Tricyclic antidepressant overdosage: experimental studies on the management of circulatory complications

Drugs and pharmaceuticals: management of intoxication and antidotes

The treatment of patients poisoned with drugs and pharmaceuticals can be quite challenging. Diverse exposure circumstances, varied clinical presentations, unique patient-specific factors, and inconsistent diagnostic and therapeutic infrastructure support, coupled with relatively few definitive antidotes, may complicate evaluation and management. The historical approach to poisoned patients (patient arousal, toxin elimination, and toxin identification) has given way to rigorous attention to the fundamental aspects of basic life support--airway management, oxygenation and ventilation, circulatory competence, thermoregulation, and substrate availability. Selected patients may benefit from methods to alter toxin pharmacokinetics to minimize systemic, target organ, or tissue compartment exposure (either by decreasing absorption or increasing elimination). These may include syrup of ipecac, orogastric lavage, activated single- or multi-dose charcoal, whole bowel irrigation, endoscopy and surgery, urinary alkalinization, saline diuresis, or extracorporeal methods (hemodialysis, charcoal hemoperfusion, continuous venovenous hemofiltration, and exchange transfusion). Pharmaceutical adjuncts and antidotes may be useful in toxicant-induced hyperthermias. In the context of analgesic, anti-inflammatory, anticholinergic, anticonvulsant, antihyperglycemic, antimicrobial, antineoplastic, cardiovascular, opioid, or sedative-hypnotic agents overdose, N-acetylcysteine, physostigmine, L-carnitine, dextrose, octreotide, pyridoxine, dexrazoxane, leucovorin, glucarpidase, atropine, calcium, digoxin-specific antibody fragments, glucagon, high-dose insulin euglycemia therapy, lipid emulsion, magnesium, sodium bicarbonate, naloxone, and flumazenil are specifically reviewed. In summary, patients generally benefit from aggressive support of vital functions, careful history and physical examination, specific laboratory analyses, a thoughtful consideration of the risks and benefits of decontamination and enhanced elimination, and the use of specific antidotes where warranted. Data supporting antidotes effectiveness vary considerably. Clinicians are encouraged to utilize consultation with regional poison centers or those with toxicology training to assist with diagnosis, management, and administration of antidotes, particularly in unfamiliar cases.

Effect of glucagon on amitriptyline-induced cardiovascular toxicity in rats

The aim of this study was to investigate the effect of glucagon on cardiovascular parameters in anesthetized rat model of tricyclic antidepressant overdose. Toxicity was induced by infusion of amitriptyline 0.94 mg/kg/min until a 40–45% of reduction in mean arterial pressure was observed. Amitriptyline infusion rats were then randomized into three groups. Control group of rats (group 1) received a bolus of 5% dextrose followed by the continuous infusion of dextrose, whereas treatment groups received 1 mg/kg (group 2) or 2 mg/kg (group 3) bolus doses of glucagon followed by continuous infusion (0.1 mg/kg/min) of glucagons for 60 min. Mean arterial pressure, heart rate, and electrocardiogram were recorded. Amitriptyline caused a significant decrease in mean arterial pressure and a prolongation in QRS, yet it did not change the heart rate. High-bolus dose of glucagon (2 mg/kg) followed by glucagon infusion significantly increased mean arterial pressure at 40, 50, and 60 min ( P < 0.05) and shortened the prolonged QRS at 50 and 60 min ( P < 0.05) when compared with control group. There was also a significant increase in heart rate. In conclusion, bolus doses followed by a continuous infusion of glucagon were found to be effective in reversing the hypotension and QRS prolongation in the rat model of amitriptyline toxicity. Further studies are needed to reveal the exact mechanism of the proposed effect.

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.

Assessing physostigmine's contraindication in cyclic antidepressant ingestions

Ingestion of cyclic antidepressant medications or prolongation of the electrocardiographic QRS interval are commonly considered as contraindications to the use of physostigmine as an antidote for antimuscarinic toxicity. This dictum seems to stem from a few well-publicized cases in which administration of physostigmine was temporally associated with the development of asystole. Before the report of these cases, physostigmine was more frequently used and had been considered a first-line antidote for both the neurologic and cardiac toxic effects of cyclic antidepressant overdose. This apparent inconsistency, and a resurgence of interest in physostigmine as an antidote, begs the question of the appropriateness of this drug's contraindication in all cyclic antidepressant ingestions. Review of the published clinical and experimental evidence provides little support for the clinical utility of using electrocardiographic criteria or the ingestion of cyclic antidepressants as contraindications to the use of physostigmine.

Treatment of severe tricyclic antidepressant overdose with extracorporeal sorbent detoxification

Tricyclic overdose can be a medical emergency, and therapy with intravenous bicarbonate is not always successful in preventing cardiac toxicity or coma. Mortality in patients developing these complications is from 1% to 15%. Extracorporeal detoxification with sorbents has been used in treatment of patients with very high drug levels and declining clinical condition. Ten patients with serious drug overdose caused by tricyclics failed to respond quickly to standard therapy and were in stage 3-4 encephalopathy. Nine of these patients were on respirator support, 5 had hypotension, and 6 had QRS widening. Average level was 1,423 microg/L at presentation. Enteral activated charcoal and intravenous (IV) bicarbonate were initiated in the emergency room. The patients were treated for 3 to 4 hours with the Liver Dialysis Unit, a hemodiabsorption device using a cellulosic plate dialyzer and sorbent suspension as dialysate. Inflow and outflow blood levels indicated that the hemodetoxifier removed modest amounts of the tricyclics, metabolites, and other consumed drugs. The clinical improvement of the patients was dramatic, with patients reaching stage 0 or 1 encephalopathy during the treatment. Ventilator support was removed at the end of treatment for 3 patients who had not already developed pneumonia, and for others was prolonged up to 48 hours because of pneumonia, rather than mental status. Average length of stay in the intensive care unit (ICU) was 4.8 days (range 1 to 7 days). None of the patients died despite their high risk for ventricular arrhythmias, seizures, and death. Clinical improvement may have been attributable to removal of free drug from the blood or to removal of drug metabolites.

Tricyclic antidepressant overdose: a review

Overdoses of tricyclic antidepressants are among the commonest causes of drug poisoning seen in accident and emergency departments. This review discusses the pharmacokinetics, clinical presentation and treatment of tricyclic overdose.

Part 8: advanced challenges in resuscitation. Section 2: toxicology in ECC. European Resuscitation Council

Use of standard ACLS protocols for all patients who are critically poisoned may not result in an optimal outcome. Care of severely poisoned patients can be enhanced by urgent consultation with a medical toxicologist. Alternative approaches required in severely poisoned patients include: o Higher doses than usual. o Drugs that are rarely used to treat cardiac arrest (amrinone, calcium, esmolol, glucagon, insulin, labetalol, phenylephrine, physostigmine, and sodium bicarbonate). o Heroic measures, such as prolonged CPR and use of circulatory assist devices. When resuscitation is unsuccessful, organ donation may still be an option.

Poisoning by sodium channel blocking agents

Poisoning by drugs that block voltage-gated sodium channels produces intraventricular conduction defects, myocardial depression, bradycardia, and ventricular arrhythmias. Human and animal reports suggest that hypertonic sodium bicarbonate may be effective therapy for numerous agents possessing sodium channel blocking properties, including cocaine, quinidine, procainamide, flecainide, mexiletine, bupivacaine, and others.

Incorrect overdose management advice in the Physicians' Desk Reference

STUDY HYPOTHESIS

Physicians may consult references such as Physicians' Desk Reference (PDR) for overdose management advice. Although PDR recommendations are approved by the US Food and Drug Administration (FDA), we hypothesized that they are often outdated and potentially hazardous.

METHODS

We surveyed physicians who consulted our poison center during a 1-month period with regard to their use of the PDR for overdose information and also compared PDR overdose treatment recommendations with those of five current major toxicology references. For the PDR overdose information review we examined data from the American Association of Poison Control Centers to identify pharmaceutical categories with the largest number of deaths. We reviewed the four leading drugs with at least 1,000 reported exposures in each category and identified 20 PDR-listed brand-name products for analysis. We obtained the consensus from five current toxicology references on contraindicated treatments, ineffective treatments, and specific recommended treatments or antidotes. Finally, we compared the overdose management advice provided in the 1994 PDR with the toxicology reference consensus.

RESULTS

Forty of 80 of physicians surveyed (50%) reported use of the PDR for overdose information in the preceding 12 months. Of the 20 PDR entries, 16 (80%) had at least one deficiency, and 5 (25%) had two or more deficiencies. Thirteen (65%) omitted an indicated specific treatment, three (15%) recommended contraindicated treatments, and four (20%) advised ineffective treatments with potential for harm. Only four entries (20%) had no deficiencies by our survey criteria.

CONCLUSION

We found serious discrepancies in overdose treatment advice in the PDR compared with a consensus of current toxicology references. Altogether, four of five PDR entries were deficient, and almost half advised ineffective or frankly contraindicated therapies. Despite FDA approval, the use of PDR overdose advice in a serious poisoning case could result in unnecessary morbidity or mortality.

Relative role of alkalosis and sodium ions in reversal of class I antiarrhythmic drug-induced sodium channel blockade by sodium bicarbonate

BACKGROUND

Hypertonic sodium salts are used to treat sodium channel-blocking drug cardiotoxicity. The relative roles of alkalinization and increased sodium concentration ([Na+]o) for various drugs are incompletely known.

METHODS AND RESULTS

The effects of four class I drugs on action potential characteristics of canine Purkinje fibers at equieffective concentrations (disopyramide 30 mumol/L, mexiletine 80 mumol/L, flecainide 7 mumol/L, imipramine 5 mumol/L) were studied in the presence of normal Tyrode solution and one altered solution (increased [Na+]o, increased bicarbonate concentration, or both) in each experiment. Combined increases in sodium and bicarbonate concentration significantly reduced the depressant effects of flecainide, imipramine, and mexiletine on phase 0 upstroke (Vmax) but did not alter the effects of disopyramide. The effects of sodium bicarbonate were entirely due to alkalinization in the case of imipramine, but both alkalinization and increased [Na+]o contributed to the interaction with flecainide and mexiletine. The reversal of Vmax depression by increased [Na+]o and pH was due in part to hyperpolarization. In addition, alkalosis directly reversed the hyperpolarizing shift in Vmax inactivation caused by flecainide and imipramine without altering the shift caused by disopyramide and mexiletine.

CONCLUSIONS

Increases in sodium bicarbonate concentration reverse the effects of class I antiarrhythmic drugs to a varying extent, with drug-specific contributions of the sodium and bicarbonate moiety. The molecular basis for this drug specificity remains to be elucidated, but it has important potential implications for the use of hypertonic sodium salts to treat cardiotoxicity caused by sodium channel-blocking drugs.

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.

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.

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.

Experimental amitriptyline intoxication: treatment of cardiac toxicity with sodium bicarbonate

Overdose with amitriptyline and other tricyclic antidepressants can result in ventricular conduction abnormalities as well as severe ventricular arrhythmias. The arrhythmogenic effects of these compounds may be attributed to their direct local anesthetic actions in blocking sodium channels in cardiac membranes. Thus tricyclic-induced ventricular arrhythmias usually do not respond well to therapy with standard Class I antiarrhythmic drugs that also have the same direct local anesthetic action and may potentiate the adverse effects of tricyclic antidepressants. Cardiac toxicity was produced in dogs by the administration of amitriptyline, both orally and IV. At serum concentrations less than 2,000 ng/mL, sinus tachycardia occurred with widened QRS complexes. At higher concentrations, QRS duration became more markedly prolonged and was followed by ventricular tachyarrhythmias. Occurrence of ventricular tachyarrhythmias was associated with QRS durations of more than 0.11 second. Sodium bicarbonate (18 to 36 mEq) administered IV over either 30 seconds or two minutes rapidly converted ventricular tachycardia to normal sinus rhythm. Conversion was associated with abbreviation of the QRS complex and was accompanied by a rise in both systolic and diastolic pressures. The duration of sodium bicarbonate effect paralleled the duration of the changes in arterial pH and plasma bicarbonate concentrations. In vitro studies in cardiac Purkinje fibers suggested that reversal of amitriptyline-induced cardiac membrane effects by sodium bicarbonate may be attributed not only to alkalinization but also to increased in extracellular sodium concentration, diminishing the local anesthetic action of amitriptyline and resulting in less sodium channel block.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenytoin: does it reverse tricyclic-antidepressant-induced cardiac conduction abnormalities?

Case reports have appeared describing a beneficial effect of phenytoin in reversing cardiac conduction abnormalities induced by tricyclic antidepressant (TCA) overdose. Controlled studies have not been published. The following questions were addressed using intravenous amitriptyline and phenytoin in a rabbit model: Can prophylaxis with phenytoin before amitriptyline poisoning forestall the onset of cardiac abnormalities? Would such prophylactic phenytoin administration allow a higher dose of amitriptyline before death occurs? Would phenytoin reverse the cardiotoxic effects of amitriptyline once in progress? Animals were used in repeated trials with one-week "washout" intervals and served as their own controls in all but the final trial. Prophylactic phenytoin did not change the potency of amitriptyline in inducing abnormal cardiac performance, nor did it allow the animals to be titrated to a higher dose of amitriptyline before death occurred. In 12 animals, phenytoin "rescue" at the point of a widened QRS or arrhythmia was attempted. Two showed improvement; the remainder did not. Because this portion of the experiment was neither blinded nor controlled, nor were respirations or blood pressure monitored, these results must be viewed cautiously. Although our results suggest that prophylactic phenytoin is not useful, its role in therapy of occasional cases requires further investigation.

Poisoning due to tricyclic antidepressant overdosage. Clinical presentation and treatment

Tricyclic antidepressants are among the commonest causes of both non-fatal and fatal drug poisoning in the world. Their toxicity is due to effects on the brain, the heart, the respiratory system and the parasympathetic nervous system. Symptoms usually appear within 4 hours of an overdose and all but the most seriously poisoned patients recover within 24 hours. The most common clinical features are dry mouth, blurred vision, dilated pupils, sinus tachycardia, pyramidal neurological signs, and drowsiness. In severe poisoning, there may be coma, convulsions, respiratory depression, hypotension and a wide range of electrocardiographic (ECG) abnormalities. The most frequent findings on the ECG are prolongation of the PR and QT intervals; the tracing may resemble bundle branch block or supraventricular or ventricular tachycardias. Treatment of poisoning due to the tricyclic antidepressants is essentially supportive, there being insufficient evidence at present to recommend the use of methods to increase elimination of the drug from the body. Gastric aspiration and lavage should be performed if more than 750 mg of drug have been taken. There must be regular monitoring for hypoxia, acidosis and hypokalaemia and these complications should be corrected enthusiastically. Convulsions should be treated with diazepam or chlormethiazole. Muscular paralysis and artificial ventilation should be employed if anticonvulsants are ineffective. Hypotension should be treated firstly by fluid replacement and then with sympathomimetic agents (dopamine or dobutamine). Antiarrhythmic drugs should only be employed if there is evidence of circulatory failure which fails to respond to correction of hypotension. Sodium bicarbonate infusions should be given to cardiotoxic patients who are acidotic and are worth trying even if the patient is not acidotic. Although physostigmine salicylate will reverse most of the features of tricyclic antidepressant poisoning, its effects are short-lived in serious toxicity and it can produce dangerous side effects; physostigmine should therefore be reserved for those patients who have complications of coma or who have resistant cardiotoxicity or convulsions. Drug screening and quantitative determination of tricyclic antidepressant serum concentrations are useful in a minority of patients who have severe, unusual or prolonged symptoms.

Tricyclic antidepressant overdose

Overdose of a tricyclic antidepressant is a serious and all-too-frequent occurrence. The diagnosis must be considered in known or suspected overdoses, and signs such as a dry axilla, tachycardia, and wide QRS must be specifically sought. Management depends upon support of vital functions and a thorough understanding of the pharmacology of the drug. Emptying the gastrointestinal tract with ipecac or lavage and hastening elimination with activated charcoal and a cathartic are extremely important measures. Cardiac arrhythmias generally respond to sodium bicarbonate, and seizures respond to intravenous diazepam. Neither physostigmine nor dialysis are considered to be treatments of choice. As in other overdoses, counseling to prevent ingestions is more than worth "a pound of the cure."

Amitriptyline plasma protein binding: effect of plasma pH and relevance to clinical overdose

Reversing ventricular ectopy with plasma alkalinization following acute tricyclic antidepressant overdose is a recognized mode of therapy. The mechanism responsible for this effect is unclear. Changes in plasma protein binding of free drug, effects of the sodium ion on the myocardium, and alterations of plasma concentrations of alpha-1-acid glycoprotein may all interact to alter toxicity of tricyclics in overdose. An in vitro investigation using equilibrium dialysis was designed to examine the effect of altering plasma pH on percentage of free amitriptyline at clinical overdose plasma concentrations. A 1973 report on this effect lacked adequate controls and was faulty in experimental protocol. The current investigation used plasma concentrations typically present in amitriptyline overdose, a sensitive gas liquid chromatographic assay to detect total and free drug, and adequate control of plasma pH. The results of two separate experiments demonstrated a significant decrease in percentage of free amitriptyline of 20% over a pH range of 7.0-7.4 (P less than 0.05) and 42% over a pH range of 7.4-7.8 (P less than 0.05). The rate of change in slope in both experiments was not significantly different (P less than 0.01) indicating similar effects of pH change on plasma protein binding of amitriptyline within the two groups.

Improvement of cardiac conduction after hyperventilation in tricyclic antidepressant overdose

Three patients wtih severe cardiotoxicity secondary to tricyclic antidepressant (TCA) overdosage were treated with induced mechanical hyperventilation. All three demonstrated marked QRS narrowing, reflecting improved intracardiac conduction, after hyperventilation therapy. Such therapy may help to prevent or abolish ventricular dysrhythmias, often a feature of life-threatening TCA overdoses.

Frequency-dependent effects of amitriptyline on ventricular conduction and cardiac rhythm in dogs

Although overdoses of tricyclic antidepressant are known to produce both sinus tachycardia and ventricular tachyarrhythmias in man, these have been assumed to occur by independent mechanisms. This study was designed to evaluate the relationship of ventricular activation frequency to the cardiotoxic effects of amitriptyline. When amitriptyline was infused into dogs with formalin-induced atrioventricular (AV) block to evaluate a broad range of pacing frequencies, the drug produced dose-related QRS prolongation that was markedly frequency dependent. Similar frequency-dependent depression of the maximum rate of depolarization (Vmax) was noted for canine Purkinje fibers superfused with amitriptyline in vitro. The time constant of recovery from amitriptyline-induced block was dose independent and averaged 228 msec in vivo and 216 msec in vitro. When amitriptyline was infused into dogs with intact AV conduction, sinus tachycardia occurred within 15 min, followed by progressive QRS prolongation and ventricular tachyarrhythmias after an average 29 min. Slowing of sinus rate by vagal stimulation (seven dogs) or intravenous metoprolol (five dogs) reproducibly reversed the QRS prolongation and ventricular tachyarrhythmias caused by amitriptyline. These studies show that amitriptyline produces frequency-related depression of ventricular conduction in vivo, with a time dependence similar to effects on the maximum rate of depolarization in vitro. Interventions that slow heart rate reverse the adverse effects of amitriptyline on ventricular conduction and cardiac rhythm.

Drug overdose in children

This paper reviews epidemiological factors, drugs taken, management and outcome of 478 children admitted to the Royal Children's Hospital with drug poisoning during the 5 years 1978-82. Accidental ingestion in young children is still common whilst self poisoning is increasing in the age group 8-17 years. Suicide attempts and self destructive ingestion accounted for 11.5% of all cases with two deaths. A significant number in this group were less than 10 years of age. Iatrogenic poisoning accounted for 11% of all cases with metoclopramide or prochlorperazine frequently involved. Benzodiazepines, major tranquillizers and antihistamines were the most commonly taken groups of drugs whilst the greatest number of admissions to Intensive Care Unit were due to tricyclic poisoning. Principles of management are discussed.

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

The beneficial hemodynamic effects of sodium bicarbonate as treatment for tricyclic antidepressant poisoning were investigated in an animal model. Seven adult dogs (17.5 to 20 kg) were poisoned by an intravenous infusion of amitriptyline. Toxicity was defined as a doubling of the initial QRS width. A continuous infusion was used to maintain toxicity for 30 minutes after which 44.5 mEq of sodium bicarbonate was administered intravenously. Five of the animals survived to completion of the experiment. Three of the surviving animals developed dysrhythmias. All dysrhythmias ceased within one minute of administration of sodium bicarbonate. An increase in mean blood pressure (P less than .05) and serum pH (P less than .05) and a decrease in mean QRS width (P less than .05) occurred following administration of sodium bicarbonate. The maintenance of toxicity for 30 minutes suggests that this model can be used for future studies of tricyclic antidepressant poisoning.

Acute self-poisoning with tricyclic antidepressants in 295 consecutive patients treated in an ICU

Clinical findings on admission to hospital and outcome in 295 consecutive patients with severe tricyclic antidepressant self-poisoning treated in an ICU are presented. Cerebral depression was observed in 92%, convulsions in 23% and respiratory failure was present in 72%. Cardiovascular function was impaired in 44% and an abnormal ECG was found in 57%. Cardiac arrest was treated in 14 patients (6%) of whom seven were resuscitated. The mortality rate was 2%. All patients were artificially ventilated. A beneficial effect of respiratory alkalosis on cardiac arrhythmias is supported.

Immediate effects of physostigmine on amitryptyline-induced QRS prolongation

Prior studies have suggested that physostigmine may be useful in reversing QRS prolongation due to amitriptyline toxicity. To investigate this question, we devised a pharmacologic model in rabbits utilizing an initial intravenous bolus of amitriptyline (4-6 mg) followed by a constant amitriptyline infusion (0.2-0.4 mg/min) empirically titered to maintain the QRS at 50% or more of control value for at least 5 min. Intravenous physostigmine (2 mg) sufficient to produce muscle fasciculations and significant (P less than 0.01) slowing of sinus rate was then administered to six animals. No significant change in QRS duration was noted at 1, 3, and 5 min intervals following physostigmine. Although no immediate antidotal effect of physostigmine on amitriptyline-induced QRS prolongation could be demonstrated, these results do not exclude a possible interaction between the membrane effects of the tricyclic antidepressants (and related agents) and the vagal branch of the autonomic nervous system.

Experimental studies on the effects of physostigmine and of isoproterenol on toxicity produced by tricyclic antidepressant agents

The IV infusion of nortriptyline and amitriptyline (0.5 mg/kg/min) in anesthetized cats produced death within 60 min of continuous infusion. The tricyclic antidepressant agents produced a quinidine-like depression of the myocardium characterized by bradycardia, depression of contractile force, conduction defects, bradyarrhythmias, and hypotension. The simultaneous IV infusion of isoproterenol (0.1 microgram/ kg/min) produced significant protection against death produced by the TCA drugs. The results suggested that the positive chronotropic, inotropic, and dromotropic actions of isoproterenol may all be contributory factors in the protection. Pretreatment with a large dose of physostigmine (0.2 mg/kg) produced a rightward shift of the nortriptyline time-mortality curve. The small degree of protection produced by the anticholinesterase drug may be due to a respiratory stimulant action rather than a cardiac action.

Treatment of imipramine overdose in children

Enuresis is a common problem often treated effectively with imipramine hydrochloride. The usefulness of this therapy carries with it, however, the risk of accidental overdose by younger siblings of these enuretic patients. Traditional support measures are effective in the treatment of the mild to moderate overdose, while separate symptomatic treatment of seizures and cardiac arrhythmias is possible as outlined herein. Physostigmine offers a single alternate treatment which is effective in the full panorama of life-threatening manifestations of an imipramine overdose.

Pharmacokinetics of drug overdose

Pharmacokinetics of drugs taken in overdose may differ from those observed following therapeutic doses. Differences are due both to dose-dependent changes and to effects of drugs or pathophysiological consequences of the overdose on kinetics. Dose-dependent changes in rate and extent of absorption, bioavailability (saturation of first-pass metabolism), distribution (saturation of protein binding sites) and metabolism are discussed. Gastrointestinal motility is affected both by specific drug actions, such as delayed gastric emptying by anticholinergic drugs, and by general nervous system depression caused by many drugs. Drug-induced circulatory insufficiency may retard tissue distribution and reduce clearance. Disturbances in blood and urine pH may alter distribution and clearance of weak acids and bases. Drug-induced renal or hepatic failure can significantly decrease clearance. Hypothermia is a common complication of drug overdose and might retard distribution and also reduce clearance. The data concerning pharmacokinetics during overdose are usually incomplete and difficult to interpret. Doses and times of ingestion are uncertain, duration of blood and urine sampling is often inadequate to distinguish absorption from distribution and elimination phases, active metabolites are not measured, protein binding is not determined and clinical features of patients not adequately described. We have, however, reviewed available data for salicylate, paracetamol (acetaminophen), barbiturates, ethchlorvynol, glutethimide, chloral hydrate, tricyclic antidepressants, lithium, phenytoin, ethanol, theophylline, digoxin, amphetamine and phencyclidine.

Phenytoin: the drug of choice in tricyclic antidepressant overdose?

Tricyclic antidepressants seem to have at least three types of effect on the heart: anticholinergic, adrenergic, and quinidine-like. Although the therapeutic emphasis in tricyclic antidepressant overdose has been on reversing the anticholinergic effects with physostigmine, there is considerable evidence suggesting that the life-threatening manifestations of tricyclic antidepressant overdose--the conduction defects, bradyarrhythmias, heart block, etc--are much more like quinidine and are more appropriately treated with phenytoin, or other drugs which enhance intracardiac conduction and myocardial contractility.

Comparison of the cardiovascular toxicity of three tricyclic antidepressant drugs: imipramine, amitriptyline, and doxepin

Experiments were conducted on puppies to compare the cardiovascular toxicity of imipramine, amitriptyline, and doxepin. The drugs were infused at weekly intervals to produce arrhythmias and/or hypotension. Imipramine was less arrhythmogenic and caused less fall in blood pressure for a given dose than amitriptyline or doxepin.

The antiarrhythmic action of amitriptyline on arrhythmias associated with myocardial infarction in dogs

The antiarrhythmic activity of amitriptyline, a tricyclic antidepressant, was evaluated in anesthetized dogs 24 h after coronary occlusion, during the period of spontaneous ventricular arrhythmias. In all experiments amitriptyline was administered i.v. in incremental doses of 0.3 mg/kg at 1 min intervals until a conversion to normal sinus rhythm was evident. Amitriptyline administration resulted in conversion of the ventricular arrhythmia to a normal sinus rhythm in 100% of the animals tested at a mean dose of 1.3 +/- 0.1 mg/kg. Smaller doses also resulted in a dose-related decrease in non-sinus nodal pacemaker activity. Lidocaine, when administered to the same group of animals, produced a reduction of ectopic pacemaker activity, but did not eliminate it at a cumulative dose of 2 mg/kg. Antiarrhythmic doses of amitriptyline did not produce significant changes in arterial blood pressure, cardiac output or electrocardiographic parameters associated with atrioventricular or intraventricular conduction. The results of this study suggest that at very low doses amitriptyline may be an effective antiarrhythmic agent in ventricular arrhythmias associated with myocardial ischemia.

The use of physostigmine as an antidote in tricyclic anti-depressant intoxication

The value of physostigmine treatment of unconsciousness due to self-poisoning by tricyclic antidepressant drugs (TAD) was evaluated in 10 patients, and the following conclusions are drawn: 1. A slow i.v. injection of 2 mg of physostigmine produces a clear-cut increase in consciousness within 15 min if a TAD (or other drugs with central anticholingeric potencies) is mainly responsible for the poisoning. This "test dose" can give valuable diagnostic information. 2. Repeated i.v. injections seem of little practical value, since they may be expected to produce a state with rapid shifts in the level of consciousness. 3. If the test dose has a positive effect, immediate i.v. infusion of 4 mg physostigmine/h will maintain a high level of consciousness. Infusion should be stopped every sixth hour for about 30 min to check whether the level of consciousness still falls upon withdrawal of therapy 4. In cases of massive TAD overdosage, i.v. injection of physostigmine may increase the risk of grand mal seizures. 5. No signs of enhanced peripheral cholinergic activity following physostigmine are seen if 30 mg of propantheline is given i.v. every sixth hour. 6. No evidence has been produced that the morality rate in TAD poisoning is lower following physostigmine treatment whan with conventional supportive care. There must be the usual preparedness for cardiac complications.