration of atropine may be critical to control life-threatening signs before proceeding with subsequent management steps.

Once vital signs are stable, a thorough history should be obtained while the animal is being further evaluated. If blood or urine samples are obtained for clinical evaluation, appropriate portions should be set aside for possible toxicologic testing. A minimum database in suspected toxicologic cases should include CBC, BUN, electrolytes, glucose, ECG and body temperature.

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.²

Gastric evacuation: approaches to gastric evacuation include induction of emesis with emetics such as syrup of ipecac and 3% hydrogen peroxide, apomorphine or xylazine and gastric lavage (GL). Syrup of ipecac and 3% hydrogen peroxide are often available in the home and should be considered for inducing emesis if there will be a delay in bringing an animal to the hospital (see Table 1 for recommended dosages). Owners may have difficulty administering syrup of ipecac to cats due to its objectionable taste. Three percent hydrogen peroxide can be administered relatively easily; if emesis does not occur within 10 minutes, the dose can be repeated once. Emesis is often more effectively induced when the stomach is full; therefore, instructing the owner to feed a small amount of food prior to induction can improve efficacy. Disadvantages of syrup of ipecac include prolonged emesis and adsorption by AC. The later is undesirable since the administration of AC may have to be delayed to allow the emetic action of syrup of ipecac to occur. In a clinical setting, apomorphine is the emetic of choice for dogs.¹ However, apomorphine is not recommended for use in cats. Alternatively, xylazine has been used as an emetic for cats. While apomorphine and xylazine induce emesis quickly, they also cause CNS depression, which is often an unwanted side effect.

GL can be employed in those cases in which gastric evacuation is indicated but administration of an emetic is contraindicated (presence of seizures, severe depression or coma, loss of normal gag reflex, hypoxia, species unable to vomit, and known, prior ingestion of corrosives or volatile petroleum products). In a conscious animal, GL requires anesthesia. Airway protection is necessary whenever GL is performed. As large a gastric tube as possible with terminal fenestrations is introduced into the stomach. Tube placement is confirmed by aspiration of gastric contents or air insufflation with a stesthescope placed over the stomach. After the tube is placed, the mouth should be kept lower than the chest. 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 procedure is repeated until the last several washings are clear; numerous cycles may be required. AC (+/- cathartic) can be administered via the tube just before its removal. The initial lavage sample should be retained for possible toxicologic analysis.

Adsorbents: realistically, the only adsorbent routinely used in companion animal medicine is AC. In rare instances, other adsorbents such as Fuller's earth may be indicated for specific toxicants such as paraquat. AC is an effective adsorbent for a number of toxicants with several notable exceptions including alcohols, corrosives, and some metals such as iron and lithium.³ AC is available as a powder, an aqueous slurry or combined with cathartics such as sorbitol. AC given repeatedly is effective in interrupting enterohepatic recycling of a number of toxicants and the continued presence of AC in the gastrointestinal tract may allow the tract to serve as a sink for trapping toxicant passing from the circulation into the intestines. There is little hazard to repeated administration of AC, although cathartics should be given only once. Substances less likely to be well adsorbed to AC include strongly ionized and dissociated salts such as sodium chloride and small, highly polar, hydrophilic compounds such as alcohols and strong acids, and metals such as lead, iron, lithium. Timing of AC administration is important.

Cathartics: both saline (sodium sulfate or magnesium sulfate or citrate) and saccharide (sorbitol) cathartics are available for use. In theory, cathartics hasten the elimination of unabsorbed toxicant via the stools. In general, cathartics are safe, particularly if used only once. However, repeated administration of magnesium-containing cathartics can lead to hypermagnesemia manifested as hypotonia, altered mental status and respiratory failure. Also, repeated administration of sorbitol can cause fluid pooling in the gastrointestinal tract, excessive fluid losses via the stool and severe dehydration. Table 1 lists the most commonly used GID agents and appropriate dosages.

In recent years, a critical reappraisal of GID approaches in human intoxications has occurred that is relevant for the management of intoxicated animals.⁴ There has been a movement away from gastric evacuation (induction of emesis or GL) followed by the administration of an adsorbent toward the administration of only the adsorbent, especially in mild to moderate intoxications. Early administration of AC alone has been shown to be as efficacious as the combination of gastric evacuation followed by AC. The case for or against the inclusion of a cathartic with AC is less clear-cut but the administration of a single dose of a cathartic along with the initial dose of AC is currently recommended. Those AC formulations that include a cathartic such as sorbitol should be administered only once followed by AC alone if repeated doses are indicated.

One newer approach to human GID is whole bowel irrigation (WBI) which involves the oral administration of large volumes of an electrolyte-balanced solution until a clear rectal effluent is produced. A polyethylene glycol solution, routinely employed to cleanse the gastrointestinal tract for surgical or radiographic procedures, is used.⁴ WBI has been shown to be efficacious in those situations in which an ingested toxicant is poorly adsorbed to AC or in which sustained-release medications have been ingested. Another potential use might be in those instances in which small metal objects or lead-based paint have been ingested. WBI has been well tolerated in human pediatric patients. The utility of WBI in veterinary medicine has not been determined, but the need to administer large volumes of liquid may limit its use.

Critical factors that govern appropriate management choices include severity of toxicant exposure, time since exposure and the type of toxicant to which the animal was exposed. Determining the severity of exposure must consider several factors including the inherent toxicity of the chemical, the dose, the species and age of animal and the presence of underlying disease conditions.

Antidotes should be administered if indicated and available. Due to the costs associated with keeping a full range of antidotes on hand, many veterinarians may not have ready access to all of those that are clinically useful. Antidotes that should be immediately available for case management include N-acetylcysteine, atropine sulfate, calcium disodium EDTA/succimer, calcitonin/ pamidronate, ethanol/fomepizole, pralidoxime and vitamin K₁. Antidotes that may be needed less frequently include D-penicillamine, deferoxamine mesylate, naloxone, antivenins, BAL and flumazenil. It is recommended that a source such as a human hospital or pharmacy be identified for obtaining the latter antidotes prior to their need.

Facilitating the removal of absorbed toxicants via the urine may be indicated in several specific situations. For example, alkalinization of the urine to a pH of 7.0 or greater with sodium bicarbonate has been shown to enhance the urinary elimination of weak acids such as ethylene glycol, salicylates, phenobarbital, and the herbicide, 2,4-D.¹ The administration of ammonium chloride to acidify the urine (pH of 5.5 to 6.5) may enhance the elimination of weak bases such as amphetamine and strychnine. However, urinary alkalinization or acidification requires close patient monitoring to avoid acid-base disturbances. AC may provide a "gut" dialysis affect as alluded to above.

Other methods for hastening elimination of an absorbed toxicant such as charcoal hemoperfusion and hemodialysis are less practical or available in veterinary medicine.

Fortunately, many intoxicated patients will recover if attention is paid to appropriate symptomatic and supportive care. For example, even if GID is not possible following the ingestion of strychnine, effective control of muscle rigidity with pentobarbital should result in complete recovery from intoxication.

Additional Reading

1.  Beasley VR and Dorman DC (1990) Management of toxicoses. In: Veterinary Clinics of North America: Toxicology of Selected Pesticides, Drugs, and Chemicals, VR Beasley (ed.), pp. 307-337. W.B. Saunders, Philadelphia, PA.

2.  Shannon MW and Haddad LM (1998) The emergency management of poisoning. In: Clinical Management of Poisoning and Drug Overdose, LM Haddad et al. (eds.), pp. 2-31. W.B. Saunders, Philadelphia, PA

3.  Howland MA (1994) Antidotes in-depth: activated charcoal. In: Toxicologic Emergencies, LR Goldfrank et al. (eds.), pp. 66-71. Appleton and Lange, Norwalk, CT

4.  Perry H and Shannon M (1996) Emergency department gastrointestinal decontamination. Pediatric Annals 25, 19-29.