Bretylium: a prototype for future development of antidysrhythmic agents

Pharmacologic and pharmacokinetic profile of class III antiarrhythmic drugs

Cardiac arrhythmias frequently respond only to drugs that have as their predominant electrophysiologic effect the prolongation of repolarization and refractoriness. According to the Singh-Vaughan Williams classification, these drugs are known as class III agents. In the last few years, interest has increased in the development of class III antiarrhythmic drugs as alternatives to sodium channel blocking agents, which mainly affect cardiac conduction. Much of this interest results from a perceived danger of using drugs with sodium channel blocking properties, particularly in patients with ischemic heart disease, based on the results of the Cardiac Arrhythmia Suppression Trial (CAST) and several other trials. This article is a review of the pharmacology, including the pharmacokinetics and pharmacodynamics, of the most commonly used and investigated class III antiarrhythmic drugs. As will be seen from the discussion, each of these drugs has novel pharmacology that makes it applicable in specific clinical situations. Their putative effects on various arrhythmogenic mechanisms and their efficacy in treating specific target arrhythmias will be addressed.

Freon inhalational abuse presenting with ventricular fibrillation

Inhalation of volatile halogenated hydrocarbons may produce life-threatening cardiac and neurological toxicity. A 15-year-old boy developed ventricular fibrillation immediately after intentional inhalation of a fluorinated hydrocarbon from an automobile air conditioner recharge unit. After the use of intravenous bretylium, a hemodynamically stable sinus tachycardia was restored. Aspiration pneumonitis and rhabdomyolysis complicated his hospital course before complete neurological recovery. The mechanism and treatment of cardiac arrhythmias after volatile fluorinated hydrocarbon inhalation are reviewed.

Effect of bretylium tosylate on ventricular fibrillation threshold during hypothermia in dogs

How bretylium tosylate affected the ventricular fibrillation threshold, electrophysiological parameters, and plasma catecholamine levels during hypothermia in dogs was studied. Threshold for ventricular fibrillation was determined by programmed electrical stimulation using a stimulation protocol that involved applying a maximum of five extrastimuli at body temperatures 37, 34, 31, 28, and 25 degrees C, and at the same temperatures during rewarming. Electrocardiogram, epicardial monophasic action potentials (MAP), and electrograms were recorded, and ventricular effective refractory period (VERP) was determined at each of the above temperatures. In one group (n = 7), a bolus dosage of bretylium tosylate (BT), 6 mg/kg body wt, was administered at 25 degrees C before rewarming. Another group (n = 4) was exposed to cooling and rewarming without addition of BT. Cooling to 25 degrees C reduced ventricular fibrillation threshold linearly, reduced heart rate, increased VERP and MAP, and slowed myocardial conduction velocity in both groups. There was no overall increase in plasma catecholamine levels during cooling. Addition of BT at 25 degrees C increased ventricular fibrillation threshold during rewarming compared with cooling. Addition of BT at 25 degrees C increased VERP by +/- 32 milliseconds and the corrected JT time by 0.06 +/- 0.02 seconds. VERP and JTc increased during rewarming with BT compared with cooling with no drug. BT had no effect on conduction velocity, and plasma catecholamine levels were not reduced. The antiarrhythmic effect of BT during hypothermia was attributed to an increased wavelength of refractoriness by its increase in the refractory period. This increased wavelength of refractoriness may prevent excitable gaps or increase circuit pathway in the setting of reentry arrhythmias.

Bretylium tosylate versus lidocaine in experimental cardiac arrest

Bretylium tosylate has been shown effective in the treatment of ventricular fibrillation and in the prevention of its recurrence. However, lidocaine is generally preferred because bretylium could have adverse hemodynamic effects related to its antiadrenergic action. To explore further the differences between these two antiarrhythmic agents, the authors compared the effects of bretylium, lidocaine, and saline on a standardized dog model of ventricular fibrillation followed by electromechanical dissociation (EMD). The protocol included three successive episodes of cardiac arrest in each animal. Three minutes before each episode of ventricular fibrillation, 5 mg/kg of bretylium tosylate (n = 11), 1 mg/kg of lidocaine (n = 9) or saline (n = 12) were administered blindly. There was no difference in the duration of cardiac arrest (bretylium, 8 min 18 sec; lidocaine, 7 min 54 sec; saline, 8 min 20 sec) or the total doses of epinephrine required to resuscitate the animals. Both bretylium and lidocaine appeared to preserve cardiac function 5 minutes after recovery, as stroke volume increased from 17.8 +/- 6.7 to 18.7 +/- 6.7 mL (NS) after bretylium and from 17.7 +/- 7.7 to 19.0 +/- 7.0 mL (NS) after lidocaine, but decreased from 19.0 +/- 5.3 to 14.6 +/- 6.0 mL (P less than .05) after saline. During the first 10 minutes of EMD, ventricular fibrillation or ventricular tachycardia recurred in 4 dogs treated with lidocaine, 3 dogs treated with saline, but no dog treated with bretylium (P less than .05 between bretylium and saline).(ABSTRACT TRUNCATED AT 250 WORDS)

A new group of defibrillatory drugs in the classification of antiarrhythmic agents

Ventricular antiarrhythmic therapy is aimed traditionally at preventing arrhythmias and ventricular fibrillation. Recently, a new approach has been introduced, where drug therapy facilitates the ability of the heart for spontaneous defibrillation. The chemical features and the electrophysiologic properties are discussed below. The introduction of this new group of defibrillating antiarrhythmic drugs implies the extension of the common classification of antiarrhythmic drugs.

Intracellular bretylium blocks Na+ and K+ currents in heart cells

The effects of intracellular and extracellular application of bretylium tosylate on outward potassium current and fast inward Na+ current have been studied in single ventricular cells of chick embryo (18-day-old) using the whole-cell voltage clamp technique. Extracellular superfusion with bretylium (3 x 10(-4) M) decreased the K+ current (IK) whereas the fast INa remained unaffected. Introducing bretylium intracellularly also decreased IK, but rapidly inhibited the fast INa. The addition of 3 x 10(-4) M bretylium to the superfusion medium in the presence of intracellular bretylium further decreased IK and caused the fast inward sodium current (INa) to recover. The decrease of IK by bretylium may account for the antifibrillatory properties of this drug in heart.

Balance among autonomic controls of heart rate in neonatal spontaneously hypertensive and borderline hypertensive rats

The ontogeny of functional sympathetic neural, adrenal medullary, and extra-adrenal components of adrenergic control of heart rate was compared in neonatal Spontaneously hypertensive (SHR), Wistar-Kyoto (WKY) and Borderline hypertensive (BHR) rats using combined sequential pharmacological blockade and surgical intervention. Baseline heart rate recorded from awake and unrestrained pups was lower in BHR than in WKY or SHR at 5 days of age. Tonic sympathetic neural control of heart rate was inferred from bradycardia after treatment with the adrenergic neuron-blocking agent, bretylium tosylate. Bradycardia after bretylium treatment was observed at 2, 5 and 8 days of age in all strains, suggesting tonic sympathetic neural control of heart rate during the first postnatal week. Parasympathetic control of heart rate was inferred from heart rate increase after treatment with the muscarinic receptor blocker, atropine methyl nitrate, in pups pretreated with bretylium. Tachycardia following atropine methyl nitrate was substantial in all 24-day-old pups. Control of heart rate by neurally mediated release of catecholamines from the adrenal medulla was inferred from bradycardia following administration of the ganglionic blocking agent, hexamethonium, to pups pretreated with bretylium and atropine methyl nitrate. Heart rate decreases after hexamethonium were found in 2-day-old WKY and BHR pups, and at 5 and 8 days in all strains. Adrenalectomy was performed in additional animals to confirm the adrenal catecholamine influence on heart rate. The influence of residual circulating catecholamines on neonatal heart rate was inferred from bradycardia following administration of the beta-adrenergic receptor blocking agent, atenolol, in pups pretreated with bretylium, methylatropine, and hexamethonium. Bradycardia was observed in pups of each strain and at all ages after atenolol treatment. Strain differences in autonomic controls of heart rate were most pronounced at 24 days of age. At 24 days of age both SHR and BHR pups showed increased adrenal catecholamine and parasympathetic influences on heart rate compared to WKY. Thus, prior to weaning, rats differing in their genetic predisposition to hypertension showed a unique pattern of autonomic control over heart rate which may be related to adult cardiovascular regulation.

Use of bretylium tosylate as prophylaxis and treatment in hypothermic ventricular fibrillation in the canine model

We conducted a study to determine if bretylium tosylate (BT) is effective in the prophylaxis and treatment of hypothermic ventricular fibrillation (VF) in the setting of various maneuvers thought to induce this lethal arrhythmia. Twenty-two mongrel dogs were cooled to 24 C after being placed in a cold room. At 24 C, a double-blinded placebo or BT solution was infused. The dogs then were removed from the cold. They underwent the following sequential maneuvers: oral endotracheal extubation and intubation, central line and nasogastric tube placement, vigorous movement, and Swan-Ganz catheter insertion. If VF ensued, arterial blood gases were drawn, and BT was given only if refractory to countershock and epinephrine. Of the dogs that were given placebo, six of 11 (55%) fibrillated with manipulation, as compared with one of 11 (9%) dogs pretreated with BT (P = .067). Three of the 11 dogs that received BT fibrillated within minutes of its infusion. In the placebo dogs that fibrillated, four required BT and two defibrillated with countershock alone or with epinephrine prior to achieving stable rhythms.

Bethanidine sulfate in paroxysmal ventricular tachycardia: toxicity and antifibrillatory actions

Programmed ventricular stimulation was used to test oral bethanidine sulfate in 10 patients with life-threatening ventricular arrhythmias. These patients had previously documented, recurrent, sustained ventricular tachycardia (VT) and/or ventricular fibrillation (VF) complicating stable heart disease. During control electrophysiologic studies, VT could be induced in all 10 patients: 6 with nonsustained VT, 3 with sustained VT, and 1 with VT/VF. After control, bethanidine 20-30 mg/kg was administered orally and beginning 60 minutes later, programmed ventricular stimulation was repeated. After bethanidine administration, VT could be induced in nine patients; in four, the VT was essentially unchanged from that induced during control studies. In four others, worse VT was induced after bethanidine. The remaining two patients had a potentially beneficial response to the drug. Bethanidine was poorly tolerated: seven patients had symptomatic orthostatic hypotension that persisted for several days despite concurrent protriptyline therapy. Furthermore, in four patients, spontaneous VT or VT/VF occurred 3-8 hours after the last dose. Nausea, vomiting, flushing, and blood pressure elevation were also noted. Bethanidine sulfate in the dosages used usually does not prevent the induction of VT by programmed ventricular stimulation and frequently causes serious toxicity. These findings suggest that the drug would be ineffective and poorly tolerated for long-term therapy in patients with serious ventricular arrhythmias.

The effect of bretylium and clofilium on dispersion of refractoriness and vulnerability to ventricular fibrillation in the ischemic feline heart

Bretylium has been shown to have a pronounced antifibrillatory effect. The purpose of this study was to examine the effects of bretylium on changes in vulnerability to ventricular fibrillation (VF) and refractoriness which occur during acute myocardial infarction. Right ventricular VF thresholds and effective refractory periods (ERP) at six left ventricular sites were measured before and serially after left anterior descending coronary occlusion in chloralose-anesthetized cats. In eight untreated animals, there was a decrease in VF thresholds of 73% (p less than 0.01) immediately after occlusion and dispersion of refractoriness (DR) (maximum difference in ERP between normal and ischemic left ventricular sites) increased from 18 +/- 4 to 50 +/- 6 msec (p less than 0.01). Five of eight animals manifested spontaneous VF within the first minutes of occlusion but none had nonsustained VF. Pretreatment with bretylium (10 to 20 mg/kg intravenously) increased resting ERP from 181 +/- 9 to 201 +/- 9 msec (p less than 0.05) and VF threshold from 32 +/- 5 to 85 +/- 7 mA (p less than 0.001). Bretylium also prevented spontaneous VF in all eight animals and abolished occlusion-related changes in VF and DR. Fourteen animals were similarly studied using clofilium, a bretylium congener which is devoid of sympatholytic effect (no effect on blood pressure response to bilateral carotid artery occlusion). Clofilium increased resting ERP and VF thresholds at both low (0.5 mg/kg intravenously) and high doses (5 mg/kg intravenously). High-but not low-dose clofilium blunted the fall in VF threshold after coronary occlusion. In addition, DR correlated with VF threshold changes at both doses.(ABSTRACT TRUNCATED AT 250 WORDS)