Class 3 antiarrhythmic action in man. Observations from monophasic action potential recordings and amiodarone treatment

An overview of QT interval assessment in safety pharmacology

Medicinal products that prolong cardiac repolarization, as assessed in terms of prolongation of the QT interval of the electrocardiogram, may trigger torsade de pointe, a potentially fatal arrhythmia. The lethality of this risk necessitates a detailed preclinical evaluation before initiating clinical trials. The strategy for assessing the potential of new chemical entities to cause QT interval prolongation involves two complementary approaches. An in vivo test provides information on the potential of the agent to prolong the QT interval under near-physiological conditions. The results are mostly descriptive, providing little insight into the mechanisms of action. In vitro experiments provide more mechanistic data, although the test procedure is far removed from the clinical situation. While both approaches have reasonable predictive value, the results may depend largely on the experimental conditions employed. Discussed in this unit are experimental issues that should be considered when testing agents for their potential to cause arrhythmias, as well as general strategies for understanding the problems associated with this cardiovascular risk.

Effect of amiodarone in the Wolff-Parkinson-White syndrome. A clinical and electrophysiological study

Six patients with Wolff-Parkinson-White (WPW) syndrome were given long-term treatment with amiodarone. Symptomatic relief was obtained in all. Tolerance to the drug was good. Reversible corneal changes appeared after some weeks' treatment in five patients. No thyroid side-effects were noticed. Prior to treatment, dual atrioventricular (AV) conduction was demonstrated on His bundle electrograms in all six patients. Recordings were made at varied heart rates, using atrial and ventricular pacing. Reciprocating tachycardia was readily provoked by properly timed extra stimuli in all patients. When amiodarone treatment had become clinically effective, a second comparative study was made in four patients after 26--85 days' treatment. Amiodarone reduced heart rate and second degree AV block appeared at a lower atrial pacing rate. It increased the refractory periods of right atrium, AV node, and the accessory pathway in proportion to the duration of treatment. Induction of tachycardia was effectively prevented by the drug. It appears that amiodarone in chronic treatment has a predictable and unique depressant action on cardiac conduction, supporting the opinion that this compound, despite side-effects, has an important role to play in the treatment of refractory arrhythmias in patients with the WPW syndrome.

Amiodarone in atrial fibrillation

Twenty-seven patients with atrial fibrillation without any concomitant conduction abnormality have been treated with oral amiodarone in a daily maintenance dose of 200 mg. The drug has been used for three purposes: 1) to block atrioventricular conduction, thereby decreasing the ventricular rate during atrial fibrillation (9 patients), 2) as prophylaxis against paroxysmal atrial fibrillation (8 patients), 3) as prophylaxis against recurrence of atrial fibrillation after DC conversion to sinus rhythm (13 patients). All patients were considered refractory to other antiarrhythmic drugs in these respects. In the second group, 4 of the 8 patients reported complete cessation of attacks and the others a marked reduction of the attack rate. In the third group, 10 of the 13 patients have maintained sinus rhythm for a longer period on treatment with amiodarone than with other drugs, resulting more than a triple prolongation of the time in sinus rhythm. In 3 patients the drug has been discontinued because of side-effects. In conclusion, amiodarone affords protection from episodes of paroxysmal atrial fibrillation, as well as from recurrence of atrial fibrillation after DC conversion to sinus rhythm. If the drug is ineffective in either of these respects, it may still be useful as a means of moderating the ventricular response in atrial fibrillation.

Antiarrhythmic efficacy of amiodarone in recurrent ventricular tachycardia evaluated by multiple electrophysiological and ambulatory ECG recordings

Thirty-three patients with recurrent drug-refractory ventricular tachycardia were treated with oral amiodarone during an average period of 6.1 months. In-hospital monitoring for two weeks or more, electrophysiological tests and ambulatory ECG were used to evaluate the results. Twenty patients are still using the drug with complete control of the arrhythmia. Eleven have failed the drug, ten due to recurrence of documented ventricular tachycardia. Only three patients failed after the first month of therapy. Two patients died, one suddenly. The drug was discontinued in a further two patients due to side-effects. Other side-effects were tolerable or manageable by dose adjustments alone. Five patients showed evidence of inadequate arrhythmia control between days 15 and 32 of therapy but subsequently responded to the drug for 4-9 months, giving further support to the concept that in some patients at least 30 days of therapy is necessary for the full effect of the drug to appear. In 16 of the 20 patients tested by arrhythmia induction study while on the drug, ventricular tachycardia could still be induced. Seven (44%) of these eventually failed the drug. Arrhythmia recurred in one of those four in whom tachycardia could not be induced. Amiodarone is a valuable drug in the management of recurrent ventricular tachycardia, refractory to other antiarrhythmic drugs.

An overview of QT interval assessment in safety pharmacology

Medicinal products that prolong cardiac repolarization unintentionally, as assessed in terms of prolongation of the QT interval of the electrocardiogram, may trigger a potentially fatal arrhythmia called torsade de pointe (TDP). This lethal risk necessitates a detailed preclinical evaluation before initiating clinical trials. There are two different and complementary approaches to assess the potential of drugs to cause QT interval prolongation. The in vivo approach provides information on the potential of the compound to prolong the QT interval under near-physiological conditions. It is mostly descriptive and not explanatory in terms of mechanisms of action. The in vitro approach provides much more mechanistic information, but is far removed from the clinical situation. While both approaches appear to possess reasonable predictive value, the results may depend largely on the experimental conditions employed. This unit reviews these issues and discusses a strategy aimed at understanding the problems associated with this cardiovascular risk.

Electrophysiological characteristics of the human atria after cardioversion of persistent atrial fibrillation

BACKGROUND

In animal models, induced atrial fibrillation shortens the atrial effective refractory period (ERP) and reverses its physiological adaptation to rate. It is not clear whether this process, known as "electrical remodeling," occurs in humans.

METHODS AND RESULTS

We determined the ERPs, at 5 pacing cycle lengths (300 to 700 ms) and in 5 right atrial sites, after internal cardioversion of chronic atrial fibrillation in 25 patients (14 in pharmacological washout and 11 on amiodarone). The ERPs were 195.5+/-18.8 ms in the washout and 206.3+/-17.9 ms in the amiodarone patients (P<0.0001). ERPs were closely correlated with the stimulation rates (r=0.95 in the washout and r=0.94 in the amiodarone group), and slope values indicating a normal (>/=0.07) or nearly normal (0.05 to 0.06) adaptation of ERP to rate were found in 77% of the 84 paced sites. The mean ERP was shorter in the lateral wall (198.1+/-17.9 ms) than in the atrial roof (203.3+/-21.5 ms) and in the septum (210.5+/-20.0 ms) (P<0.03). After 4 weeks of sinus rhythm, the mean ERP, determined again in 8 patients (4 in wash-out and 4 on amiodarone), was significantly increased compared with the basal study (221. 4+/-21.4 versus 197.8+/-18.3 ms, P<0.0001).

CONCLUSIONS

After cardioversion of chronic atrial fibrillation, (1) atrial ERP adaptation to rate was normal or nearly normal in the majority of the cases, (2) a significant dispersion of refractoriness between different right atrial sites was present, and (3) ERPs were significantly increased after 4 weeks of sinus rhythm in both washout and amiodarone patients.

Spontaneous conversion and maintenance of sinus rhythm by amiodarone in patients with heart failure and atrial fibrillation: observations from the veterans affairs congestive heart failure survival trial of antiarrhythmic therapy (CHF-STAT). The Department of Veterans Affairs CHF-STAT Investigators

BACKGROUND

In a multicenter, double-blind, placebo-controlled study, the long-term effects of amiodarone on morbidity and mortality in patients with congestive heart failure (CHF) and atrial fibrillation (AF) were evaluated during a 4-year period.

METHODS AND RESULTS

Of 667 patients with CHF, 103 (15%) had AF at baseline. Of these, 51 were randomized to amiodarone and 52 to placebo. The group with sinus rhythm and the group in AF were comparable except for a higher proportion of AF in patients with nonischemic versus ischemic cardiomyopathy (41% versus 27%, P<0.005). The mean ventricular response (VR) during AF over 24 hours was reduced by amiodarone at 2 weeks (20%, P=0.001), at 6 months (18%, P=0.001), and at 12 months (16%, P=0.006). Maximal VR was reduced 22% (P=0.001) at 2 weeks, 19% (P=0.001) at 6 months, and 14% (P=0.001) at 12 months. Sixteen of 51 patients on amiodarone and 4 of 52 on placebo converted to sinus rhythm during the study (chi2=9.23, P=0.002). During follow-up, 11 of 268 patients in sinus rhythm on amiodarone at baseline and 22 of the 263 in sinus rhythm on placebo developed AF; the difference was significant (chi2=12.88, P=0.005). Analysis of total mortality during follow-up showed a significantly lower mortality rate (P=0. 04) in patients in AF at baseline who subsequently converted to sinus rhythm on amiodarone than in those who did not convert to sinus rhythm on the drug.

CONCLUSIONS

In patients with CHF, amiodarone has a significant potential to spontaneously convert patients in AF to sinus rhythm, with patients who convert having a lower mortality rate than those who do not. The drug prevented the development of new-onset AF and significantly reduced the VR in those with persistent AF.

Amiodarone: the expanding antiarrhythmic role and how to follow a patient on chronic therapy

Amiodarone was introduced as an antiarrhythmic compound in the early 1970s and was approved in the U.S. for the treatment of refractory ventricular arrhythmias in late 1984. Since that time the drug has become the most widely studied antiarrhythmic compound with expanding potential indications, including maintaining stability of sinus rhythm, secondary prevention in the survivors of myocardial infarction, and prolongation of survival in certain subsets of patients with congestive heart failure. Intravenous amiodarone was introduced in the U.S. in 1995 for the control of recurrent destabilizing ventricular tachycardia or ventricular fibrillation resistant to conventional therapy. The level of comfort in its use has risen considerably in the recent past. This has stemmed from the reasonably decisive evidence that class I agents increase mortality in patients with structural heart disease. In contrast, amiodarone either reduces mortality or its effect is neutral; this is consistent with its low to negligible proarrhythmic actions. The drug does not aggravate heart failure and it may even increase left ventricular ejection fraction and improve exercise capacity. Above all, it is becoming increasingly evident from wider experience and from controlled clinical trials that the side-effect profile of the drug is not as compelling an issue as it appeared to be when first used in much higher doses. Therefore, the overall objective of amiodarone therapy is to use the lowest dose that produces a defined therapeutic end point without causing serious side effects. Careful clinical surveillance in conjunction with monitoring of certain laboratory parameters and indices of efficacy at regular intervals permits the drug to be used effectively in a large number of patients who fail to respond to, or are intolerant of other antiarrhythmic compounds. Many experienced clinicians have begun to consider the use of amiodarone as first-line therapy in certain disorders of rhythm, especially in patients with severely compromised ventricular function.

Controlling Paroxysmal Atrial Fibrillation by a Combination of Amiodarone and Flecainide: Description of a Case With 15-Year Follow-up

A 77-year-old man with no known cardiac disease has had paroxysmal atrial fibrillation for 35 years with disabling symptoms and poor exercise tolerance when not in sinus rhythm, and he did not respond to conventional therapy. Fifteen years ago he was placed on amiodarone. His arrhythmia converted to atrial flutter with a flutter rate below 200 beats/min; DC cardioversion at 3 months led to transient sinus rhythm. At 5 months he converted spontaneously to sinus rhythm and had very few recurrences until 4 years later when he began to experience further frequent recurrences when the dose was reduced from 400 mg/day to 200 mg/day. Redosing at the higher dose led to skin discoloration; amiodarone was then replaced with sotalol, which the patient did not tolerate. After 9 months with efforts to rate control with various agents, amiodarone was reintroduced at 400 mg/day, which achieved full control, but to obviate the development of skin changes, flecainide was added at a dose of 100 mg twice a day, and the dose of amiodarone was gradually reduced to 200 mg/day. This combination regimen has produced no side effects or organ toxicity, although a degree of hypogonadism developed. It responded well to testosterone replacement. On the combination regimen, there have been no symptomatic arrhythmia recurrences over 8 years. Amiodarone and flecainide may have additive or synergistic effects in maintaining sinus rhythm in atrial fibrillation; the antiarrhythmic property of amiodarone is likely to minimize or nullify the proarryhthmic reactions of flecainide during combination therapy. This combination regimen may allow the extension of the use of flecainide in controlling refractory atrial flutter and fibrillation in patients with structural cardiac disease. The efficacy and safety of the combination regimen of the two drugs should be addressed in controlled clinical trials.

Antiarrhythmic actions of amiodarone: a profile of a paradoxical agent

Amiodarone, a complex compound with variegated electropharmacologic and pharmacokinetic properties and an equally complex side-effect profile, continues to have a critical role in the control of ventricular and supraventricular tachyarrhythmias as the use of class I agents has declined. Such is also the case with sotalol. Unlike other so-called class III agents, amiodarone non-competitively blocks sympathetic stimulation, and its effects on repolarization are not associated with reverse use dependency. Rarely does it produce torsades de pointes despite its propensity to induce significant bradycardia and marked prolongation of the QT interval. During long-term therapy with the drug, there is no impairment of ventricular function; in fact, there are significant increases in the left ventricular ejection fraction during protracted amiodarone therapy in patients with heart failure. Long-term amiodarone administration consistently demonstrates marked efficacy in a wide spectrum of arrhythmias. The major limitation of amiodarone during long-term therapy is its unusual side-effect profile, although the increasing trend for low-dose drug therapy has demonstrated a major decline in the overall incidence of serious adverse reactions. Amiodarone is effective in controlling symptomatic ventricular tachycardia and fibrillation (VT/VF) in > 60-70% of patients when conventional agents (especially class I) are ineffective or not well tolerated. The efficacy of amiodarone compared with that of an implantable cardioverter-defibrillator in patients with VT/VF and in survivors of cardiac arrest remains uncertain when total mortality is used as the primary endpoint of comparison. Amiodarone suppresses ventricular ectopy and markedly suppresses nonsustained VT. It prevents inducible VT/VF in a small number of patients, but slows VT rate in a larger number. The role of the drug in prolonging survival in the postmyocardial infarction patient is unclear, although preliminary data from blinded studies suggest that the drug decreases arrhythmia-related mortality. Similarly, in heart failure, amiodarone has the potential to reduce total mortality but appears to be selectively effective in nonischemic rather than in ischemic cardiomyopathy. Intravenous amiodarone was recently introduced in the United States for the control of recurrent destabilizing VT or VF resistant to conventional therapy. There is also evolving data indicating that the drug might be the most potent agent in maintaining sinus rhythm in patients with atrial fibrillation or flutter converted chemically or electrically to sinus rhythm. However, blinded controlled comparative studies involving sotalol, quinidine, or pure class III drugs have not been carried out. The available data nevertheless suggest that, barring its side-effect profile, amiodarone is a desirable prototype of a broad-spectrum antifibrillatory and antiarrhythmic compound.

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.

Monophasic action potentials: concepts to practical applications

Monophasic action potential (MAP) recordings reproduce the repolarization time course of intracellular action potentials with high accuracy and provide precise information on the local activation time. With the advantage of in vivo application and the development of the safer and simpler contact catheter technique, MAP recording has become the method of choice for evaluating myocardial repolarization changes. This review aims to provide information on practical application of MAP recording in the clinical setting. MAPs can easily be recorded from the endocardium with the contact catheter technique in the electrophysiology laboratory and from the epicardium with electrode probes during open heart surgery. The technical aspects are described in detail. The rate dependence of myocardial excitability and repolarization and the effect of antiarrhythmic drugs on MAP duration and effective refractory period are thoroughly reviewed. The use of MAPs in detecting myocardial ischemia, in studying early afterdepolarization and triggered arrhythmias, in measuring dispersion of repolarization, in identifying intracardiac conduction and the development of the T wave, and in verifying the arrhythmogenic effect of mechanoelectric feedback are presented. Computerized automatic analysis of MAPs and the limitations of the MAP technique are also discussed.

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 and post-MI patients

Amiodarone is a viable drug for preventing sudden cardiac death, particularly during the first year after MI. If larger trials confirm the aforementioned prospective trials of Ceremuzynski et al, Cairns et al, and the BASIS trial, the efficacy of amiodarone would outweigh the risk of its side effects during the first year after MI. Based on the long-term observation from the BASIS trial, the duration of amiodarone therapy need not be more than 1 year--which, as we have learned, is when these post-MI patients would benefit most from the drug. It is also likely that the effects of amiodarone would complement those of aspirin and angiotensin converting enzyme inhibitors. The SAVE, CONSENSUS II, and SOLVD trials demonstrated that captopril and enalapril did not reduce the mortality rate during the first year after MI, nor did they reduce the sudden cardiac death rate. Their beneficial effects became evident only during the second year and thereafter. Unlike other antiarrhythmic agents of various classes, amiodarone possesses antiarrhythmic properties but does not exert deleterious effects on ventricular function. More studies are needed to determine if the benefit of amiodarone could be enhanced by combination therapy (eg, angiotensin converting enzyme inhibitors, aspirin, or beta-blockers). Whether amiodarone will provide the same protection for patients who have poor left ventricular function or congestive heart failure is not known. The European and VA cooperative studies should help answer this question. If it turns out that amiodarone is beneficial, one must then determine whether higher doses of the drug will offer more protection, and, if so, if that greater protection would be offset by increased toxicity. How much amiodarone should be given to offer the most protection with the least risk? Another intriguing research question is this: If we treat patients with amiodarone for more than 1 year, would the drug continue to improve the mortality rate in subsequent years? Other studies are needed in patients at very high risk of sudden cardiac death (ie, those who have a low ejection fraction and high-density VPDs). A study comparing amiodarone and sotalol in high-risk patients for sudden cardiac death is also needed. These clinical studies should be carried out with basic science research investigating the actions of amiodarone at the molecular and cellular level in order to give us a better understanding of how the drug works.

Efficacy and safety of short intravenous amiodarone in supraventricular tachyarrhythmias

The safety and efficacy of short intravenous therapy with amiodarone were evaluated in 44 patients (24 males, 20 females), aged 21-84 years, with supraventricular tachyarrhythmias newly arisen in less than 24 hours. The study group consisted of 15 patients with paroxysmal supraventricular tachycardia, 8 patients with atrial flutter and 21 patients with atrial fibrillation. They were treated with a single infusion of amiodarone up to 2 hours after the restoration of a stable sinus rhythm, or up to a maximum dose of 2400 mg in 24 hours. Our study shows that 88.6% of all supraventricular tachyarrhythmias reverts to sinus rhythm in less than 24 hours: 100% of paroxysmal supraventricular tachycardia, 75% of atrial flutter, and 85.7% of atrial fibrillation. Intravenously administered amiodarone proves to take effect rapidly (0.5 to 22 hours). The plasma amiodarone concentrations at sinus rhythm restoration showed a wide range (405-3800 ng/ml). Piecewise analysis suggested that the probability of sinus rhythm was 14.4-fold greater in paroxysmal supraventricular tachycardia. No linear statistical relationship was detectable between the log-dose-body mass index and log-QTc. Total amiodarone dose and left atrial volume are inversely correlated with a statistically significant difference. The toxicity in our short intravenous course with amiodarone was not relevant. We conclude that short high-dose intravenous amiodarone shows efficacy and safety in all newly occurring supraventricular tachyarrhythmias.

Monophasic action potential recordings: evaluation of antiarrhythmic drugs

The use of MAP recording techniques has been said to have bridged the gap between basic in vitro investigation of the transmembrane action potential and observations made in situ from the beating heart. With regard to antiarrhythmic agents, MAP recordings are particularly useful in evaluating drugs which prolong repolarization. The simultaneous measurement of MAP and ERP at the same site permits the comparison of drug effects on repolarization and refractoriness. The ability to safely and reliably record the MAP contributes importantly to the evaluation and classification of antiarrhythmic drug effects in vivo and may ultimately lead to more rational selection of drug therapy for individual patients. Antiarrhythmic drug effects demonstrated with MAP recordings have generally shown good agreement with the Vaughan Williams classification of electrophysiological actions. An important key to drug efficacy may be that some drugs prolong refractoriness beyond their effect on repolarization. Conversely, a potential explanation for proarrhythmia may lie in slowing of conduction without the concomitant protective effect of postrepolarization refractoriness. The phenomenon of use dependence, which has been demonstrated for many drugs, suggests why an agent that prevents induction of arrhythmia during programmed stimulation in the electrophysiology laboratory may not prevent spontaneous arrhythmia initiation at slower heart rates. The paramount task of clinical electrophysiology is the successful treatment of rhythm disturbances. The more detailed and quantitative evaluation of drug effects afforded by MAP recordings may ultimately result in the more effective use of antiarrhythmic drugs in general and to more precise tailoring of therapy for individual patients.

Method and theory of monophasic action potential recording

MAP recordings have been at the cradle of cardiac electrophysiology but only recently, through safer and simpler technology, have gained wider access to clinical electrophysiology. In contrast to conventional electrode catheter recordings, MAP recording devices provide precise information not only of the local activation time but of the entire local repolarization time course as well. Although the MAP does not reflect the absolute amplitude or upstroke velocity of transmembrane action potentials, it delivers highly accurate information on the action potential duration and configuration, including early afterdepolarizations as well as relative changes in transmembrane diastolic and systolic potential changes. Based on available data, the MAP probably reflects the transmembrane voltage of cells within a few millimeters of the exploring electrode. MAPs can be recorded by catheter technique from the endocardial surface and by special probes from the epicardium in the operating room. The contact electrode technique is preferable over suction electrodes because it is safer and simpler to use in patients and because it produces more stable, longer-lasting signals. A modified contact MAP catheter incorporates pacing electrodes and permits simultaneous assessment of action potential duration and refractoriness. This not only facilitates the use of MAP catheters in routine electrophysiological studies but also is important for assessing the voltage-independent effects of antiarrhythmic drugs on refractoriness. MAP recordings offer the opportunity to study, in the in situ heart, a variety of pertinent electrophysiological phenomena including, for example, effects of cycle length changes and antiarrhythmic drugs on action potential duration or the role of afterdepolarizations in the genesis of triggered arrhythmias. Due to vigorous heart beating, movement artifacts may occur and need to be distinguished from true abnormalities in the action potential time course. With these limitations in mind, MAP recordings are a valuable addition to clinical electrophysiological studies.

Electropharmacology of amiodarone therapy initiation. Time courses of onset of electrophysiologic and antiarrhythmic effects

The time courses of onset of the electrophysiologic and antiarrhythmic effects of amiodarone were determined with serial electrophysiologic studies in 34 patients with inducible ventricular tachycardia. A standardized oral loading dosage was used for all patients (1,200 mg/day for 14 days; 800 mg/day for 7 days; and 400 mg/day thereafter). Eleven patients had the studies performed at baseline and after 2, 6, 10, and 20 weeks. Subsequently, 23 patients had studies at baseline and after 2 and 10 weeks. Changes in atrial, sinus, and atrioventricular nodal properties and in conduction intervals were maximal within 2 weeks (early effects). For example, atrioventricular nodal Wenckebach cycle length increased between baseline (369 +/- 80 msec) and 2 weeks (498 +/- 78 msec) (p less than 0.001) but did not change further after 10 weeks (500 +/- 89 msec). However, ventricular Class III effects required 10 weeks to become maximal (late effects). For example, the QT interval during atrial pacing increased between baseline (355 +/- 36 msec) and 2 weeks (406 +/- 37 msec) (p less than 0.001) and increased further after 10 weeks (436 +/- 45 msec) (p less than 0.001). Antiarrhythmic effects also followed different time courses of onset. Suppression of ventricular premature beats was maximal within 2 weeks. However, suppression of ventricular tachycardia inducibility and slowing of ventricular tachycardia rate was not maximal for 10 weeks. Correlations between serum desethylamiodarone concentrations and some late effects suggest that the mechanism of the time delay to maximal ventricular Class III effects may involve desethylamiodarone.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Inhibitory actions of amiodarone on the isolated rabbit heart and aorta

1. The inhibitory actions of amiodarone on the isolated rabbit heart and aorta have been studied. 2. Amiodarone inhibited vasopressin- and ergonovine-induced coronary spasm, starting from a concentration of 10(-7) M which did not affect myocardial contractility to 10(-4) M, which decreased myocardial contractility. 3. Sinus node activity was largely unaffected even when the highest dose of 10(-4) M was used. 4. Amiodarone did not modify the smooth muscle contraction in rabbit aorta strips precontracted with noradrenaline or potassium. 5. Comparison with other inhibitors of the cardiovascular system (alpha- and beta-blockers, nitrates, calcium entry blockers) points out a peculiar pharmacological profile of amiodarone and indicates some doubts about its presumed anti-adrenergic properties.

Mechanisms of depressed conduction from long-term amiodarone therapy in canine myocardium

Amiodarone therapy leads to a significant impairment in myocardial conduction, yet it causes only a modest decrease in the maximum rate of depolarization of the action potential (dV/dT). To determine whether the decrease in dV/dT solely accounts for the impaired myocardial conduction or whether passive membrane properties may also be involved, we studied 21 ventricular epicardial tissues from 14 beagles; six dogs received long-term treatment (3-6 weeks) of amiodarone orally, and the remaining dogs served as controls. Amiodarone therapy was associated with a decrease in conduction velocity (0.41 +/- 0.15 vs. 0.56 +/- 0.05 m/sec; p less than 0.01). There was a trend toward a decrease in dV/dT and a significant decrease in the space constant (0.69 +/- 0.27 vs. 1.05 +/- 0.25 mm; p = 0.01), of which the latter correlated closely with the decrease in conduction velocity measured in the amiodarone-treated tissues (r = 0.85, p less than 0.05). These data indicate that the decrease in myocardial conduction velocity caused by amiodarone is primarily due to effects on overall resistance to passive current flow rather than effects on the inward sodium current.

New antiarrhythmic drugs

While controversy still exists as to the precise indications for the treatment of all forms of ventricular arrhythmia, advances in the number and, more importantly, type of antiarrhythmic drugs can provide the clinician with a rational basis for selecting antiarrhythmic drug therapy. A host of new agents with different pharmacokinetic and electrophysiological actions are now available, and can be compared or contrasted to conventional antiarrhythmic agents such as quinidine, procainamide, disopyramide, lignocaine (lidocaine) and bretylium. This review summarises the electrophysiological, haemodynamic, pharmacokinetic, and efficacy and safety data of mexiletine, tocainide, flecainide, encainide, propafenone, amiodarone, sotalol, pirmenol, cibenzoline (cifenline) and ethmozine (moracizine, moricizine), and aims to provide a basis on which clinicians can compare and contrast these agents and form an algorithm for selection of antiarrhythmic drug therapy in the treatment of patients with ventricular arrhythmias.

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 predictors of long-term clinical outcome with amiodarone for refractory ventricular tachycardia secondary to coronary artery disease

Fifty-four patients with a previous myocardial infarction and drug-refractory symptomatic ventricular tachycardia (VT) were treated with amiodarone on a long-term basis (range 6 to 54 months, mean 26) irrespective of the results of programmed ventricular stimulation, which was performed after high-dose oral amiodarone loading for more than 4 weeks. VT was rendered noninducible in 6 of 54 patients (11%) taking oral amiodarone. During a mean follow-up of 32 months, these 6 patients remained free of VT or sudden cardiac death. Forty-eight patients (89%) continued to have VT inducible by programmed ventricular stimulation. However, they could be separated into 2 groups: VT-modified (20 patients) and VT-unchanged (28 patients). In the VT-modified group, the induced VT with amiodarone was slowed or rendered nonsustained, and only 3 of 20 (15%) patients during a mean follow-up of 23 months had well tolerated VT recurrences. In the VT-unchanged group, 16 of 28 patients (57%) had recurrences of VT or ventricular fibrillation during a mean follow-up of 24 months. Sudden cardiac death occurred in 6 of these 16 patients. Thus, programmed ventricular stimulation in patients with VT taking long-term amiodarone may have prognostic implications.

Anaesthesia and amiodarone

Two recent reports support and one report disputes the existence of dangerous interactions between the new benzofuran antiarrhythmic amiodarone and the anaesthetic state. We have reviewed our experience with 17 anaesthetics administered to 16 patients taking amiodarone. Haemodynamics and serum amiodarone levels were evaluated where available. Twelve cases involved cardio-pulmonary bypass; of these, three patients died. There were no deaths in the non-cardio-pulmonary bypass group. The charts of 30 patients with poor left ventricular function, who were not receiving amiodarone but who were undergoing coronary artery bypass surgery, were reviewed to establish a comparison group. Interactions were manifested in three forms: nodal rhythm and/or complete heart block developed in ten of 15 patients (one patient had a preoperative pacemaker inserted for the sick sinus syndrome), poor cardiac output requiring intra-aortic balloon pump augmentation developed in six of 12 cardio-pulmonary bypass patients, or, a state of alpha adrenergic blockade leading to a low systemic vascular resistance despite alpha agonist therapy developed in two of 16 patients. We conclude that dangerous, fatal interactions may occur in patients taking amiodarone who undergo general anaesthesia with cardio-pulmonary bypass. Anaesthesia for non-cardiac surgery may be associated with haemodynamically significant bradyarrhythmias. We recommend aggressive invasive monitoring, including pulmonary artery catheterization and consideration of an atrio-ventricular pacemaker in high risk patients.

Simultaneous recording of atrial and ventricular monophasic action potentials: monophasic action potential duration during atrial pacing, ventricular pacing, and ventricular fibrillation

A newly developed transvenous suction electrode was used in dogs to record monophasic action potentials (MAPs) from the right atrium and right ventricle simultaneously. Continuous MAP recordings could be made from the same endocardial site for test periods of 1.5 hours. Left ventricular pacing at increasing heart rates resulted in a statistically significant decrease of right ventricular MAP duration. A high degree of correlation was found between right ventricular MAP duration at 90% of repolarization and the QT interval during both right atrial and left ventricular pacing. At the onset of ventricular fibrillation (VF), right ventricular MAP duration shortened to 25% of the value obtained during left ventricular pacing at a cycle length of 250 ms. A cyclic alternation in amplitude of the right ventricular MAPs was observed during VF. Fast Fourier Transform Analysis of right ventricular MAPs during VF showed a significant dominant frequency at 12 Hz, with no levels of interest beyond this frequency. This observation might prove to be useful in elaborating a new algorithm for the automatic detection of ventricular fibrillation.

Right ventricular monophasic action potentials in healthy young men

The right ventricular repolarization phase was studied in 48 healthy men between 20 and 40 years of age. The assessment of the repolarization time included the measurement of ventricular effective refractory periods and monophasic action potentials during constant ventricular stimulation. Computer-based analysis of the monophasic action potential allowed the duration at 90% and 50% repolarization, the amplitude, the maximal upstroke velocity and the total rise time of the depolarization to be determined. These results may serve as reference values in further studies on ventricular repolarization using the same monophasic action potential recording technique.

Amiodarone: electrophysiologic actions, pharmacokinetics and clinical effects

Interest in amiodarone has increased because of its remarkable efficacy as an antiarrhythmic agent. The purpose of this report is to review what is known about the electrophysiologic actions, hemodynamic effects, pharmacokinetics, alterations of thyroid function, response to treatment of supraventricular and ventricular tachyarrhythmias and adverse effects of amiodarone. Understanding the actions of amiodarone and its metabolism will provide more intelligent use of the drug and minimize the development of side effects. The mechanism by which amiodarone suppresses cardiac arrhythmias is not known and may relate to prolongation of refractoriness in all cardiac tissues, suppression of automaticity in some fibers, minimal slowing of conduction in fast channel-dependent tissue, or to interactions with the autonomic nervous system, alterations in thyroid metabolism or other factors. Amiodarone exerts definite but fairly minor negative inotropic effects that may be offset by its vasodilator actions. Amiodarone has a reduced clearance rate, large volume of distribution, low bioavailability and a long half-life that may last 2 months in patients receiving short-term therapy. Therapeutic serum concentrations range between 1.0 and 3.5 micrograms/ml. The drug suppresses recurrences of cardiac tachyarrhythmias in a high percent of patients, in the range of 80% or more for most supraventricular tachycardias and in about 66% of patients with ventricular tachyarrhythmias, sometimes requiring addition of a second antiarrhythmic agent. Side effects, particularly when high doses are used, may limit amiodarone's usefulness and include skin, corneal, thyroid, pulmonary, neurologic, gastrointestinal and hepatic dysfunction. Aggravation of cardiac arrhythmias occurs but serious arrhythmias are caused in less than 5% of patients. Amiodarone affects the metabolism of many other drugs and care must be used to reduce doses of agents combined with amiodarone.

Prognosis after sudden cardiac death without associated myocardial infarction: one year follow-up of empiric therapy with amiodarone

Thirty-three consecutively referred patients with cardiac arrest from ventricular arrhythmias unassociated with a new acute myocardial infarction (AMI) were commenced on amiodarone therapy and followed for a minimum of 12 months. The dose of amiodarone was adjusted to maximum tolerance and not according to the incidence of asymptomatic ventricular premature complex activity. Eight patients died including five sudden deaths. Five out of the eight deaths occurred either within 3 months of therapy or when the dose of amiodarone was less than 400 mg/day. The majority of patients were found to have corneal microdeposits or either thyroid or liver function abnormalities, although none had any clinical manifestation. Ten patients had neurologic side effects. In summary, although the overall cardiac mortality seemed to be reduced by amiodarone therapy and the drug appears to be well tolerated by patients, its role in the prophylaxis against recurrent ventricular fibrillation may be enhanced by a regimen of higher loading and maintenance doses.

The clinical results of amiodarone in cardiac arrhythmias: optimal dosing

Amiodarone hydrochloride is a relatively new antiarrhythmic agent, the properties of which differ in a significant manner electrophysiologically, pharmacokinetically and structurally from those of conventional as well as other investigational antidysrhythmic compounds. It is also pharmacologically unique in so far as its fundamental action on cardiac muscle following chronic therapy differs markedly from that found during the intravenous administration; its I.V. action is dominated by the lengthening of intranodal (AV) conduction time and the effective refractory period of the AV node, the electrophysiologic basis for which is unclear but accounts for the slowing of the ventricular response in atrial flutter and fibrillation and the variable conversion rate of narrow QRS reentrant paroxysmal supraventricular tachycardia. Intravenous amiodarone is ineffective in most other arrhythmias; it does not lengthen repolarization, nor does it prolong the effective refractory period of atria, ventricle, His-Purkinje system or the accessory pathways of the heart in the WPW syndrome. In contrast, chronically administered amiodarone lengthens repolarization and the effective refractory period of all cardiac tissues as a function of dose and duration of therapy consistent with its wide spectrum of antiarrhythmic activity in the prophylactic control of supraventricular and ventricular tachyarrhythmias. The nature of the slow onset of action of the oral drug is not well-understood; it may be due to the slow formation of active metabolites or the gradual and selective inhibition of T3 action on the myocardium since the effects of amiodarone on cardiac repolarization are identical to those of hypothyroidism and are negated by the concomitant administration of thyroxine. Serum reverse T3 levels increase as a function of dose and duration of amiodarone therapy and tentative data indicate that serial measurements of rT3 levels may provide a reliable index for gauging efficacy and toxicity of amiodarone during chronic therapy. The role of serum drug and metabolite levels appears less reliable in this regard. The exceedingly long and variable elimination half-life of amiodarone necessitates individualized loading and maintenance dosage regimens, and the latency of onset of antiarrhythmic action during oral therapy is not shortened by intravenous bolus injections or sustained infusions. However, the judicious choice of oral dosage as discussed herein permits the development of an effective prophylactic regimen for most patients with supraventricular and ventricular tachyarrhythmias; when the lowest dosage regimen to control a particular arrhythmia is identified, limiting side

The effect on the rat isolated atria of amiodarone in the presence of either ouabain or verapamil

Amiodarone causes a decrease in the rate of contraction of the rat isolated atria and has a negative inotropic action in the paced preparation. Interactions occur between amiodarone and ouabain and amiodarone and verapamil. It is possible that the clinically reported drug interaction with amiodarone may have a component of direct interactions on the myocardium rather than solely changes in plasma protein binding.

Amiodarone: historical development and pharmacologic profile

Although synthesized as a coronary dilator for use as an antianginal agent over 20 years ago, amiodarone hydrochloride has recently drawn much attention as a potent antiarrhythmic compound for the control of a variety of cardiac dysrhythmias. The rapidly expanding clinical and experimental data continue to emphasize the unusual electrophysiologic, pharmacologic, and especially pharmacokinetic properties of this benzofuran derivative. The compound is a potent coronary dilator and has minimal negative inotropic propensity of a direct nature while exhibiting a mild degree of noncompetitive sympathetic antagonism. Pharmacokinetically, it has a long elimination half-life with a correspondingly long and variable latency of onset of therapeutic effect. Electrophysiologically, the drug has the propensity to lengthen the action potential duration and hence the voltage-dependent effective refractory period in all cardiac tissues after long-term, rather than short-term, administration. It has little effect on depolarization, conduction velocity, or the slow response. The precise ionic mechanisms mediating its effects on repolarization are not known. Clinically, the electrophysiologic effects of the drug differ significantly when it is given by mouth over a longer period and when it is given intravenously, a difference that remains to be explained in terms of mechanism. These differences, however, account for the varying spectrum of the drug's action after single intravenous doses (when its antiarrhythmic effects are essentially explained by the drug's action on the atrioventricular node and possibly its antiadrenergic actions) in comparison to long-term oral administration, which predictably suppresses ectopic activity and lengthens the effective refractory period in all cardiac tissues. These features may account for the drug's remarkable efficacy in the control of supraventricular and ventricular tachyarrhythmias. The safe and rational therapeutic uses of amiodarone as an antiarrhythmic agent presuppose detailed understanding of its manifold pharmacodynamic and pharmacokinetic properties.

Acute effects of amiodarone upon the canine sinus node and atrioventricular junctional region

Amiodarone was selectively perfused into the sinus node artery and atrioventricular node artery of 51 dogs. Amiodarone had an immediate negative chronotropic and dromotropic effect. Threshold concentration was 2.5 micrograms/ml. 25 and 50 micrograms/ml of amiodarone injected into the sinus node artery slowed the heart by 25.6 +/- 3.1 and 33.7 +/- 2.6 beats/min (mean +/- 1 SEM), respectively. Amiodarone 25 and 50 micrograms/ml injected into the AV node artery during AV junctional rhythm slowed the AV junctional pacemaker by 12.2 +/- 1.8 and 17.4 +/- 1.7 beats/min, respectively. Injections of amiodarone into the AV node artery during sinus rhythm regularly increased AV conduction time sometimes causing 2 degrees AV block at the highest concentration used. Impaired conduction was exclusively measured at the level of the A-H interval in the His electrogram. Neither atropine nor propranolol prevented the negative chronotropic effects of amiodarone. Amiodarone had no significant effect on sinus node response to either stellate stimulation or intranodal administration of norepinephrine. The negative chronotropic action of amiodarone was significantly enhanced when amiodarone was administered in a perfusate containing low (0.6 mM) instead of normal calcium. Taken collectively these observations indicate that amiodarone has immediate depressant electrophysiologic effects on both the sinus node and the AV junction and that these early effects might involve the blockade of the slow channel.

Electrophysiologic effects of chronic amiodarone therapy in patients with ventricular arrhythmias

Detailed electrophysiologic studies were performed in nine patients with chronic refractory ventricular arrhythmias before and after 7 to 20 weeks (mean 11 weeks) of amiodarone therapy. The amiodarone dose at the time of the repeat study ranged from 400 to 800 mg/day. The drug reduced the sinus rate (p less than 0.001) and prolonged the sinoatrial conduction time (p less than 0.05) with some prolongation of the corrected sinus node recovery time. Intra-atrial conduction was slightly prolonged both in sinus rhythm and during atrial pacing. Anterograde conduction through the AV node was significantly prolonged both in sinus rhythm (p = 0.001) and during atrial pacing (p less than 0.005), and Wenckebach AV block was seen at significantly lower atrial pacing rates after the drug (p less than 0.005). The HV interval was prolonged both in sinus rhythm (p less than 0.05) and during atrial pacing (p = 0.001), and so was the QRS width during atrial pacing (p less than 0.005) and the QT interval in sinus rhythm (p less than 0.005) and during atrial pacing (p less than 0.005). Significant prolongation of the refractory periods in the atrium, AV node, and ventricular muscle were also seen following the drug. We concluded that the significant electrophysiologic effects of this drug throughout the heart during chronic oral use attest to its clinical effectiveness in patients with atrial and ventricular arrhythmias. With due care and despite its effects on the HV interval and QRS width, it can be used in patients with intraventricular conduction defects complicating severe organic heart disease.

Prolongation of the human cardiac monophasic action potential by sotalol

Sotalol and propranolol are nonselective beta-adrenergic blocking agents. Sotalol at low concentration, unlike propranolol, prolongs the duration of the transmembrane action potential. In a double-blind study, the electrophysiologic effects of intravenous sotalol (0.30 or 0.60 mg/kg; n = 9) were compared with intravenous propranolol (0.15 or 0.20 mg/kg; n = 8) in 17 patients with use of bipolar suction electrodes in the right atrium and right ventricle to determine whether sotalol prolongs the monophasic action potential duration in man. After administration of sotalol, there were significant increases (paired t test) in the Q-T interval (p less than 0.001), right atrial effective refractory period (p less than 0.05), right ventricular effective refractory period (p less than 0.005), right atrial monophasic action potential duration at 90% repolarization (p less than 0.01), and right ventricular monophasic action potential duration at 90% repolarization (p less than 0.005). Prolongation of the monophasic action potential duration was dependent on plasma sotalol concentration. There were no significant changes in these variables after propranolol. The spontaneous cycle length and Wenckebach cycle length increased significantly in both groups, and the mean blood pressure decreased in both, although not significantly after propranolol. In summary, sotalol but not propranolol prolonged atrial and ventricular effective refractory periods and lengthened the monophasic action potential and the Q-T interval of human myocardium after intravenous infusion. The ability to acutely prolong repolarization at therapeutic plasma concentration is unique among known competitive beta-adrenergic receptor antagonists.

Amiodarone for control of sustained ventricular tachyarrhythmia: clinical and electrophysiologic effects in 51 patients

We evaluated the electrophysiologic effects of amiodarone and its ability to control ventricular arrhythmia in a selected group of 51 patients with refractory sustained ventricular arrhythmia. Amiodarone in doses of 400 to 800 mg/day prolonged refractoriness in the atria, atrioventricular (AV) node, and ventricle as well as conduction through the AV node and His-Purkinje system. Although it had no effect on measurements of sinus nodal function (sinus nodal recovery time and sinoatrial conduction time), it prolonged the sinus cycle length and 2 patients required a permanent pacemaker for symptomatic sinus bradycardia. Amiodarone did not alter the ease of inducibility in any consistent manner, and only 5 of 43 patients (12%) who had inducible ventricular tachycardia before amiodarone therapy had none induced during amiodarone treatment. The clinical effectiveness of amiodarone could be evaluated in 46 patients followed up for 8.6 +/- 6 months (range 0.5 to 22). It provided effective therapy in 23 patients (50%), partly effective therapy in 13 (28%), and was ineffective in 10 (22%). Adverse effects were noted in 28 of 51 patients (55%), and in 11 of these (22%) the drug had to be discontinued because of adverse effects. We conclude that amiodarone is a useful agent for the treatment of refractory sustained ventricular arrhythmia. Its use should be reserved for patients with life-threatening sustained arrhythmia because of the significant incidence of adverse effects. Furthermore, good clinical response can be observed in patients receiving amiodarone in spite of continued inducibility.

Evaluation of drug-induced changes in myocardial repolarisation using the paced evoked response

The use of the pace evoked response system in the assessment of drug-induced changes in myocardial repolarisation is reported. Using a conventional pacing electrode lead for both pacing and sensing, this system records the dominantly local repolarisation which follows a controlled (paced) depolarisation from the same site. Measurements of the latency of the ventricular evoked response at matched heart rates before and after drug administration permit the accurate direct comparison of the effects of drugs with class 3 mode of action on cardiac muscle repolarisation. Using this method we have evaluated the effect on the timing of the evoked T wave of two drugs which are known to prolong phase 2 of the action potential. Intravenous amiodarone (5 mg/kg) prolonged the stimulus-peak evoked T wave interval by an average of 39-4 ms (15% of control values); three hours after oral bethanidine (2 mg/kg) this interval increased by an average of 25.8 ms (10% of control values). The effect of therapeutic interventions on the latency of the local paced evoked response provides a simple, accurate assessment of their effect on the cellular action potential duration and constitutes a new tool in electrophysiological investigations.