Decontaminating poisoned pets (Proceedings)

To Treat or Not to Treat, That is the Question

A number of factors should be considered in deciding the best approach to case management.

These factors include: Substance ingested, amount of substance ingested, co-ingestants, time post ingestion, prior intervention(s), species/age of animal, and concurrent health problems (particularly problems that might interfere with detoxification or elimination of the ingested substance, e.g., liver or kidney dysfunction)

In asymptomatic, recently exposed animals, it is important to attempt to determine the severity of exposure to the toxicant (exposure assessment). Such an assessment will assist you 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 a reported toxic dose (less than or equal to 1/10th 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 symptomatic animals, the first priority is to stabilize the patient and then to consider whether decontamination procedures are warranted. In such situations, an exposure assessment may need to be postponed until a later time.

Figure 1. Treatment alogrithm for initial management of small animal intoxications. AC = activated charcoal, WBI = whole bowel irrigation.

*The 1/10th of an LD50 rule of thumb is only a General Guideline. Non-toxic doses are dependent on a number of factors including the nature of the dose-response curve for the exposed species. Extrapolations from available data are often required.

In many situations, even where there is a strong suspicion of intoxication, no specific toxicant can be identified and therefore, no exposure determined. Fortunately, with many toxicant exposures, appropriate symptomatic and supportive care will result in a positive case outcome. Sometimes 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.

Prioritizing Your Priorities

Once a determination is made that an animal has been exposed to (or potentially 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. Obviously, each situation is unique and one or more of the steps may be eliminated or their priority changed depending on the circumstances of the case. For example, there may not be an antidote for a given toxicant or a way to significantly enhance its elimination once it has been systemically absorbed. In some situations with a known exposure, 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.

Prevent Continued Absorption of Toxicant

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 such as syrup of ipecac, 3% hydrogen peroxide, apomorphine or xylazine or 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. Both are undesirable since the administration of AC may have to be delayed to allow emesis to stop and AC to be retained in the stomach or the emetic action of syrup of ipecac to occur.

Table 1: Common gastrointestinal and systemic decontamination agents.

In a clinical setting, apomorphine is the emetic of choice for dogs. 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. Fortunately, drugs are available to reverse some or all of the effects of apomorphine (naloxone) and xylazine (yohimbine, atipamezole or tolazidine). The effectiveness of emetics in most cases decreases as the time from ingestion increases. In fact, in most cases, emetics may not be all that useful if the time since ingestion is > 1 hour.

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 stethoscope 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. 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. AC is available as a powder, an aqueous slurry or combined with cathartics such as sorbitol. AC given repeatedly (multiple dose activated charcoal or MDAC) is effective in interrupting entero-hepatic 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.

Experimental studies have shown that delaying administraton of AC from 30 minutes post ingestion to 60 minutes post ingestion results in a mean increase in systemic drug absorption from 36% (100 – 54 = 36) of the ingested dose to 63% (100 – 37 = 63).

A relatively new product, Bio-Sponge™, has been used by some equine clinicians to adsorb some natural toxicants. The product is designed to be used to adsorb bacterial toxins. The advantage of its use over AC in poisoning cases has not been investigated in detail.

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.

In recent years, a critical reappraisal of the role of GID in human intoxications has occurred that is relevant for the management of intoxicated animals. 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.

Enhance Elimination of an Absorbed Toxicant

AC may provide a "gut" dialysis affect as alluded to above. Facilitating the removal of absorbed toxicants via ion-trapping in 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. 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 (urinary acidification is generally not recommended). Some textbooks or articles will recommend diuresis to hasten elimination of an absorbed toxicant. However, the efficacy of this approach has not been studied in detail. Obviously, enhancing renal elimination of a toxicant depends on the ability of the kidney to eliminate the toxicant under normal circumstances. Other methods for hastening elimination of an absorbed toxicant such as charcoal hemoperfusion and hemodialysis are less practical or available in veterinary medicine. Some tertiary veterinary care centers now have the capability to perform hemodialysis; this can be used to enhance toxicant removal in some cases.

Provide Symptomatic and Supportive Care

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.

References

Management of Toxicoses by Beasley VR and Dorman DC (1990) In: Veterinary Clinics of North America: Toxicology of Selected Pesticides, Drugs, and Chemicals, pp. 307-337.

Establishing a Minimum Database in Small Animal Poisonings by Kevin Fitzgerald in Small Animal Toxicology by Peterson and Talcott, 48-55, 2001.

Supportive Care of the Poisoned Patient by Matthew Mellema and Steve Haskins in Small Animal Toxicology by Peterson and Talcott, 56-84, 2001.

Toxicologic Decontamination by Michael Peterson in Small Animal Toxicology by Peterson and Talcott, 85-98, 2001.

Decontamination Strategies by Marcy Rosendale in Veterinary Clinics of North America: Small Animal Practice 32, 2002.

Treatment by Robert Poppenga in Clinical Veterinary Toxicology, 13-20, 2004.