Ventricular fibrillation in a conscious canine preparation of sudden coronary death--prevention by short- and long-term amiodarone administration

Amiodarone Treatment in the Early Phase of Acute Myocardial Infarction Protects Against Ventricular Fibrillation in a Porcine Model

Ventricular fibrillation (VF) occurring in the first minutes to hours of acute myocardial infarction (AMI) is a frequent cause of death and treatment options are limited. The aim was to test whether early infusion of amiodarone 10 min after onset of AMI reduced the incidence of VF in a porcine model. Eighteen female Danish landrace pigs were randomized to a control and an amiodarone group. AMI was induced by ligation of the mid-left anterior descending artery for 120 min followed by 60 min of reperfusion. VF occurred in 0/8 pigs treated with amiodarone compared to 7/10 controls (P < 0.01). Amiodarone treatment prolonged RR intervals, reduced dispersion of action potential duration in the infarcted area and mean number of ectopic beats. No negative effects on cardiac output and blood pressure were observed with amiodarone. Amiodarone qualifies as a potential drug candidate to prevent VF in the first minutes to hours of AMI.

In vivo models for the evaluation of antithrombotics and thrombolytics

The development and application of animal models of thrombosis have played a crucial role in the discovery and validation of novel drug targets and the selection of new agents for clinical evaluation, and have informed dosing and safety information for clinical trials. These models also provide valuable information about the mechanisms of action/interaction of new antithrombotic agents. Small and large animal models of thrombosis and their role in the discovery and development of novel agents are described. Methods and major issues regarding the use of animal models of thrombosis, such as positive controls, appropriate pharmacodynamic markers of activity, safety evaluation, species specificity, and pharmacokinetics, are highlighted. Finally, the use of genetic models of thrombosis/hemostasis and how these models have aided in the development of therapies that are presently being evaluated clinically are presented.

Identification by automated screening of a small molecule that selectively eliminates neural stem cells derived from hESCs but not dopamine neurons

Background

We have previously described fundamental differences in the biology of stem cells as compared to other dividing cell populations. We reasoned therefore that a differential screen using US Food and Drug Administration (FDA)-approved compounds may identify either selective survival factors or specific toxins and may be useful for the therapeutically-driven manufacturing of cells in vitro and possibly in vivo.

Methodology/Principal Findings

In this study we report on optimized methods for feeder-free culture of hESCs and hESC-derived neural stem cells (NSCs) to facilitate automated screening. We show that we are able to measure ATP as an indicator of metabolic activity in an automated screening assay. With this optimized platform we screened a collection of FDA-approved drugs to identify compounds that have differential toxicity to hESCs and their neural derivatives. Nine compounds were identified to be specifically toxic for NSCs to a greater extent than for hESCs. Six of these initial hits were retested and verified by large-scale cell culture to determine dose-responsive NSC toxicity. One of the compounds retested, amiodarone HCL, was further tested for possible effects on postmitotic neurons, a likely target for transplant therapy. Amiodarone HCL was found to be selectively toxic to NSCs but not to differentiated neurons or glial cells. Treated and untreated NSCs and neurons were then interrogated with global gene expression analysis to explore the mechanisms of action of amiodarone HCl. The gene expression analysis suggests that activation of cell-type specific cationic channels may underlie the toxicity of the drug.

Conclusions/Significance

In conclusion, we have developed a screening strategy that allows us to rapidly identify clinically approved drugs for use in a Chemistry, Manufacture and Control protocol that can be safely used to deplete unwanted contaminating precursor cells from a differentiated cell product. Our results also suggest that such a strategy is rich in the potential of identifying lineage specific reagents and provides additional evidence for the utility of stem cells in screening and discovery paradigms.

In vitro effects of acute amiodarone and dronedarone on epicardial, endocardial, and M cells of the canine ventricle

Amiodarone (AM) is an antiarrhythmic agent widely used in the treatment of ventricular and supraventricular arrhythmias. Dronedarone (DR) is a new compound with a pharmacological profile similar to that of AM, but iodine free. We previously demonstrated that chronic AM treatment reduces transmural dispersion of repolarization (TDR) in the canine heart. We used standard microelectrode technique to evaluate the effects of acute AM (100 microM) and DR (30 microM) on epicardial (EPI), endocardial (ENDO), and M region tissues obtained from the left ventricular wall of the canine heart. Amiodarone (100 microM, 120 min of exposure) produced little change in the action potential duration of ENDO and EPI tissues, but it shortened the action potential of M cells, especially at slow rates, leading to a decrease in TDR. Similar results were observed with DR. Acute AM (100 microM) and DR (30 microM) eliminated d-sotalol-induced early afterdepolarizations (EADs) and triggered activity in 3 of 3 and 2 of 6 M cell preparations, respectively. The reduction of TDR and the elimination of EAD-induced triggered activity differentiates AM and DR from other class III agents. These effects may explain the efficacy and low arrhythmogenicity of acute AM and suggest a potential safe use of DR as an antiarrhythmic agent.

Combined treatment with L-carnitine and a pan-caspase inhibitor effectively reverses amiodarone-induced injury in cultured human lung epithelial cells

Amiodarone is an effective class III antiarrhythmic drug, however, the pulmonary toxicity is one of the most life-threatening complications of its use. The present study was designed to determine the mechanisms underlying pulmonary toxicity of amiodarone. In cultured human lung epithelial cells A549, amiodarone caused cell injury characterized by mitochondrial membrane depolarization, ATP depletion, enhanced propidium iodide (PI) uptake and increase in the number of Annexin-V positive cells, although the population of PI-stained cells appeared earlier and was not identical to that of Annexin-V stained cells, suggesting that the apoptosis and necrosis appeared in different cells. The apoptosis was accompanied with the activation of caspase-2, -3 and -8 but not caspase-9, and reversed by these caspase inhibitors. However, the caspase inhibitors had no influence on mitochondrial membrane potential or PI uptake after exposure of A549 cells to amiodarone. In contrast, mitochondrial cofactors such as L-carnitine and acetyl-L: -carnitine attenuated mitochondrial membrane depolarization, abrogated cellular ATP depletion and reversed PI uptake without affecting Annexin-V positive cells. These finding suggest that different intracellular events operate to cause apoptosis and necrosis after exposure of pulmonary epithelial cells to amiodarone.

Antiarrhythmic Properties of a Prior Oral Loading of Amiodarone in In Vivo Canine Coronary Ligation/Reperfusion-Induced Arrhythmia Model: Comparison With Other Class III Antiarrhythmic Drugs

Amiodarone, which is generally classified as class III antiarrhythmic drug in the Vaughan Williams classification, is widely used for the treatments of refractory arrhythmias. However, we previously reported that intravenous infusion of amiodarone (6.67 mg/kg per hour) did not suppress arrhythmias induced by coronary ligation/reperfusion in dogs. In this study, we examined effects of a prior oral loading of amiodarone on arrhythmias induced by coronary ligation/reperfusion. Sixteen female beagle dogs (8.5 - 12.5 kg) were divided into two groups; one group was given amiodarone (40 mg/kg, orally, n = 8), and the other was given empty gelatin capsules (n = 8) 2 h before the operation. Dogs were anesthetized with pentobarbital and artificially ventilated. The left chest was opened, and the left anterior descending coronary artery was ligated for 30 min and then reperfused. The mean plasma concentration of amiodarone was over 1.3 mug/ml. Although the prior oral loading of amiodarone did not change the QT interval, amiodarone suppressed the number of ectopic beats during coronary ligation and the incidence of ventricular fibrillation during coronary ligation and reperfusion periods (P<0.05 vs control group). In conclusion, a prior oral loading of amiodarone suppressed arrhythmias induced by coronary ligation/reperfusion with a dose that did not prolong the QT interval. This antiarrhythmic property of amiodarone is different from those of the other class III drugs in that antiarrhythmic effects were accompanied by QT prolongation in our all previous studies.

Combined amiodarone and silymarin treatment, but not amiodarone alone, prevents sustained atrial flutter in dogs

Amiodarone/Silymarin Treatment for Sustained Atrial Flutter.

INTRODUCTION

Because amiodarone generates free radicals that may mediate amiodarone's toxicity, simultaneous therapy with an antioxidant might be beneficial if the antioxidant did not impair amiodarone's antiarrhythmic action. We tested whether simultaneous administration of a flavonoid antioxidant, silymarin, altered the electrophysiologic (EP) actions of amiodarone in 62 open chest dogs with electrically induced atrial flutter created by a Y-shaped right atrial incision.

METHODS AND RESULTS

Fifteen dogs received oral amiodarone (600 mg/day); 15 dogs received amiodarone (600 mg/day) and silymarin (70 mg bid); and 8 dogs received silymarin (70 mg bid) alone. All dosing was for 8 weeks; 24 control dogs received no drugs prior to induction of atrial flutter. Atrial flutter was induced by rapid right atrial pacing, and EP measurements were made before (presurgical) and after (postsurgical) creation of a Y-shaped right atrial incision. There was no difference in the frequency of induction of atrial flutter lasting >30 minutes among amiodarone-treated (8/15 [53%]), silymarin-treated (4/6 [67%]), and control (15/21 [71%]) groups, whereas the frequency of induction in the amiodarone+silymarin dogs (2/15 [13%]) was significantly reduced (P = 0.008) compared with the other three groups. Both amiodarone and amiodarone+silymarin treatment prolonged the presurgical and postsurgical right atrial effective refractory period (P = 0.012) compared with control; however, there was no significant difference in either parameter between the amiodarone+silymarin-treated and amiodarone-treated groups. The increase in atrial flutter mean cycle length (postsurgical minus presurgical) was significantly (P = 0.005) less in the amiodarone+silymarin-treated and control dogs compared with the amiodarone-treated dogs (16 +/- 11 msec for amiodarone+silymarin; 24 +/- 8 msec for control; and 42 +/- 14 msec for amiodarone treatment). Amiodarone+silymarin treatment resulted in a longer postsurgical right atrial refractory period (155 +/- 13 msec) than atrial flutter mean cycle length (154 +/- 19 msec), consistent with reduction and/or elimination of the excitable gap. Silymarin alone did not exert significant EP or antiarrhythmic action.

CONCLUSION

Amiodarone exerted no preventative antiarrhythmic action in this atrial flutter model, probably because it could not reduce the excitable gap of atrial flutter. However, an antioxidant, silymarin, without a direct antiarrhythmic action, when administered together with amiodarone, potentiated amiodarone's antiarrhythmic actions and prevented sustained atrial flutter by reduction and/or elimination of the excitable gap.

Acute electropharmacological effects of intravenously administered amiodarone assessed in the in vivo canine model

Acute hemodynamic and electrophysiological effects of amiodarone were assessed simultaneously, using the halothane-anesthetized, closed-chest in vivo canine model in comparison with those of solvent ethanol alone. Intravenous administration of the solvent (n = 8) induced no significant change in any of the cardiovascular parameters. On the other hand, intravenous amiodarone in the canine antiarrhythmic dose of 3.0 mg/kg (n = 6) exerted negative chronotropic, inotropic and dromotropic effects in addition to the transient hypotensive action followed by an increase of the total peripheral vascular resistance. Amiodarone also prolonged both the ventricular repolarization phase and the effective refractory period, where the increment was greater in the latter than in the former, indicating the shortening of the electrical vulnerable period of the ventricle. More importantly, appearance of the electrophysiological effect on repolarization took more time and higher dose compared with the effect on refractoriness, which could be detected at a one tenth the dose. These results support the previous knowledge that intravenously administered amiodarone possesses class I, III and IV actions and suggest that shortening of the electrical vulnerable period may be one of the unique antiarrhythmic properties of intravenous amiodarone against re-entry type arrhythmias.

A comparison of the antifibrillatory effects of desethylamiodarone to amiodarone in a swine model

We evaluated the effect of two different doses of desethylamiodarone (DEA) and amiodarone on the ventricular fibrillation threshold (VFT). We ascertained the VFT in 24 pigs randomized to intravenous DEA, amiodarone, or vehicle. Ventricular fibrillation was induced by pacing the right ventricle by using a primary drive train at a cycle length of 270 ms for eight beats of 2-ms duration each. A secondary train of 20 pulses of 4-ms duration (100 Hz) immediately followed this over a total duration of 200 ms synchronized to the primary drive train. The intensity of the secondary train stimuli current was initially 2 mA and was increased by 2-mA increments until sustained VF with hemodynamic collapse was induced. The minimal current strength needed to induce sustained VF was defined as the VFT measured in mA. DEA (10 mg/kg) increased the VFT significantly over baseline from 13.5+/-4.9 to 23.2+/-8.8 mA (p = 0.0076). Amiodarone, 10 mg/kg, increased the VFT significantly over baseline (mean +/- SD) from 14.4+/-3.6 to 23.8+/-6.1 mA (p = 0.0016). An additional dose of amiodarone (15 mg/kg) increased the VFT to 38.5+/-15.9 mA, which is significantly greater than the VFT derived from lower-dose amiodarone (p = 0.046). We showed that DEA (10 mg/kg) has a similar antifibrillatory effect as 10 mg/kg of amiodarone. We also demonstrated a dose-dependent effect on VFT for amiodarone.

Chronic amiodarone reduces transmural dispersion of repolarization in the canine heart

INTRODUCTION

Amiodarone is a potent antiarrhythmic agent used in the management of both atrial and ventricular arrhythmias. In addition to its beta-blocking properties, amiodarone is known to block the sodium, potassium, and calcium channels in the heart. Its complex electropharmacology notwithstanding, the reasons for the high efficacy of the drug remain unclear. Also not well understood is the basis for the low incidence of proarrhythmia seen with amiodarone relative to other agents with Class III actions. The present study was designed to examine the effects of chronic amiodarone in epicardial, endocardial, and M cells of the canine left ventricle.

METHODS AND RESULTS

We used standard microelectrode techniques to record transmembrane activity from endocardial, epicardial, mid-myocardial, and transmural strips isolated from the canine left ventricle. Tissues were obtained from mongrel dogs receiving amiodarone orally (30 to 40 mg/kg per day) for 30 to 45 days or from untreated controls. Chronic amiodarone produced a greater prolongation of action potential duration in epicardium and endocardium, but less of an increase, or even a decrease at slow rates, in the M region, thereby reducing transmural dispersion of repolarization. In addition, chronic amiodarone therapy suppressed the ability of the IKr blocker, d-sotalol, to induce a marked dispersion of repolarization or early afterdepolarization activity.

CONCLUSION

Our data demonstrate for the first time a direct effect of chronic amiodarone treatment to differentially alter the cellular electrophysiology of ventricular myocardium so as to produce an important decrease in transmural dispersion of repolarization, especially under conditions in which dispersion is exaggerated. These results may contribute to our understanding of the effectiveness of amiodarone in the treatment of life-threatening arrhythmias as well as to our understanding of the low incidence of proarrhythmia attending therapy with chronic amiodarone in comparison with other Class III agents.

Parenteral antiarrhythmic drug therapy in ventricular tachycardia/ventricular fibrillation: evolving role of class III agents--focus on amiodarone

More effective intravenous antiarrhythmic agents are required for treatment of patients with refractory malignant ventricular arrhythmias. More recently, a great deal of interest has been focused on use of intravenous amiodarone for these patients. Uncontrolled early studies showed that intravenous amiodarone was effective in 42% to 81% of treated patients. Recent large cooperative trials have documented the efficacy of intravenous amiodarone in these patients and have shown an efficacy comparable to bretylium in patients with refractory sustained ventricular tachycardia or fibrillation.

Safety and diagnostic yield of noninvasive ventricular stimulation performed via tiered therapy implantable defibrillators

Extensive electrophysiological testing is critical for the effective utilization of sophisticated tachycardia detection and termination algorithms available in tiered therapy ICDs. To evaluate the safety and diagnostic yield of electrophysiological testing via noninvasive ventricular stimulation, we performed 294 electrophysiological studies in 154 patients (age 65 +/- 10; left ventricular ejection fraction 0.36 +/- 0.15) with tiered therapy ICDs. Stimulation was performed under methohexital anesthesia. A total of 918 sustained ventricular tachyarrhythmias were induced (3.1 +/- 2.5 per procedure): monomorphic VT, 550; ventricular flutter, 74; and VF, 246. The results of invasive and noninvasive programmed stimulation were compared for 79 patients who had both studies under similar treatment. Overall concordance was 83%, and did not differ significantly between patients who had the noninvasive stimulation via epicardial or endocardial pacing leads. VF could be induced in 206 of 257 studies (82%), and it was less likely to be induced in patients on amiodarone (74% vs 85%; P = 0.02), or beta blockers (55% vs 83%; P = 0.017). No patient presented a serious complication. Minor complications occurred during 39 studies: transient laryngospasm in 1, unintended delivery of an ICD shock to a conscious patient in 4; induction of sustained atrial fibrillation in 8; need for external rescue defibrillation shocks in 13; and delivery of inappropriate shocks for supraventricular rhythms in 14 studies. Noninvasive ventricular stimulation performed under methohexital anesthesia is safe. Its diagnostic yield compares favorably with that of conventional electrophysiological studies. VF can be induced in a majority of patients. There is good correlation between invasive and noninvasive programmed stimulation for induction of VT. Noninvasive ventricular stimulation may emerge as standard procedure for the initial programming and follow-up of ICDs.

Effects of amiodarone on the circadian rhythm and power spectral changes of heart rate and QT interval: significance for the control of sudden cardiac death

Effects of chronic amiodarone therapy on the circadian rhythmicity and power spectral changes of heart rate and QT intervals from Holter recordings were evaluated in three groups of patients: group 1 baseline (n = 10); group 2, treated for 3 to 6 months (n = 11); and group 3, treated for > 1 year (n = 13). Amiodarone reduced heart rate, which reached steady state at 3 to 6 months; bradycardia was evident during the entire 24 hours. The corrected QT (QTc) interval increased as a function of treatment duration. It was 457 +/- 39, 530 +/- 28 (p < 0.001), and 581 +/- 36 (p < 0.0002) msec for groups 1, 2, and 3, after 6 months, respectively. The circadian rhythmicity of QTc was abolished in group 3. Power spectral analysis showed a tendency for amiodarone to reduce both R-R and QT interval variabilities, suggesting inhibition of autonomic control on the heart by the drug. The effectiveness of amiodarone against ventricular arrhythmias may result in part from the sustained bradycardia in concert with continuous uniform prolongation of myocardial repolarization.

Antifibrillatory effects of clofilium in the rabbit isolated heart

1. This study was designed to determine whether clofilium exhibits antifibrillatory activity in a pinacidil + hypoxia-induced model of ventricular fibrillation (VF) in Langendorff-perfused hearts. 2. Ten minutes after exposure to vehicle or clofilium (0.1, 1.0 and 10.0 microM), hearts were exposed to pinacidil (1.25 microM), then subjected to 12 min of hypoxia and reoxygenated. Onset to VF was recorded. Additional groups of hearts were pretreated with UK-68,798 (1.0, 3.0 and 10.0 microM), a delayed rectifier channel blocker, and 5-hydroxydecanoate (10 microM), a known ATP-dependent K+ channel blocker, and subjected to an identical protocol. 3. Clofilium decreased the incidence of VF in a concentration-dependent manner; 7/9 control hearts developed VF vs 1/9 hearts (P = 0.007, Fisher's Exact) treated with 10.0 microM clofilium. In addition, 5-hydroxydecanoate protected hearts from VF, while UK-68,798 pretreatment did not. 4. In a separate group of hearts, electrically-induced VF was converted to sinus rhythm in 10/11 hearts after clofilium was introduced as a bolus. 5. Clofilium is capable of preventing VF in the rabbit isolated heart in a concentration-dependent manner. We have data to suggest that the ability of clofilium to attenuate the effects of pinacidil+hypoxia in our model may include blockade of metabolically active K+ channels, i.e., KATP (glibenclamide-sensitive) channel.

New antiarrhythmic drugs in pediatric use: amiodarone

Amiodarone, a class III antiarrhythmic agent, prolongs action potential duration and refractoriness of all cardiac structures. The drug is more rapidly metabolized in pediatric patients than in adults, but its kinetics are still unique compared with other drugs. Due to the unusual pharmacokinetic characteristics of amiodarone, treatment has to be started by administering loading doses, and there is a significant delay both in the achievement of the full anti-arrhythmic effect and in the development of side effects. Amiodarone is a highly effective agent in pediatric patients with automatic and reentrant supraventricular tachycardia as well as in refractory atrial flutter. Efficacy in ventricular tachycardia has been shown to be variable depending on the underlying anatomical substrate. The incidence of side effects is lower than that observed in adult studies with similar duration of therapy but their incidence is still significant. Amiodarone treatment is associated with a significant risk of proarrhythmic effects, requiring hospitalization of the patient during the loading period.

Antiarrhythmic versus antifibrillatory actions: inference from experimental studies

Pathophysiology of the coronary circulation is a major contributor to altering the myocardial substrate, rendering the heart susceptible to the onset of arrhythmias associated with sudden cardiac death. Antiarrhythmic drug therapy for the prevention of sudden cardiac death has been provided primarily on the basis of trial and error and in some instances based on ill-suited preclinical evaluations. The findings of the Cardiac Arrhythmia Suppression Trial (CAST) requires a reexamination of the manner in which antiarrhythmic drugs are developed before entering into clinical testing. The major deficiency in this area of experimental investigation has been the lack of animal models that would permit preclinical studies to identify potentially useful or deleterious therapeutic agents. Further, CAST has emphasized the need to distinguish between pharmacologic interventions that suppresses nonlethal disturbances of cardiac rhythm as opposed to those agents capable of preventing lethal ventricular tachycardia or ventricular fibrillation. Preclinical models for the testing of antifibrillatory agents must consider the fact that the superimposition of transient ischemic events on an underlying pathophysiologic substrate makes the heart susceptible to lethal arrhythmias. Proarrhythmic events, not observed in the normal heart, may become manifest only when the myocardial substrate has been altered. We describe a model of sudden cardiac death that may more closely simulate the clinical state in humans who are at risk. The experimental results show a good correlation with clinical data regarding agents known to reduce the incidence of lethal arrhythmias as well as those showing proarrhythmic actions.

Low-dose amiodarone for atrial fibrillation

Concerns about proarrhythmia risk and inefficacy associated with class I antiarrhythmic drugs have revived interest in low-dose amiodarone (maintenance dose 200-400 mg/day) for suppression of atrial fibrillation. In nonrandomized trials of amiodarone for atrial fibrillation refractory to conventional agents, amiodarone has been successful in maintaining sinus rhythm in 53-79% of patients during a mean follow-up of 15-27 months. Intolerable side effects, including pulmonary toxicity, are in the range of 1-12% per year and resolve following amiodarone withdrawal in the majority of cases. Proarrhythmia risk associated with amiodarone, even in the setting of left ventricular dysfunction, is extremely low. In patients with congestive heart failure, in whom other pharmacologic options are limited by proarrhythmia risk and negative inotropism, preliminary experience with amiodarone is especially promising. Randomized trials are needed, directly comparing amiodarone to conventional antiarrhythmic therapy for atrial fibrillation suppression and comparing amiodarone to warfarin for thromboembolism prevention in patients with atrial fibrillation refractory to conventional antiarrhythmic drugs.

Amiodarone therapy does not compromise subsequent heart transplantation

OBJECTIVES

The objective of this study was to determine the frequency of pulmonary complications, feasibility of early hospital discharge and requirements for postoperative inotropic and chronotropic support in patients receiving amiodarone therapy before heart transplantation.

BACKGROUND

Although many patients waiting for heart transplantation will die of arrhythmias before a donor heart is found, the use of amiodarone has been limited by concern about increased complications in the perioperative period.

METHODS

The 29 patients receiving amiodarone at the time of heart transplantation at University of California, Los Angeles Medical Center between October 1986 and September 1990 were compared with 29 control recipients to evaluate postoperative morbidity. Patients were receiving amiodarone for recurrent ventricular tachyarrhythmias (n = 11), atrial fibrillation (n = 2) or complex ventricular ectopic activity (n = 16). The average daily dose was 360 +/- 230 mg/day for an average of 11 +/- 22 months before transplantation. Amiodarone and control groups had a similar ejection fraction (0.18 +/- 0.07 vs. 0.20 +/- 0.08), frequency of coronary disease, age and gender. There were three more status I patients in the control group. OKT3 was given to only two patients receiving amiodarone and 12 control patients at high risk for renal dysfunction.

RESULTS

Postoperatively, the duration of assisted ventilation was 21 +/- 19 h after amiodarone therapy versus 26 +/- 2 h in the control group (20 +/- 18 h vs. 15 +/- 9 h after excluding patients receiving OKT3), discharge arterial oxygen saturation was > 95% in both groups. Two patients in the amiodarone group with a smoking history of > 100 pack-years developed bilateral pulmonary infiltrates of brief duration. Although patients receiving amiodarone required atrial pacing more frequently (eight vs. two patients) and had a lower heart rate at discharge (75 +/- 18 vs. 86 +/- 11 beats/min), the duration of inotropic support (2.1 +/- 1.5 vs. 3.5 +/- 2.5 days) and of hospital stay (10 +/- 3 vs. 15 +/- 10 days) was not higher in the amiodarone than in the control group. The mortality rate at 30 days was similar in the two groups (6.8% vs. 3.4%, p = NS).

CONCLUSIONS

Amiodarone therapy before heart transplantation may contribute to occasional pulmonary complications but does not significantly increase perioperative morbidity or mortality with the regimens used in this retrospective study.

Amiodarone. An overview of its pharmacological properties, and review of its therapeutic use in cardiac arrhythmias

Amiodarone, originally developed over 20 years ago, is a potent antiarrhythmic drug with the actions of all antiarrhythmic drug classes. It has been successfully used in the treatment of symptomatic and life-threatening ventricular arrhythmias and symptomatic supraventricular arrhythmias. In patients with left ventricular dysfunction amiodarone does not usually produce any clinically significant cardiodepression and the drug has relatively high antiarrhythmic efficacy. Preliminary studies indicate that amiodarone may have a beneficial effect on mortality and survival in certain groups of patients with ventricular arrhythmias, an action probably related to both its antiarrhythmic and antifibrillatory effects. The adverse effect profile of amiodarone is diverse, involving the cardiac, thyroid, pulmonary, hepatic, gastrointestinal, ocular, neurological and dermatological systems. Interstitial pneumonitis and hepatitis are potentially fatal, but the vast majority of adverse events are less serious, and some may be dose dependent. Pretreatment monitoring, regular assessments and the use of minimum effective doses are, therefore, necessary. Thus, with appropriate monitoring to control its well recognised adverse effects amiodarone has an important place as an effective 'broad spectrum' antiarrhythmic drug which has, so far, been used when other treatments have proved ineffective. More recent preliminary data also suggest that it may also have a beneficial effect in the prevention of sudden death in some patients.

Pharmacology and pharmacokinetics of amiodarone

Amiodarone is a unique antiarrhythmic agent originally developed as a vasodilator. Classified electrophysiologically as a Type III antiarrhythmic, it also has both nonspecific antisympathetic and direct, fast channel-membrane effects. Hemodynamic effects of orally administered amiodarone (a negative inotropic agent) are usually negligible, and are usually compensated for by induced vasodilation. Effects on thyroid and hepatic function may help to explain some of the unique pharmacologic as well as toxicologic effects of the drug. Amiodarone is poorly bioavailable (20-80%) and undergoes extensive enterohepatic circulation before entry into a central compartment. The principal metabolite, mono-n-desethyl amiodarone is also an antiarrhythmic. From this central compartment, it undergoes extensive tissue distribution (exceptionally high tissue/plasma partition coefficients). The distribution half-life of amiodarone out of the central compartment to peripheral and deep tissue compartments (t1/2 alpha) may be as short as 4 hours. The terminal half-life (t1/2 beta) is both long and variable (9-77 days) secondary to the slow mobilization of the lipophilic medication out of (primarily) adipocytes. A pharmacokinetically based loading scheme is described, and data suggesting a role for routine amiodarone plasma levels are presented.

Practical aspects of the use of amiodarone

Amiodarone is a class III antiarrhythmic drug with sympatholytic properties, which prolongs the refractory period of all cardiac tissues, depresses sinus node automaticity and atrioventricular nodal conduction. It is active on all cardiac arrhythmias, its use being limited by the risk of side effects, mainly extracardiac, which are dependent upon dosage and duration of treatment. The use of an oral loading dose of intravenous infusion to rapidly obtain an efficacious blood concentration may minimise the delay in onset of drug action. Amiodarone progressively accumulates in cardiac muscle and is eliminated slowly, allowing drug free periods (2 days a week) on long term therapy. Pharmacokinetic interaction with digoxin, class I antiarrhythmic drugs and warfarin must be considered during combination therapy, as well as the potentiation of electrophysiological effects when amiodarone is co-administered with combined calcium antagonists and beta-blockers.

Reassessment of benefit-risk ratio and treatment algorithms for antiarrhythmic drug therapy after the cardiac arrhythmia suppression trial

The Cardiac Arrhythmia Suppression Trial (CAST) has led to serious reconsideration of both the benefit-risk ratio of antiarrhythmic drug therapy and the appropriate therapeutic approach to various cardiac arrhythmias. Class IC drugs, such as encainide and flecainide, should not be used to treat asymptomatic postinfarction arrhythmias. Furthermore, because the CAST raises serious questions about the concept of treating asymptomatic but "potentially malignant" (prognostically important) arrhythmias guided by ambulatory monitoring, the prophylactic use of any of the antiarrhythmic agents (except beta blockers) must be considered inappropriate and potentially harmful until otherwise established by specific clinical trials. For prophylaxis of malignant ventricular arrhythmias (sustained ventricular tachycardia or ventricular fibrillation), treatment may still begin with standard agents in classes IA, IB, or both, preferably guided by electrophysiologic testing alone or in combination with noninvasive testing. Class IC therapy may be most useful in those patients in this group who do not have such high-risk characteristics for proarrhythmia as a history of multiple myocardial infarctions (MIs), congestive heart failure, or low ejection fraction. Amiodarone is moderately effective for treating these arrhythmias but is reserved as second- or third-line therapy because of its potential organ toxicity. Sotalol, a beta blocker with class III activity, is often effective and relatively well tolerated in these patients and may become a preferred drug when approved. For symptomatic but nonmalignant ventricular arrhythmias, a more conservative approach is more appropriate than in the past, with therapy reversed for those with debilitating symptoms. An initial trial of beta blockade is often appropriate before class I agents are considered.(ABSTRACT TRUNCATED AT 250 WORDS)

Do antiarrhythmic drugs work? Some reflections on the implications of the Cardiac Arrhythmia Suppression Trial

Despite major advances in our understanding of the mechanisms of cardiac arrhythmias and how antiarrhythmic drugs appear to work, there remains much doubt whether these agents reduce arrhythmic mortality except in certain subsets of patients. The results of the Cardiac Arrhythmia Suppression Trial (CAST) have indicated that certain antiarrhythmic drugs not only "fail to work" but may substantially increase mortality. The effects of Class Ic agents in CAST and the meta-analysis of randomized antiarrhythmic trials in the survivors of acute infarction suggest that drugs that act primarily by delaying conduction are particularly deleterious in the survivors of acute infarction. Whether these data have a wide applicability in terms of all ventricular arrhythmias is unclear, but beta-blockers remain the only class of agents that in control trials have been shown to reduce sudden death. The effect appears to be related to beta-blockade and not to suppression of premature ventricular contractions (PVCs). Beta blockers appear to act by preventing ventricular fibrillation. It is reasonable to assume that PVC suppression per se is unlikely to produce a reduction in sudden death. Uncontrolled data with amiodarone suggests that it has the potential to prolong survival by controlling arrhythmias. The effects of amiodarone and beta blockers, both significantly attenuating adrenergic stimulation, provide pharmacologic probes to define the crucial determinants of efficacy of a compound for mortality reduction in high risk survivors of myocardial infarction. The focus must now shift from antiectopic and antiarrhythmic agents that delay conduction to those that exert antifibrillatory actions by sympathetic antagonism and those that exhibit the added property of lengthening myocardial refractoriness.

Implications of the Cardiac Arrhythmia Suppression Trial for antiarrhythmic drug treatment

The Cardiac Arrhythmia Suppression Trial (CAST) is a randomized, placebo-controlled, double-blind, multicenter clinical trial involving 27 centers and more than 100 hospitals in North America and Europe to test the 1-tailed hypothesis that suppression of ventricular arrhythmias in patients with left ventricular dysfunction after myocardial infarction will reduce arrhythmic death. Since April 18, 1989, the CAST is enrolling patients aged less than 80 years with greater than or equal to 6 ventricular premature complexes and left ventricular ejection fraction less than or equal to 40%. Sustained ventricular tachycardia, class IV congestive heart failure or class IV angina pectoris are exclusion criteria. During a prerandomization period, antiarrhythmic drugs are titrated to suppress ventricular arrhythmias. If greater than or equal to 80% suppression is achieved during open-label titration, patients are randomized to the effective dose or to a matched placebo. If only partial suppression (1 to 79%) is achieved, patients are eligible for a substudy that randomizes them to the best dose found during open-label titration or to placebo. The only patients not randomized to treatment are those with increased arrhythmias or drug intolerance during titration. On April 18, 1989, encainide and flecainide were removed from the CAST because these drugs increased the death rate 2.5-fold. There were no imbalances in baseline risk variables between the encainide/flecainide group and the placebo group that might explain the adverse treatment effect. There was remarkable uniformity of the adverse effect across all subgroups. There were no subgroups that benefited from treatment; all were either harmed or not evaluable.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical implications of new studies in the treatment of benign, potentially malignant and malignant ventricular arrhythmias

For purposes of clinical management, ventricular arrhythmias have been divided into risk categories of benign, prognostically important (potentially malignant) and malignant. Benign arrhythmias occur in the setting of structurally normal hearts and do not require therapy unless associated with debilitating symptoms. Malignant arrhythmias such as sustained ventricular tachycardia or fibrillation deserve aggressive therapy to prevent recurrence. Arrhythmias occurring in the presence of organic heart disease (often ischemic disease) are frequently asymptomatic but prognostically important as a risk factor for sudden death or cardiac arrest. The common empiric practice to treat such arrhythmias (by about 40 to 50% of cardiologists in the United States) needs to be reassessed in the face of the Cardiac Arrhythmia Suppression Trial. For malignant arrhythmias, class IA agents (procainamide and quinidine) continue to be the standard of treatment, and class IB agents (e.g., mexiletine) may be used as alternative or additive therapy. Class IC agents are used as second-line therapy, especially in the setting of ischemic heart disease. Class III therapy with amiodarone is reserved for refractory patients because of potential toxicity. Sotalol, a new class II-III agent, may become a first-line drug. For prognostically important arrhythmias, beta blockers remain the agents of choice, class IC agents are contraindicated, and class IA or IB drugs, or both, should be used conservatively (i.e., only for symptomatic arrhythmias). For symptomatic but benign arrhythmias requiring treatment, beta blockers are safe although not always effective. Class IA, IB and IC agents may then be considered. In these patients, the proarrhythmic potential of quinidine and class IC agents remains a concern.(ABSTRACT TRUNCATED AT 250 WORDS)

Class III antiarrhythmic action linked with positive inotropy: acute electrophysiological and inotropic effects of amiodarone in vitro

Negative inotropy is an adverse feature of most antiarrhythmic drugs. Positive inotropy, however, has been demonstrated for some drugs with class III antiarrhythmic action. Although amiodarone exerts its antiarrhythmic effect by an interplay of different actions on cardiac cells, it has been regarded to be the prototype class III drug due to its prolongation of action potential duration. The present study was designed to test the hypothesis that class III antiarrhythmic action and positive inotropy may be linked. We compared the effects of amiodarone in Cordarone and its solvent Tween 80 on automaticity, refractoriness and inotropy. Two series of experiments were done; one with spontaneously beating rat atria to study the effects on sinus node function, and one with electrically stimulated left atria to study the effects on excitability, refractoriness and inotropy. Amiodarone 1 x 10(-4) M decreased spontaneous heart rate by 13% and prolonged sinus node recovery time by 105%. Without affecting the excitability amiodarone prolonged the effective refractory period by 12%. At the same time contractile force increased by 12%. Lower concentration of amiodarone (5 x 10(-6) M) or Tween 80 had no significant effects. In conclusion, amiodarone exerts acute electrophysiological and inotropic effects in vitro. The class III antiarrhythmic action of amiodarone is linked with positive inotropy.

Efficacy and toxicity of amiodarone for the treatment of supraventricular tachyarrhythmias

Amiodarone is an effective agent for all types of supraventricular tachyarrhythmias regardless of mechanism and may, in fact, control a high percentage of supraventricular tachyarrhythmias refractory to conventional antiarrhythmic agents. However, its toxicity should temper enthusiasm for the use of the medication in non-life-threatening arrhythmias. As always, when recommending specific therapies the potential benefit should be weighed in light of the related risk. In patients with life disordering, drug-refractory atrial fibrillation, it seems reasonable to attempt control with amiodarone. Likewise in patients with ectopic atrial tachycardias refractory to conventional agents, this seems reasonable as well. Other and better therapies are available for patients with life-threatening arrhythmias associated with the Wolff-Parkinson-White syndrome. While amiodarone is moderately effective in these patients, the advent of improved surgical techniques and the relatively low risk of an operation make surgery the treatment of choice. The role of IV amiodarone, acutely, in the treatment of supraventricular tachyarrhythmias remains to be defined.

The effects of amiodarone on repolarization and refractoriness of cardiac fibers

The effects of superfusion (acute) and chronic amiodarone pretreatment on repolarization in dog Purkinje and guinea pig papillary muscle fibers were studied using standard microelectrode techniques. In dog Purkinje fibers superfusion with 5 and 50 micrograms/ml amiodarone shortened action potential duration, slowed restitution of premature action duration, and decreased the range of premature action potential durations. In Purkinje fibers from pretreated dogs action potential duration and range of premature action potential durations did not differ significantly from the corresponding control values but restitution was slowed. In guinea pig papillary muscle superfusion with 20 micrograms/ml amiodarone did not change action potential duration and restitution kinetics but in the muscle fibers from pretreated animals both the action potential duration and the range of premature action potential durations were increased. We concluded that the designation of 'class III action' applied to chronic amiodarone treatment in ventricular but not to chronic treatment in Purkinje fibers, and not to acute treatment in either fibers.

Amiodarone in the management of cardiac arrhythmias: current concepts

This article reviews current information on the clinical pharmacology, therapeutic utility, and adverse reactions of amiodarone, with emphasis on guidelines for its rational use.

Electrophysiologic effects of desethylamiodarone, an active metabolite of amiodarone: comparison with amiodarone during chronic administration in rabbits

During acute superfusion studies by means of the standard microelectrode technique, we previously showed that both amiodarone and its major metabolite, desethylamiodarone, had a modest effect on the lengthening of the action potential duration (APD) at high drug concentrations and produced a rate-dependent block of the sodium channel in cardiac muscle. In this study the comparative electrophysiologic effects of the two compounds in rabbits treated chronically with these compounds were determined with particular reference to repolarization and sinus node automaticity. The changes were correlated with those in serum and tissue drug levels and in thyroid hormone indices. After 1 week neither compound had a significant effect on atrial or sinus nodal potentials; after 3 weeks, amiodarone increased the atrial APD at 90% repolarization time by 10.5% (p less than 0.05) and the effective refractory period (ERP) by 6.7% (p less than 0.05). The corresponding figures for desethylamiodarone were 13% (NS) and 18% (NS). The sinus cycle length was increased 12% (NS) by amiodarone and 27.9% (p less than 0.05) after the metabolite. In animals treated for 6 weeks, amiodarone increased the ventricular APD at 90% repolarization by 58.8% (p less than 0.01) and desethylamiodarone by 42.0% the corresponding figures for the ERP were 63.4% (p less than 0.01 and 47.4% (p less than 0.01), respectively. At the stimulation frequency used, neither compound exerted a significant effect on Vmax. Both amiodarone and desethylamiodarone significantly decreased serum triiodothyronine and increased reverse triiodothyronine levels but had no effect on thyroxine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of acute intravenous and chronic oral amiodarone on defibrillation energy requirements

Amiodarone is commonly used with the automatic implantable defibrillator to treat recurrent ventricular tachyarrhythmias. The effects of acute intravenous and chronic oral amiodarone on the energy requirements for successful defibrillation were evaluated in 12 dogs chronically instrumented with right atrial spring and left ventricular patch defibrillation electrodes. Multiple shocks of varying energy were applied in balanced random order to construct curves of percent successful defibrillation vs energy (DF curves) on each test day. Dogs were studied on days 1, 11, 18, 25, and 32. On day 11, DF curves were determined before and after infusing saline (n = 6) or amiodarone (n = 6), 10 mg/kg loading and 0.33 mg/kg/min maintenance doses. Dogs administered intravenous amiodarone were continued on oral drug (300 mg twice daily) for the remainder of the study. Data were analyzed by logistic regression and the energy required for 50% (E50) and 80% (E80) successful defibrillation were compared. Differences between controls and animals receiving chronic oral amiodarone were not significant on any day. After acute intravenous infusion, dogs given amiodarone had a 21.7 +/- 12.8% decrease in E50 (p less than 0.01) and a 19.7 +/- 17.8% decrease in E80 (p less than 0.05), while controls had an 11.4 +/- 30.5% increase (p = NS) in E50 and 6.30 +/- 30.5 increase in E80 (p = NS). It is concluded that the energy required for successful defibrillation is decreased by acute intravenous amiodarone, while chronic oral administration has no significant effect.

Antiarrhythmic effect of desethylamiodarone in conscious rats

The effect of desethylamiodarone, a metabolite of amiodarone, was studied in the early phase of arrhythmias induced by coronary artery ligation in conscious rats. Desethylamiodarone pretreatment improved survival without altering the occurrence of different types of arrhythmias during the first 20 min after coronary ligation. It was concluded that desethylamiodarone may contribute to the antiarrhythmic effect seen after chronic amiodarone treatment.

Effect of flecainide acetate on prevention of electrical induction of ventricular tachycardia and occurrence of ischemic ventricular fibrillation during the early postmyocardial infarction period: evaluation in a conscious canine model of sudden death

The antiarrhythmic and antifibrillatory effects of flecainide acetate during the early postinfarction period were evaluated in a conscious canine model of sudden cardiac death. Ventricular tachycardia remained inducible early after infarction in eight of nine dogs receiving an intravenous loading dose of flecainide (2.0 mg/kg body weight) and seven of eight dogs receiving saline vehicle. In both the drug and vehicle groups, there was no significant change in the ventricular refractory period or in the cycle length of the induced ventricular tachycardia. With a maintenance intravenous infusion of flecainide, 1.0 mg/kg per h for 4 hours, the subsequent occurrence of acute posterolateral ischemia resulted in the development of ventricular fibrillation and sudden death in seven of eight flecainide-treated and eight of eight vehicle-treated dogs. Seven additional postinfarction dogs with noninducible tachycardia during pretreatment programmed stimulation, and thereby considered to be at "low risk" for the development of ischemic ventricular fibrillation, were also given flecainide in an intravenous loading and maintenance dosing regimen. The subsequent occurrence of posterolateral ischemia resulted in the development of ventricular fibrillation in three of these seven dogs. These findings suggest that flecainide acetate may not possess pharmacologic properties useful in managing ventricular tachycardia or in preventing ischemic ventricular fibrillation in the presence of recent myocardial damage.

Electropharmacology of amiodarone: absence of relationship to serum, myocardial, and cardiac sarcolemmal membrane drug concentrations

Plasma concentrations are often of major consideration in the evaluation of therapeutic efficacy of cardiovascular drugs. This approach is based on the assumptions that the concentration of the drug in the cardiac muscle is in equilibrium with the plasma drug level and that pharmacologic efficacy is proportional to the myocardial drug concentration. The more pronounced pharmacologic efficacy of amiodarone following chronic administration, despite low plasma drug concentrations, and the lesser effects of the drug after acute intravenous administration, when drug levels are maximum, has not been explained on the basis of the pharmacokinetic behavior of the drug. Data obtained from the transmembrane action potential recordings from rabbit ventricular myocardium were therefore correlated with drug concentrations in the serum, myocardium, and myocardial sarcolemma following acute intravenous administration and after 4 weeks of oral administration of 20 mg/kg/day of amiodarone. Following 15 minutes of acute drug administration, when amiodarone concentrations were maximal in the serum (4.72 +/- 1.23 micrograms/ml), cardiac muscle (34.5 +/- 7.6 micrograms/gm), and sarcolemma (1.94 mg/gm protein), the electrophysiologic changes were insignificant. However, following chronic treatment, when levels of amiodarone were low in the serum (0.05 +/- 0.01 micrograms/ml amiodarone, 0.25 +/- 0.08 micrograms/ml desethylamiodarone), cardiac muscle (1.91 +/- 0.9 micrograms/gm amiodarone, 1.35 +/- 1.33 micrograms/gm desethylamiodarone), and myocardial membranes (0.043 mg/gm protein [amiodarone], 0.097 mg/gm protein [desethylamiodarone], there was a 54.3% increase in action potential duration at 90% repolarization (p less than 0.01) and 65% increase in the effective refractory period (p less than 0.01) of rabbit ventricular myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Amiodarone-induced ultrastructural changes in canine myocardial fibers

Chronic clinical toxicity with amiodarone, a unique antiarrhythmic drug, is associated with intracellular myelinoid inclusion bodies in skin, cornea, lung, liver and lymph nodes. The present study was designed to develop a canine animal model to study amiodarone concentration and onset of formation of myelinoid inclusion bodies in the cardiac tissue. Amiodarone was given intravenously in a single dose (40 mg/kg body weight) or multiple doses (40 mg/kg IV + 10 mg/kg IV X 7 days). Amiodarone concentration in the heart was high after a single dose but electron microscopic examination showed a normal ultrastructure. Numerous myelinoid inclusion bodies were found in the myofibrils of the left atria and right and left ventricles after only 7 days of amiodarone treatment. Myelinoid inclusion bodies were identified in several subcellular locations including intercalated disc but most were in close proximity of the mitochondria, sometimes touching and indenting the mitochondrial membrane. The antiarrhythmic effect of the schedule of intravenous amiodarone for 7 days used in this study was minimal, and this correlated with unexpectedly low myocardial levels of the drug and its metabolite. The results are consistent with our previous data of an antiarrhythmic effect with a single intravenous dose of 40 mg/kg body weight associated with high myocardial levels of amiodarone. We conclude that a single large dose of amiodarone with high tissue level may not cause myelinoid inclusion bodies, but they can be readily identified in all heart chambers after only 1 week of amiodarone treatment. This model would be useful to study amiodarone-induced ultrastructural changes in the heart.

Antiarrhythmic effect of acute and chronic amiodarone treatment in conscious rats

The effect of amiodarone after acute and chronic pretreatment was studied in the early phase of arrhythmias induced by coronary artery ligation in conscious rats. Both acute and chronic amiodarone pretreatments improved survival during the first 20 min after coronary ligation. Only chronic amiodarone pretreatment reduced significantly the incidence of ventricular fibrillation. It was concluded that chronic amiodarone pretreatment seemed to be more effective than the acute one.

Effect of amiodarone on ventricular fibrillation and defibrillation thresholds in the canine heart under normal and ischemic conditions

The main goal of this project was to study the effect of amiodarone upon ventricular fibrillation and defibrillation thresholds (VFT and VDT) in the canine heart under normal and ischemic conditions. Both parameters were assessed in 11 dogs, each experiment consisting of three consecutive phases: (a) control, which resulted in VFT = 27.9 (S.D. = 15.5) and VDT = 43.9 (S.D. = 4.5); (b) drug, with VFT = 63.5 (S.D. = 32.8) and VDT = 57.8 (S.D. = 11.3), and (c) coronary occlusion, with VFT = 44.7 (S.D. = 21.6) and VDT = 57.1 (S.D. = 11.0). These values are overall means scaled to ventricular weight (microA/g for VFT and mA/g for VDT). Both VFT and VDT in (b) and (c) were, on the average, greater than in (a) and these differences were statistically significant (P less than 0.01, paired t-test). The animals were kept normothermic (37.3 degrees C, S.D. = 0.6) within normal values for the acid-base state. We concluded that amiodarone increased the VFT significantly. Similarly, it increased VDT. However, although the latter change was statistically significant, we believe it would not be important from a physiological or clinical point of view.

[Amiodarone therapy--behavior of serum and fatty tissue concentrations]

Thirty-eight patients with refractory supraventricular and ventricular tachyarrhythmias were administered a mean oral dosage of 400 mg amiodarone daily (200-600 mg). A high-pressure liquid chromatography method was used to measure serum concentrations of amiodarone and its metabolite desethylamiodarone after one week, one month, three months, and then at 6-month intervals. In 24 patients subcutaneous fatty tissue concentrations were also measured. The mean follow-up was 9 months (4 days to 29 months). A linear correlation was found between amiodarone and its metabolite in serum (r = 0.56, p less than 0.001) as well as in subcutaneous fatty tissue (r = 0.67, p less than 0.001). While serum concentrations were dose dependent, tissue concentrations accumulated during chronic therapy (p less than 0.01, both). Clinical efficacy was achieved in 84% of the patients. No statistically significant difference was found between responders and non-responders as regards serum and subcutaneous fatty tissue concentrations. Side effects of amiodarone occurred in 63%. The incidence of adverse effects was related to significantly higher serum and subcutaneous fatty tissue concentrations of amiodarone and its metabolite (p less than 0.001, both). Thus, although the determination of serum and subcutaneous fatty tissue concentrations does not seem to be helpful for assessing clinical efficacy of this antiarrhythmic drug, these values may predict the occurrence of adverse effects.

Antiarrhythmic and electrophysiologic actions of clofilium in experimental canine models

Clofilium was studied in three experimental models. In non-ischemic and chronically infarcted canine hearts, clofilium (0.5-2 mg/kg) produced a dose-dependent increase in electrical ventricular fibrillation threshold (VFT), but prolonged the effective refractory period (ERP) of normal myocardium in only the non-ischemic heart. When chronically infarcted hearts were subjected to programmed electrical stimulation, 1 mg/kg of clofilium inhibited the re-induction of either ventricular tachycardia or ventricular fibrillation in 5 of 6 animals and slowed the rate of the induced tachycardia in the sixth. Clofilium, however, failed to alter ventricular refractory periods of normal myocardium at either twice diastolic threshold current (176 +/- 5 ms control vs. 187 +/- 9 ms post-clofilium, P greater than 0.05) or at 10 mA (134 +/- 6 ms control vs. 137 +/- 13 ms post-clofilium, P greater than 0.05). In addition, chronic administration of clofilium (2 mg/kg, i.v., followed by 1 mg/kg every 12 h) was ineffective in decreasing mortality in a canine model of sudden coronary death. Of 10 saline-treated conscious animals subjected to an electrically-induced intimal lesion of the left circumflex coronary artery in the presence of a previous ischemic insult, all 10 died suddenly of ventricular fibrillation within 173 +/- 45 min after current application. Under similar conditions, 7 clofilium-treated animals died suddenly within 249 +/- 88 min (P greater than 0.05) after current application while 3 animals survived (P greater than 0.10). Clofilium did, however, elevate the effective refractory period in these animals (150 +/- 3 ms saline-treated vs. 195 +/- 7 ms clofilium-treated). It is concluded from our data that there is little relationship between clofilium's electrophysiologic actions in normal myocardium and antiarrhythmic effects. Furthermore, simple prolongation of refractoriness in normal non-ischemic myocardium may be insufficient for the prevention of ventricular fibrillation which develops in response to a transient ischemic event superimposed on a chronically injured myocardium.

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)