Localization of the neurally mediated arrhythmogenic properties of digitalis.

Available evidence suggests that the propensity of digitalis glycosides to produce cardiac arrhythmias is due in part to their neuroexictatory effects. We have performed experiments in cats which support the existence of a neurogenic component in the etiology of digitalis-induced ventricular arrhythmias. Our data further indicate that the locus of this neural effect lies within an area of the medulla 2 millimeters above to 2 millimeters below the obex. These findings, when considered with the effects of polar cardiac glycosides that do not cross the blood-brain barrier, suggest that the area postrema may be the site of neural activation.

Differing sensitivities of Purkinje fibers and myocardium to inhibition of monovalent cation transport by digitalis.

The extent of inhibition of monovalent cation active transport in Purkinje fibers and myocardium in response to toxic and inotropic doses of digitalis were studied in the dog to elucidate the factors underlying the different relative sensitivities of these tissues to the toxic arrhythmogenic effects of digitalis. Monovalent cation transport inhibition was assessed by measuring uptake of the K(+) analog Rb(+) in samples of myocardium and Purkinje fibers after in vitro ouabain exposure and after acute or chronic administration of digoxin in vivo. The active uptake of Rb+ was determined as the difference between total uptake and uptake in the presence of 1.0 mM ouabain. Mean active uptake of Rb(+) by Purkinje fibers from control hearts was 1.62+/-0.11 (SEM) nmol/mg wet wt per 15 min, significantly greater than the value of 0.49+/-0.05 for myocardium (P < 0.001). Concentration-effect curves for inhibition of monovalent cation active transport by in vitro exposure to graded concentrations of ouabain showed that the concentration for half-maximal inhibition of monovalent cation transport, IC(50), for Purkinje fiber transport was 0.4 muM, significantly less than the value of 1.4 muM for myocardial samples. Dogs were given toxic doses of digoxin (0.3 mg/kg i.v.). At onset of sustained ventricular tachycardia, they were killed and monovalent cation transport measured in myocardial and Purkinje fiber samples. Active Rb(+) uptake was inhibited by 44% in myocardial samples, whereas a significantly greater inhibition of 76% was noted in Purkinje fibers (P < 0.01). Similar data were obtained for both transmural myocardial biopsy samples and subendocardial trabecular samples obtained from regions adjacent to Purkinje fibers. Another group of dogs received a nontoxic dose of 0.02 mg/kg i.v. daily for 6 d. Myocardium showed a 17% reduction in Rb(+) active uptake compared to control animals receiving no drug, whereas Purkinje fiber transport was reduced in these dogs to a significantly greater extent averaging 44% (P < 0.001). Thus, both toxic and inotropic (non-toxic) doses of digoxin inhibited monovalent cation transport in Purkinje fibers to a greater extent than in myocardium. This difference in apparent sensitivity of monovalent cation transport to digoxin may contribute to the previously reported tendency of digitalis toxic arrhythmias to arise in Purkinje fibers.

Studies on the possible mechanisms of lidoflazine arrhythmogenicity.

Lidoflazine is a calcium channel blocking agent that is effective and safe in the treatment of angina pectoris, but has been reported to be associated with sudden death when administered for the treatment of supraventricular arrhythmias. Studies were performed in dogs to determine if lidoflazine caused a rise in serum digoxin concentration that could cause arrhythmias or if it was directly arrhythmogenic. Dogs received chronic injections of digoxin and then digoxin in combination with lidoflazine. No increase in digoxin concentration was found. Dogs also underwent programmed electrical stimulation while not receiving medications and then after incremental doses of lidoflazine administered intravenously. Lidoflazine did not cause spontaneous ventricular tachycardia and did not lower the threshold of ventricular tachycardia induction. Combined administration of lidoflazine and digoxin did not facilitate arrhythmia induction. These studies do not support a digoxin-lidoflazine interaction or a direct arrhythmogenic action of lidoflazine.

Comparative study of encainide and quinidine in the treatment of ventricular arrhythmias.

The antiarrhythmic efficacy and safety of oral encainide hydrochloride and quinidine sulfate were compared in a nine center double-blind crossover study in 187 outpatients with benign or potentially lethal ventricular arrhythmias. Patients with at least 30 premature ventricular complexes/h were randomized to receive either encainide, 25 mg four times/day, or quinidine, 200 mg four times/day, for 2 weeks. These doses were continued for another 2 weeks if a 75% or greater reduction in premature ventricular complexes was observed. If this reduction was not seen, encainide was increased to 50 mg four times/day or quinidine to 400 mg four times/day for an additional 2 weeks. Both drugs produced a statistically significant reduction in premature ventricular complex frequency compared with baseline values. Encainide produced a statistically significant greater mean reduction in total premature ventricular complexes than did quinidine during the initial dose phase and after dose adjustment. More patients required dose increases of quinidine (60%) than of encainide (51%). Early discontinuation of treatment resulting in advancement to the next study period occurred in 12 patients taking encainide and 38 patients taking quinidine (p less than 0.05). PR and QRS intervals increased significantly during encainide treatment, as did QTc and JT intervals during quinidine treatment. No adverse reactions resulted from these electrocardiographic changes. Adverse reactions were more common with quinidine than with encainide.(ABSTRACT TRUNCATED AT 250 WORDS)

QT prolongation and the antiarrhythmic efficacy of amiodarone.

Amiodarone is an antiarrhythmic agent known to cause prolongation of action potential duration which is reflected in the electrocardiogram as a prolongation of the QT interval. Prolongation of the QT interval in patients dying suddenly was compared with that in patients who remained alive to determine whether a difference existed between these two groups. The electrocardiogram and amiodarone levels were evaluated in 33 patients who presented with cardiac arrest and symptomatic ventricular tachycardia in whom no other antiarrhythmic agent was found effective in preventing induction of ventricular tachycardia during electrophysiologic studies. There were 30 men and 3 women (mean age 52 +/- 10 years). Twenty-three are alive after a mean follow-up period of 12 +/- 7 months. Ten died: six suddenly, three of non-cardiac causes and one of congestive heart failure. Using a two-way analysis of variance, the percent change in QT, QTc, JT and JTc intervals before and after amiodarone therapy was analyzed. Marked prolongation in the QT interval was present in patients who remained alive with amiodarone therapy. A significant difference in percent QT prolongation was seen between the latter patients and those who died suddenly (p less than 0.005). No difference was observed in the percent change in QRS interval between the two groups. The levels of amiodarone (2.5 versus 3.2 micrograms/ml) and its metabolite (desethylamiodarone) were not significantly different between the living patients and those who died suddenly. These findings suggest that a prolongation of the QT interval may be a marker for the therapeutic antiarrhythmic effect of amiodarone.

Multicenter trial of sotalol for suppression of frequent, complex ventricular arrhythmias: a double-blind, randomized, placebo-controlled evaluation of two doses.

Sotalol is a unique beta-adrenergic blocking agent with additional actions characteristic of Vaughn-Williams class III antiarrhythmic agents in experimental models. To test the efficacy of sotalol to suppress ventricular arrhythmias, a 6 week parallel, placebo-controlled out-patient study of two doses (320 and 640 mg/day, in two divided doses) was performed in four hospitals in 56 patients with chronic premature ventricular complexes at a frequency of 30/h or more (mean +/- SE, 528 +/- 60/h) on 48 hour ambulatory electrocardiographic recording. During a placebo week, no change occurred in arrhythmia frequency (532 +/- 76/h). Subsequent sotalol therapy significantly reduced median arrhythmia frequency in patients receiving both low (n = 19) and high (n = 18) doses compared with that in patients receiving placebo (by 77 and 83%, respectively, versus 6%; p less than 0.001). Twenty-two (59%) of 37 sotalol-treated patients, 11 in each group, reached the prospectively defined criterion of efficacy (greater than or equal to 75% arrhythmia reduction) versus 2 (11%) of 19 placebo control patients (p less than 0.001). Sotalol reduced the median frequency of couplets by 94% (p less than 0.0001) and that of runs by 89% (p less than 0.0007). The electrocardiographic effects of sotalol included reductions in heart rate (by 17 to 27%) and increases in the QTc (by 6 to 9%) and PR (by 6%) intervals. Ejection fraction was unchanged. The most common adverse side effect was fatigue, but drug discontinuation was required in only three patients taking 640 mg/day. No proarrhythmic events or biochemical abnormalities were observed. In summary, sotalol displays significant antiarrhythmic activity of moderately high degree with good tolerance in doses of both 320 and 640 mg/day. Its antiarrhythmic actions are distinguished from those reported for other beta-blockers by its effects on the QTc interval and its moderately high degree of antiarrhythmic activity.

Cibenzoline plasma concentration and antiarrhythmic effect.

The relationship between plasma concentrations of cibenzoline and its antiarrhythmic effect was evaluated in patients receiving the drug orally as part of an ascending multiple dose efficacy and tolerance study. Twenty-five patients participated in a 3-day placebo period, 3 days of 32.5 mg cibenzoline every 6 hr, 3 days of 65 mg cibenzoline every 6 hr, 3 days of 81.25 mg cibenzoline every 6 hr, and 3 final placebo days. Arrhythmia frequency was monitored by 24-hr Holter monitoring and blood samples were drawn during and after dosing. Percent reduction in baseline premature ventricular complex (PVC) frequency for the 25 subjects demonstrated considerable interpatient variability in antiarrhythmic response. Cibenzoline plasma concentrations over 300 ng/ml were associated with some decrease in PVC frequency in virtually all cases. The relationship between plasma concentration and PVC frequency was studied more rigorously in eight of the 25 patients and that for ventricular couplet (VC) frequency was studied in six. For these analyses, PVC and VC frequency data were averaged over 6-hr intervals and plotted against trough cibenzoline concentrations. The data from each patient were fitted with a concentration-effect function (Hill equation) by means of least squares regression. With the exception of two extreme values, the concentration corresponding to 90% reduction in PVC frequency (C90) ranged from 215 to 405 ng/ml. In five of the six patients with arrhythmia in whom VC data were also evaluated, the individual C90 for VCs were considerably less than those for PVCs. The agreement between the observed concentration-response relationships and those predicted by curve-fitting the data suggests that the antiarrhythmic effect of cibenzoline is proportional to its plasma concentration, and that the Hill equation provides an accurate mathematic description of the concentration-response relationship.

Effects of actodigin on the heart.

Actodigin is a new semisynthetic cardiac glycoside reported to have a rapid onset and brief duration of action in dogs. Five patients with congestive heart failure in normal sinus rhythm were given incremental doses of actodigin. Overall, there was no significant change in heart rate, aortic or pulmonary artery pressure, systemic vascular resistance, cardiac index, and stroke volume. This lack of response to actodigin is consistent with previous reports of acute administration of other cardiac glycosides. Four patients with atrial fibrillation and a rapid ventricular rate were given similar doses of actodigin. The ventricular rate was readily controlled. After drug administration was stopped, the ventricular rate quickly returned toward predrug levels. Thus, the rapid onset and brief duration of action of actodigin may be useful in the initial management of atrial fibrillation.

Pharmacokinetics of oral cibenzoline in arrhythmia patients.

The pharmacokinetics of oral cibenzoline were studied in 30 arrhythmia patients as part of an ascending multiple-dose efficacy study. The elimination half-life of the drug following repetitive dosing ranged from 7.6 to 22.3 hours, with a harmonic mean of 12.3 hours (n = 24), and increased with age and decreasing renal function. The drug exhibited apparent dose proportional and linear pharmacokinetics over the range of doses studied. Multivariate analysis revealed that the patients' age and serum creatinine concentration accounted for 71% of the variability in the range of beta values (terminal elimination rate constant), and that 69.5% of the intersubject variability in the steady-state trough plasma concentrations could be accounted for by the patients' age, weight and serum creatinine concentration. These data suggest that, although there is some intersubject variability in the elimination and accumulation of cibenzoline, much of the variability can be explained by the patients' age, weight and renal function.

New directions in antiarrhythmic drug therapy.

Cardiac arrhythmia causing sudden cardiac death is a serious worldwide public health problem. Antiarrhythmic agents have been available for therapy, but the conventional agents cause a high degree of intolerable side effects. The recent development of many new experimental antiarrhythmic agents has increased our capacity to effectively treat cardiac arrhythmias. Using a multifaceted approach of programmed electrical stimulation studies, drug level determinations, exercise testing and 24-hour ambulatory Holter monitoring, it can reasonably be decided which patient needs therapy and if therapy is going to be effective. Both aspects of the sudden death equation, ectopy frequency (triggering mechanism) and the ability to propagate sustained ventricular tachycardia (substrate), may be examined. Careful follow-up is needed to determine continued drug efficacy and the presence of side effects that may compromise patient compliance with therapy. If side effects intervene that may cause continued therapy to be intolerable, changing the antiarrhythmic agent, as opposed to decreasing the dosage to an ineffective range, may be appropriate. A comprehensive approach to arrhythmia management may begin to reduce the high incidence of sudden death due to fatal arrhythmias.

Relation of left ventricular mass and QT dispersion in patients with systematic hypertension.

This study evaluated the correlation of left ventricular hypertrophy and QT dispersion in patients with systemic hypertension. QT dispersion, determined using the standard electrocardiogram, showed an increase as left ventricular mass determined by echocardiography increased in hypertensive patients.

Effect of quinidine on differing sensitivities of Purkinje fibers and myocardium to inhibition of monovalent cation transport by digitalis in dogs.

Studies have implicated quinidine in increasing serum digoxin levels, resulting in serious arrhythmias. Arrhythmias caused by digitalis intoxication are thought to originate in Purkinje fibers. Thus, the extent of inhibition of monovalent cation-active transport in Purkinje fibers and myocardium may explain the enhanced toxicity of the combined administration of digoxin and quinidine. Monovalent cation transport was assessed by measuring the uptake of the potassium analog rubidium in samples of myocardium and Purkinje fibers after in vitro exposure to ouabain and after long-term administration of digoxin and quinidine or digoxin alone. A group of dogs received chronic digoxin administration and achieved a steady-stage digoxin administration and achieved a steady-stage digoxin level of 2.1 +/- 0.3 ng/ml. Quinidine administered intravenously caused a 134% increase in the serum digoxin level. The transport in myocardium was unchanged, while it was reduced to 40% of control levels in Purkinje fibers. The difference in sensitivity between Purkinje fibers and myocardium may explain the finding that digitalis-toxic arrhythmias arise in Purkinje fibers and that quinidine, when combined with digitalis, increases the incidence of such arrhythmias.

QT interval dispersion in healthy subjects and survivors of sudden cardiac death: circadian variation in a twenty-four-hour assessment.

Twenty-four-hour acquisition of QT dispersion (QTd) from the Holter and the circadian variation of QTd were evaluated in 20 survivors of sudden cardiac death (SCD), in 20 healthy subjects, and in 14 control patients without a history of cardiac arrest who were age, sex, diagnosis and therapy matched to 14 SCD patients. Computer-assisted QT measurements were performed on 24-hour Holter recordings; each recording was divided into 288 5-minute segments and templates representing the average QRST were generated. QTd was calculated as the difference between QT intervals in leads V1 and V5 for each template on Holter. The 24-hour mean QTd was significantly greater in SCD patients (40 +/- 28 ms) than in healthy subjects (20 +/- 10 ms) and control patients (15 +/- 5 ms) (p <0.05). There was a circadian variation in QTd with greater values at night (0 to 6 A.M.) than at daytime (10 A.M. to 4 P.M.) in healthy subjects (25 +/- 13 vs 15 +/- 8 ms, p <0.001) and control patients (18 +/- 10 vs 12 +/- 4 ms p <0.05), whereas in SCD patients there was no significant difference between night and day values (45 +/- 31 vs 37 +/- 28 ms, p = NS). It is concluded that QTd measured by Holter was greater in SCD patients than in healthy subjects and matched control patients during the entire day. QTd has a clear circadian variation in normal subjects, whereas this variation is blunted in SCD patients. QTd measured on Holter differentiates survivors of cardiac arrest and may be a useful tool for risk stratification.

Safety and efficacy of flestolol, a new ultrashort-acting beta-adrenergic blocking agent, for supraventricular tachyarrhythmias.

Flestolol, a new ultrashort-acting (half-life 6.9 minutes) beta-blocking drug, was administered by intravenous infusion to 18 patients with new-onset atrial fibrillation or flutter and rapid ventricular response (120 beats/min or more for at least 30 minutes). Drug dose of flestolol was progressively increased until at least 1 of 3 endpoints was achieved: at least a 20% reduction in heart rate from baseline, heart rate 100 beats/min or less, or conversion to normal sinus rhythm. Flestolol was then administered as a maintenance infusion up to 24 hours. When flestolol was discontinued, patients were monitored for 1 additional hour. The mean ventricular response at baseline of 133 +/- 12 beats/min decreased to 103 +/- 20 beats/min at the end of flestolol titration (p less than 0.0001). Fourteen patients (78%) achieved defined endpoints. All 14 patients who continued to receive maintenance infusion had a sustained response. When flestolol was discontinued, ventricular response increased 33 +/- 23% within 60 minutes. The only adverse effect seen was hypotension in 2 patients. Flestolol is effective in slowing ventricular response in new-onset atrial fibrillation and flutter, maintains a therapeutic effect during continuous infusion and rapidly loses therapeutic effect when discontinued.

Prophylactic tocainide or lidocaine in acute myocardial infarction.

Twenty-nine patients with acute myocardial infarction (AMI) were studied in a randomized double-blind trial of intravenous lidocaine and tocainide, followed by either oral tocainide or placebo without regard to previous therapy, for the prophylaxis of arrhythmias associated with acute infarction. No patient had symptomatic ventricular tachycardia or fibrillation, although 1 patient taking lidocaine was withdrawn from therapy because of breakthrough arrhythmias. One patient in each group died from mechanical complications of AMI. Tocainide was administered to 16 patients and lidocaine to 13. Seven of the 13 patients receiving lidocaine had ventricular tachycardia or accelerated idioventricular rhythm, compared with 2 of 16 receiving tocainide (p less than 0.05). Adverse effects were noted in 11 of the 13 patients receiving lidocaine and 6 of the 16 patients receiving tocainide. The infusions used provided therapeutic levels of lidocaine or tocainide and the transition to oral tocainide was accomplished safely with maintenance of therapeutic antiarrhythmic levels. Thus, tocainide appears to be at least as efficacious and may be safer than lidocaine for the prophylaxis of ventricular arrhythmias associated with AMI. The transition to oral tocainide is well tolerated and can be accomplished with minimal difficulty.

Flecainide: long-term treatment using a reduced dosing schedule.

Flecainide was initially prescribed at a dose of 200 mg twice daily, but after early toxicity in patients with ventricular tachycardia (VT), the dosage was reduced to 100 mg twice daily. The effects of flecainide were studied in 40 patients (29 men and 11 women, mean age 62 +/- 2 years, ejection fraction 45 +/- 3%) who underwent programmed electrical stimulation to determine the efficacy of flecainide in preventing VT chronically at the reduced dose. Sustained VT was induced in 21 patients and nonsustained VT in 19. Flecainide prevented VT induction in 26 patients (65%). At a mean dose of 1.5 +/- 0.1 mg/kg, prolongation occurred in the effective refractory period of the first (280 +/- 5 vs 249 +/- 5 ms) and second (254 +/- 6 vs 209 +/- 9 ms) extrastimuli (p less than 0.01). In the patients protected by flecainide, the effective refractory periods increased by a 17 +/- 2% and 21 +/- 3%, in contrast to only a 7 +/- 3% and 6 +/- 4% increase in the nonprotected group (p less than 0.05), despite a higher mean dose (1.9 +/- 0.1 vs 1.35 +/- 0.1 mg/kg). Twenty-one patients were discharged on flecainide therapy, 100 mg twice daily, and were followed for a mean of 11 months. Sixteen patients are alive and well, 1 died suddenly, 1 died from a noncardiac cause and 1 had a "breakthrough" arrhythmia. Two were switched to quinidine therapy by their referring physicians, but were without problems while receiving flecainide.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiologic evaluation of the antiarrhythmic effects of N-acetylprocainamide for ventricular tachycardia secondary to coronary artery disease.

Antiarrhythmic properties of N-acetylprocainamide (NAPA), an active metabolite of procainamide, were studied in 12 patients with coronary artery disease who presented with cardiac arrest or documented sustained ventricular tachycardia (VT). Programmed electrical stimulation (PES) studies were performed in 10 men and 2 women, aged 52 to 80 years (mean 63), who had a left ventricular ejection fraction of 16 to 69% (mean 33). All patients tested had inducible VT provoked by PES without antiarrhythmic therapy. Patients were then tested with procainamide, 1,000 mg administered intravenously. VT could be provoked after procainamide treatment in 8 of 10 patients. Twenty-four to 36 hours later NAPA was administered, 18 mg/kg body weight intravenously, and PES was performed after 20 minutes. NAPA did not significantly change heart rate, mean arterial blood pressure, electrocardiographic intervals and AH or HV conduction times. The QT interval lengthened, but not significantly. The mean serum NAPA levels were 15.7 +/- 4 micrograms/ml in the group protected by NAPA and 16.2 +/- 4 micrograms/ml in the group not protected by NAPA. Five patients were discharged with NAPA therapy, 1.5 g orally every 8 hours. Two patients have been maintained with chronic NAPA therapy (10 +/- 3 months), and 2 patients had breakthrough VT on follow-up Holter monitoring and alternative therapy was given. One patient died while taking oral therapy. NAPA demonstrates antiarrhythmic efficacy in preventing induction of VT by PES in a high-risk group of patients. During chronic oral therapy in some patients, NAPA appears to be well tolerated, with antiarrhythmic efficacy that may be enhanced with further upward dose titration.

Lorcainide therapy for the high-risk patient post myocardial infarction.

Nonsustained ventricular tachycardia (VT) in the late period (7 to 21 days) after myocardial infarction (MI) is reported to be a predictor of sudden death. Patients with 3-beat VT on Holter monitoring in the late infarction period would be suspected to demonstrate electrical instability on electrophysiologic studies. Forty-seven patients were identified as having at least 3-beat VT on Holter monitoring. Eighteen patients refused electrophysiologic studies or were not referred. Eight patients died; 3 were sudden deaths in 13 +/- 5 months, a 17% incidence. Twenty-nine patients underwent invasive electrophysiologic studies and 28 had inducible VT, 18 sustained and 10 nonsustained. Lorcainide prevented VT induction in 21 of the 28 patients, whereas 12 of the 22 patients studied on procainamide were protected. Lidocaine, tested in 21 patients, prevented VT induction in only 5. Lorcainide and procainamide prolonged refractoriness in those patients protected at programmed electrical stimulation (PES), whereas the QT interval was prolonged in patients in whom VT could still be induced. Twenty-seven of the 28 patients were placed on drugs predicted to be effective by PES studies, 19 on lorcainide. After a mean follow-up of 12.5 +/- 4 months the patient with noninducible arrhythmia is alive and 26 of the 28 patients with inducible arrhythmia are alive and well. Two patients died, 1 of stroke and 1 of pump failure after a second MI. No sudden deaths were observed in this group. Two patients had breakthrough arrhythmias and were treated by alternative antiarrhythmic therapy that was also effective on initial electrophysiologic studies.(ABSTRACT TRUNCATED AT 250 WORDS)