Bicarbonate therapy for the cardiovascular toxicity of amitriptyline in an animal model

A Literature Review of the Use of Sodium Bicarbonate for the Treatment of QRS Widening

Sodium bicarbonate is a well-known antidote for tricyclic antidepressant (TCA) poisoning. It has been used for over half a century to treat toxin-induced sodium channel blockade as evidenced by QRS widening on the electrocardiogram (ECG). The purpose of this review is to describe the literature regarding electrophysiological mechanisms and clinical use of this antidote after poisoning by tricyclic antidepressants and other agents. This article will also address the literature supporting an increased serum sodium concentration, alkalemia, or the combination of both as the responsible mechanism(s) for sodium bicarbonate's antidotal properties. While sodium bicarbonate has been used as a treatment for cardiac sodium channel blockade for multiple other agents including citalopram, cocaine, flecainide, diphenhydramine, propoxyphene, and lamotrigine, it has uncertain efficacy with bupropion, propranolol, and taxine-containing plants.

Clinical Course, Therapy, Outcome and Analytical Data in Amitriptyline and Combined Amitriptyline/Chlordiazepoxide Overdose

A total of 103 cases of amitriptyline (AT) overdose (group 1) and 81 cases of overdose with a fixed combination of AT and chlordiazepoxide (CDE) (group 2), treated at our Intensive Care Unit or reported to our Poison Information Center between 1985-1990, were evaluated with respect to clinical course, symptoms and outcome, as well as efficacy of therapy. The mean amount of AT was considerably higher in group 1 compared to group 2 (13 mg kg-1 vs 7.7 mg kg-1 ). The most frequent symptoms in both groups were impaired consciousness, anticholinergic symptoms, seizures, arrhythmia and hypotension. Respiratory insufficiency necessitated respirator therapy in 63 of the patients. Two patients in group 1 and one patient in group 2 did not survive.

Therapy included primary detoxification by gastric lavage and repeated administration of activated charcoal. In four of eight patients with cardiac conduction disturbances, hypertonic sodium bicarbonate led to a significant reduction in QRS duration and AV interval. Physostigmine was effective in eight of 14 patients with pronounced anticholinergic symptoms. No effect was observed in the other six patients. Haemoperfusion, which was performed in five patients, led to rapid improvement of coma after initiation of therapy in four patients. The clinical efficacy of haemoperfusion in AT overdose despite the high volume of distribution of AT deserves further investigation.

The rather high average overdose of AT implies that large package sizes of AT were available to the patients. A major step towards prevention of serious AT overdose would be the prescription of package sizes containing a total amount of less than 500 mg AT.

Different analytical methods (enzyme immunoassay, fluorescence polarization immunoassay, and gas chromatography/mass spectrometry) for rapid detection of AT and its metabolites in plasma and urine were evaluated. Commercially available immunoassays like EMIT and ADX were highly reliable and sensitive in the detection of AT overdose. Five previously unknown metabolites or derivatives of AT could be detected in cases of AT overdose with the aid of a gas chromatography/mass spectrometry screening procedure.

Delayed-onset seizure and cardiac arrest after amitriptyline overdose, treated with intravenous lipid emulsion therapy

In recent years, intravenous lipid emulsion (ILE) therapy has emerged as a new rescue antidote for treatment of certain toxicities, including cyclic antidepressants, and as the primary treatment of toxic manifestations after local anesthetic exposure. We present a case of a 13-year-old girl who developed delayed seizures and cardiac arrest after amitriptyline ingestion. As part of the treatment, she was treated with ILE therapy. The patient's laboratories were not interpretable for several hours after the lipid emulsion. The patient developed pancreatitis after the ILE therapy. This case is unique; not only is it one of the first reported cases of lipid emulsion being used in a pediatric patient, but in that the patient developed delayed toxicity and iatrogenic harm from the ILE.

Management of the Cardiovascular Complications of Tricyclic Antidepressant Poisoning

Experimental studies suggest that both alkalinisation and sodium loading are effective in reducing cardiotoxicity independently. Species and experimental differences may explain why sodium bicarbonate appears to work by sodium loading in some studies and by a pH change in others. In the only case series, the administration of intravenous sodium bicarbonate to achieve a systemic pH of 7.5-7.55 reduced QRS prolongation, reversed hypotension (although colloid was also given) and improved mental status in patients with moderate to severe tricyclic antidepressant poisoning. This clinical study supports the use of sodium bicarbonate in the management of the cardiovascular complications of tricyclic antidepressant poisoning. However, the clinical indications and dosing recommendations remain to be clarified. Hypotension should be managed initially by administration of colloid or crystalloid solutions, guided by central venous pressure monitoring. Based on experimental and clinical studies, sodium bicarbonate should then be administered. If hypotension persists despite adequate filling pressure and sodium bicarbonate administration, inotropic support should be initiated. In a non-randomised controlled trial in rats, epinephrine resulted in a higher survival rate and was superior to norepinephrine both when the drugs were used alone or when epinephrine was used in combination with sodium bicarbonate. Sodium bicarbonate alone resulted in a modest increase in survival rate but this increased markedly when sodium bicarbonate was used with epinephrine or norepinephrine. Clinical studies suggest benefit from norepinephrine and dopamine; in an uncontrolled study the former appeared more effective. Glucagon has also been of benefit. Experimental studies suggest extracorporeal circulation membrane oxygenation is also of potential value. The immediate treatment of arrhythmias involves correcting hypoxia, electrolyte abnormalities, hypotension and acidosis. Administration of sodium bicarbonate may resolve arrhythmias even in the absence of acidosis and, only if this therapy fails, should conventional antiarrhythmic drugs be used. The class 1b agent phenytoin may reverse conduction defects and may be used for resistant ventricular tachycardia. There is also limited evidence for benefit from magnesium infusion. However, class 1a and 1c antiarrhythmic drugs should be avoided since they worsen sodium channel blockade, further slow conduction velocity and depress contractility. Class II agents (beta-blockers) may also precipitate hypotension and cardiac arrest.

Review of prehospital sodium bicarbonate use for cyclic antidepressant overdose

OBJECTIVE

To describe the clinical presentation of patients with cyclic antidepressant (CA) and use of sodium bicarbonate (NaHCO(3)) in the treatment of this overdose in the prehospital setting.

METHODS

A three year retrospective observational review of records was performed using the San Diego County Quality Assurance Network database for prehospital providers. All adult patients who were treated with NaHCO(3) by paramedics for a CA overdose were included. Demographic data, presenting cardiovascular and neurological symptoms, paramedic treatments, and any changes in status were reviewed.

RESULTS

Twenty one patients were treated by paramedics with NaHCO(3) for CA overdose. Seventeen patients (80%) presented with mental status changes, including 11 presenting with a GCS<8. Seven of the 21 (33%) presented with a cardiac arrhythmia expected to possibly respond to NaHCO(3) treatment. Seven of the 21 (33%) were hypotensive, and five (24%) patients had reported seizure activity. Only 2 of the 21 patients (10%) treated with NaHCO(3) had recorded improvements after administration of the drug, while the other 19 remained stable without any deterioration. Sixteen of 21 patients (76%) were given NaHCO(3) for indications on standing order, while five patients were treated outside the standing order indications by base physician order with none of the five patients having any change in status ater treatment.

CONCLUSIONS

After prehospital NaHCO(3) use in patients with CA overdose, there were no complications reported, two patients improved in status and the others remained unchanged. Base hospital physician orders of NaHCO(3) for indications beyond the standing orders were not associated with changes in patient status.

Tricyclic antidepressants directly depress human myocardial mechanical function independent of effects on the conduction system

OBJECTIVES

To measure the effect of tricyclic antidepressant drugs (TCAs) on human myocardial contractility.

METHODS

Human atrial tissue was obtained during cardiac bypass surgery. The tissue was harvested, suspended in a Tyrode buffer at 37 degrees C, and perfused with a 95%/5% oxygen-carbon dioxide mixture. Developed force was continuously measured using a force transducer and recorded by computer. After an equilibration period, escalating doses of amitriptyline or desipramine were added to the bath. All strips were exposed to the following five concentrations of each drug: 0 (control) 0.4, 4, 40, and 400 microM. The results for each experiment were expressed as the difference between the developed force measured prior to the addition of each concentration of drug and the developed force measured after a 30-minute exposure to the drug.

RESULTS

Desipramine decreased the developed force by 27%, 49%, and 74% at concentrations of 0.4, 40, and 400 microM, respectively. Amitriptyline decreased the developed force by 38% at the 40-microM concentration and by 89% at the 400-microM concentration. Untreated strips retained 94% of baseline developed force at 150 minutes.

CONCLUSIONS

Tricyclic antidepressants depress human myocardial function in a dose-dependent fashion independent of the effects on the cardiac conduction system. While previous work has demonstrated the effect of therapies for the reversal of impaired cardiac conduction following TCA poisoning, to the best of the authors' knowledge, no reports have documented the effects of therapy on direct myocardial depression. Additional therapies targeted at reversing the direct cardiodepressive effects of TCA may improve outcome following TCA poisoning.

Plasma alkalinization for tricyclic antidepressant toxicity: a systematic review

OBJECTIVE

To review the evidence that plasma alkalinization improves the outcome in tricyclic antidepressant toxicity.

METHODS

Medline search from 1966 to October 2000 (articles in all languages were included) and examination of bibliographies. Published papers including animal studies, in vitro studies, human case reports, case series and retrospective studies were reviewed.

RESULTS

Our search identified 115 publications, all of which were retrieved. Human studies included eight case reports, four case series, one controlled study and two retrospective chart reviews. No randomized controlled human trials were found. Twelve animal studies were identified that investigated pH manipulation or saline load and their effects on physiological parameters in tricyclic antidepressant toxicity.

CONCLUSIONS

The practice of alkalinization for tricyclic antidepressant toxicity is based on animal studies, case reports and opinion. The mechanism of action appears to be multifaceted and may vary between different tricyclic antidepressants. Significant interspecies variation makes extrapolation from animal studies to humans difficult. Alkalinization therapy appears reasonable in patients with compromising dysrhythmias and shock when supportive interventions have been ineffective; however, the available evidence does not support prophylactic alkalinization in the absence of life-threatening cardiovascular toxicity.

The effect of sodium bicarbonate on propranolol-induced cardiovascular toxicity in a canine model

STUDY OBJECTIVE

To evaluate the potential utility of sodium bicarbonate in an established model of acute propranolol toxicity.

METHODS

Two minutes after the completion of a propranolol infusion (10 mg/kg), a bolus of 1.5 mEq/kg of sodium bicarbonate solution (1 mEq/mL) followed by an infusion of 1.5 mEq/kg over the next 26 minutes (n = 6) or an equivalent timing and volume of 5% dextrose solution (n = 6) was administered in each dog. Targeted cardiovascular parameters included heart rate, mean arterial pressure, left ventricular dP/dtmax, and QRS interval.

RESULTS

Propranolol infusion significantly depressed heart rate (p < 0.0001), mean arterial pressure (p < 0.0001), dP/dtmax (p < 0.0001) and prolonged the QRS interval (p < 0.0001). Sodium bicarbonate failed to significantly improve these targeted parameters when compared to control animals.

CONCLUSION

In this canine model of propranolol toxicity, intravenous sodium bicarbonate appears to be an ineffective single therapy. Furthermore, these results may suggest a different mechanism of sodium channel blockade for propanolol than that of type IA antiarrhythmic agents.

Effect of calcium chloride and 4-aminopyridine therapy on desipramine toxicity in rats

BACKGROUND

Hypotension is a major contributor to mortality in tricyclic antidepressant overdose. Recent data suggest that tricyclic antidepressants inhibit calcium influx in some tissues. This study addressed the potential role of calcium channel blockade in tricyclic antidepressant-induced hypotension.

METHODS

Two interventions were studied that have been shown previously to improve blood pressure with calcium channel blocker overdose. CaCl2 and 4-aminopyridine. Anesthetized rats received the tricyclic antidepressant desipramine IP to produce hypotension, QRS prolongation, and bradycardia. Fifteen min later, animals received CaCl2, NaHCO3, or saline. In a second experiment, rats received tricyclic antidepressant desipramine IP followed in 15 min by 4-aminopyridine or saline.

RESULTS

NaHCO3 briefly (5 min) reversed hypotension and QRS prolongation. CaCl2 and 4-aminopyridine failed to improve blood pressure. The incidence of ventricular arrhythmias (p = 0.004) and seizures (p = 0.03) in the CaCl2 group was higher than the other groups.

CONCLUSION

The administration of CaCl2 or 4-aminopyridine did not reverse tricyclic antidepressant-induced hypotension in rats. CaCl2 therapy may possibly worsen both cardiovascular and central nervous system toxicity. These findings do not support a role for calcium channel inhibition in the pathogenesis of tricyclic antidepressant-induced hypotension.

Does a sodium-free buffer affect QRS width in experimental amitriptyline overdose?

STUDY OBJECTIVES

We carried out this study to determine the effects of pH alteration on QRS width with administration of tromethamine, a non-sodium-containing buffering agent, in experimental amitriptyline overdose.

DESIGN

Prospective, nonblinded trial.

PARTICIPANTS

Adult mongrel dogs.

INTERVENTIONS

Pentobarbital-anesthetized dogs were overdosed with amitriptyline 5 mg/kg followed by infusion at 1.0 mg/kg/minute until the QRS width doubled, then decreased to .5 mg/kg/minute until the end of the experiment. At two defined points of toxicity, the dose of tromethamine required to raise the pH to 7.50 +/- 4 was given. pH and QRS width at a speed of 100 mm/second were measured over a 30-minute period after each tromethamine dose. Data were analyzed with non-linear-regression analysis.

RESULTS

At toxicity 1 the mean pH was 7.32, with a QRS width of 11.6 mm. Two minutes after the tromethamine dose the pH rose to 7.51, with narrowing of the QRS width to 8.4 mm. At toxicity 2 the pH was 7.40, with QRS width of 10.6 mm. Two minutes after tromethamine, the pH rose to 7.49 and the QRS width decreased to 9.7 mm. Regression analysis showed a correlation between pH and QRS width; as pH increased, QRS width decreased (P = .0001).

CONCLUSION

Cardiac toxicity of amitriptyline overdose, as manifested by QRS widening, is reversible by pH changes alone.

Experimental amitriptyline poisoning: treatment of severe cardiovascular toxicity with cardiopulmonary bypass

STUDY OBJECTIVE

To compare cardiopulmonary bypass (CPB) with more conventional therapy in the treatment of severe amitriptyline poisoning.

DESIGN

Prospective, randomized, controlled, laboratory investigation.

INTERVENTIONS

Profound cardiovascular toxicity was induced in 20 anesthetized Yorkshire swine (72 +/- 8.3 kg) by amitriptyline infusion at 0.5 mg/kg/min. Ventilation was adjusted to keep arterial pH at 7.50 +/- 0.05 and the PCO2 at 35 mm Hg. The swine were randomized in a 1:1 ratio to one of two groups, CPB or control. Both groups received amitriptyline infusion until they experienced near-lethal toxicity, defined as a systolic blood pressure below 30 mm Hg for one minute. The control group was then given supportive treatment, including IV fluids, sodium bicarbonate, vasopressors, and standard pharmacologic (advanced cardiac life support) interventions. Control animals failing to respond to supportive measures after five minutes were given open-chest cardiac massage for 30 minutes or until the return of spontaneous circulation. The CPB group received only mechanical support by CPB for 90 to 120 minutes. No sodium bicarbonate, antiarrhythmics, or cardiotonic agents were provided to the CPB group during this resuscitation.

RESULTS

All 20 animals experienced cardiac conduction delays, dysrhythmias, and progressive hypotension within 30 minutes of receiving IV amitriptyline at 0.5 mg/kg/min. The ten swine receiving CPB as treatment for cardiovascular toxicity were able to completely correct the dysrhythmias, cardiac conduction abnormalities, and hypotension produced by the amitriptyline; however, only one of ten control animals could be resuscitated (P = .0001). Nine of ten swine treated with CPB were easily weaned off bypass without any pharmacologic intervention; however, one required norepinephrine to be weaned. All 11 resuscitated swine were able to be salvaged.

CONCLUSION

CPB improved survival in our swine model of severe amitriptyline poisoning.

Recovery from severe cyclic antidepressant overdose with hypertonic saline/dextran in a swine model

OBJECTIVE

To determine the effect of a hypertonic saline and dextran (HSD) solution on blood pressure and QS duration during severe cyclic antidepressant (CA) toxicity in swine.

METHODS

Ten domestic swine weighing 20-24 kg were anesthetized and placed on mechanical ventilation. Nortriptyline solution was infused intravenously to achieve hypotension (systolic blood pressure equal to 50% of baseline) and a QRS duration of 120 msec. After reaching toxicity, the animals received in a randomized fashion either 10 mL/kg of a 7.5% saline/6% dextran solution or an equal volume of 0.9% saline as a rapid intravenous bolus. The animals were observed for one hour or until they died. Blood pressure and ECG were recorded continuously. Arterial pH was maintained in the physiologic range by controlled ventilation.

RESULTS

Mean systolic blood pressure 10 minutes after treatment was 45 +/- 8 torr in the normal- saline group compared with 115 +/- 12 torr in the HSD group (p < 0.05). Mean QRS duration 10 minutes after treatment was 180 +/- 8 msec in the normal-saline group; it was 88 +/- 13 msec in the HSD group (p < 0.05). All normal-saline--group animals died within 20 minutes, and four of the five animals in the HSD group survived to 60 minutes (p < 0.05). The mean peak sodium concentration was 157 mmol/dL (mEq/dL) in the HSD group, and this was transient.

CONCLUSION

In this swine model of severe CA toxicity, a solution of 7.5% saline/6% dextran significantly reversed hypotension and QRS prolongation. HSD also improved survival to 60 minutes.

Effect of hypertonic sodium bicarbonate in the treatment of moderate-to-severe cyclic antidepressant overdose

The objective of this study was to characterize the effect of intravenous hypertonic sodium bicarbonate (NaHCO3) administration in patients with moderate-to-severe cyclic antidepressant (CA) overdose. We reviewed charts of all 91 patients given the diagnosis of CA overdose in the University of California Los Angeles (UCLA) Emergency Medicine Center (EMC), who either died in the EMC or were admitted to the medical intensive care unit (MICU), and who received NaHCO3 in the EMC between 1980 and 1988. Twenty-four other patients with the same EMC diagnosis were admitted to the MICU during this period but did not receive NaHCO3. The response of blood pressure, electrocardiographic parameters, and mental status to serum alkalinization with NaHCO3 were evaluated. Major morbidity and mortality were recorded for all patients. Hypotension was corrected within 1 hour in 20 of 21 (96%) patients, QRS prolongation corrected in 39 of 49 (80%), and mental status improved in 40 of 85 (47%). There was one death, in a patient who was moribund on arrival to the EMC. No complications were attributable to the administration of NaHCO3. NaHCO3 seems to improve hypotension and normalize QRS duration rapidly in most patients treated, and improve mental status changes in almost one half. Serum alkalinization with NaHCO3, in conjunction with appropriate supportive care, seems to limit major morbidity and mortality effectively in patients with serious CA overdose.

Profound alkalemia during treatment of tricyclic antidepressant overdose: a potential hazard of combined hyperventilation and intravenous bicarbonate

Two patients with cardiovascular and neurologic toxicity from intentional tricyclic antidepressant overdose received bicarbonate infusions in association with hyperventilation for alkalinization. Both patients developed profound alkalemia. One patient died, and the other patient's alkalemia resolved prior to her death. Bicarbonate infusions have become the standard of care for symptomatic tricyclic antidepressant toxicity. Severe alkalemia (pH greater than 7.60) in other settings has been reported to correlate with higher rates of mortality. Careful monitoring of the pH is imperative when bicarbonate therapy is used. It is probably prudent to keep the pH level in the range 7.45 to 7.60. Capnography may also be useful in monitoring patients during alkalinization.

Hemodynamic and electrophysiological actions of cocaine. Effects of sodium bicarbonate as an antidote in dogs

BACKGROUND

Cocaine abuse has been implicated as a cause of death due to sudden cardiac arrest.

METHODS AND RESULTS

We examined the hemodynamic and electrophysiological effects of cocaine administered as a series of 5-mg/kg i.v. boluses coupled with a continuous infusion in anesthetized dogs. Sodium bicarbonate (50 meq i.v.) was administered as a potential antidote in 11 of 15 dogs, and intravenous 5% dextrose was given in the remaining four. In a dose-dependent fashion, cocaine significantly decreased blood pressure, coronary blood flow, and cardiac output; increased PR, QRS, QT, and QTc intervals and sinus cycle length; and increased ventricular effective refractory period and dispersion of ventricular refractoriness. No afterdepolarizations were noted in the monophasic action potential recording. Nonsustained monomorphic ventricular tachycardia occurred spontaneously in two dogs, and sustained ventricular tachycardia could be induced by programmed stimulation at the end of the dosing protocol in five of 11 animals. Sodium bicarbonate promptly decreased cocaine-induced QRS prolongation to nearly that measured at baseline but had no effect on the other electrocardiographic or hemodynamic variables. In one dog, sodium bicarbonate administration was associated with reversion of ventricular tachycardia to sinus rhythm.

CONCLUSIONS

We conclude that high-dose cocaine possesses negative inotropic and potent type I electrophysiological effects. Sodium bicarbonate selectively reversed cocaine-induced QRS prolongation and may be a useful treatment of ventricular arrhythmias associated with slowed ventricular conduction in the setting of cocaine overdose.

Tricyclic antidepressant poisoning

OBJECTIVE

To review poisoning with tricyclic antidepressants.

DATA SOURCE

English language literature search using Australian Medlars Service (1977-1989), manual search of journals and review of bibliographies in identified articles.

STUDY SELECTION

Approximately 250 articles, abstracts and book chapters were selected for analysis.

DATA EXTRACTION

The literature was reviewed and 93 articles were selected as representative of important advances.

DATA SYNTHESIS

The major features of overdose are neurological, cardiac, respiratory and anticholinergic. Life-threatening complications develop within six hours of overdose or not at all. All patients seen within six hours of overdose should have their stomachs emptied. All patients should receive activated charcoal. Coma, convulsions, respiratory depression and hypotension are treated with standard resuscitation techniques and drugs. Treat patients with significant cardiotoxicity or cardiac arrest with alkalinisation by sodium bicarbonate or hyperventilation, aiming for an arterial pH of 7.45-7.55. Lignocaine is used for ventricular arrhythmias. Other antiarrhythmic drugs are contraindicated (Class 1A, Class 1C), potentially lethal (Class II), of no benefit (phenytoin) or of unproven efficacy (Class III and Class IV). Physostigmine has no role at all. Haemodialysis and haemoperfusion are of no benefit.

CONCLUSION

The death rate of those who reach hospital is 2%-3%. Most of these deaths are cardiac in origin, and are caused by direct depression of myocardial function rather than cardiac arrhythmias.

Plasma catecholamine levels in cyclic antidepressant overdose

Cyclic antidepressant overdose is a major cause of morbidity and mortality in self-poisoned patients. The major cause of mortality with cyclic antidepressant overdose is cardiotoxicity. We determined plasma catecholamine levels in 41 symptomatic acute overdose patients to identify interactions between QRS duration (a marker for cardiotoxicity) and a presumed hyper-adrenergic state. Using a linear multivariable regression analysis, QRS duration correlated with the presence of cyclic antidepressant, plasma norepinephrine levels, the ratio of norepinephrine to epinephrine level, and pulse rate (p less than 0.001, r2 = 0.42). Commensurate physiologic changes were not found in the presence of elevated catecholamine levels in the cyclic antidepressant overdose group. One possible explanation for the blunted systemic response to the elevated catecholamine levels is adrenergic desensitization. Investigation of serial catecholamine levels during cyclic antidepressant overdose may lead to modification of our current theories of cardiotoxicity and therapy.

Tricyclic antidepressant toxicosis

Accidental ingestions of TCAs by companion animals often occur. During the past 4 years, over 450 cases have been reported to the IAPIC. At least 7% of the animals that displayed clinical signs of toxicosis eventually died. Overdoses of TCAs adversely affect the cardiovascular, parasympathetic, and central nervous systems. The cardiovascular system is involved most seriously and ventricular arrhythmias with severe hypotension are believed to be the primary cause of death. Animals that ingest a potentially lethal dose (over 15 mg/kg) may die within 1 to 2 hours if appropriate treatment is not administered. Treatment involves the use of initial life-supportive measures (control of seizures, maintenance of an airway, ventilation, and so on), detoxification of the animal (enterogastric lavage, activated charcoal, etc.), and the intravenous use of sodium bicarbonate (2-3 mEq/kg) to control signs of acidosis, hypotension, tachycardia, bradycardia, and other cardiac conduction abnormalities. The animal must then be monitored closely for the return of the clinical signs and sodium bicarbonate therapy should be repeated as needed. In addition, to enhance removal of the TCAs from the gastrointestinal tract and, ultimately, from the body, activated charcoal should be repeated at 3-hour intervals until the animal is asymptomatic.