Frequency-dependent electrophysiological effect of ibutilide on human atrium and ventricle

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Effects of intravenous amiodarone and ibutilide on action potential duration and atrial conduction kinetics in patients with persistent atrial fibrillation

Class III antiarrhythmic drugs have been shown to be effective for termination of atrial fibrillation (AF). The aim of this study was to determine the steady state and non-steady state effects of amiodarone and ibutilide on the atrial monophasic action potential (MAP) duration (MAPD), effective refractory period (ERP), and intra-atrial conduction time (IACT) in human persistent AF.Fourteen patients with persistent AF who underwent internal atrial defibrillation were included in the study. The atrial MAP was recorded at the high right atrium. IACT was measured from the pacing spike to the distal coronary sinus. MAPD and IACT were assessed during the steady state and at the shortest diastolic interval (DI) at a basic cycle length (CL) of 600 msec and after a premature stimulus. Amiodarone did not affect MAPD or the ERP at the basic CL, but it increased MAPD at the shortest DI. Amiodarone increased IACT at both the basic CL and the shortest DI. Ibutilide increased the MAPD and ERP at the basic CL and at the shortest DI. Ibutilide did not affect IACT at the basic CL or the shortest DI.Ibutilide increases atrial MAPD not only in the steady state but also at the shortest DI, but it does not affect IACT in patients with persistent AF. Amiodarone does not affect MAPD or ERP, but it increases IACT in the steady state, and it increases MAPD and IACT at the shortest DI.

Rate-dependent electrophysiologic effects of the class III antiarrhythmic drugs nifekalant, amiodarone, and ibutilide on the atrium in patients with persistent atrial fibrillation

Persistent atrial fibrillation (AF) is characterized by electrical remodeling, ie, marked decreases in the atrial effective refractory period (ERP), ERP rate adaptation, and atrial conduction velocity. Little information is available on the effects of class III antiarrhythmic drugs on the remodeled atrium. We studied the effects of the class III antiarrhythmic drugs nifekalant, ibutilide, and amiodarone on rate-dependent changes in atrial action potential duration in patients with persistent AF. Right atrial (RA) monophasic action potential duration (MAPD) and intra-atrial conduction time (IACT) were measured at pacing cycle lengths (CLs) of 800, 700, 600, 500, 400, 350, 300, and 250 ms before and after administration of nifekalant (0.4 mg/kg + 0.3 mg/kg/hr, iv), amiodarone (5 mg/kg, iv), or ibutilide (0.01 mg/kg, iv) in 31 patients after successful internal cardioversion of chronic AF of > 2 months duration. Nifekalant and ibutilide significantly increased RA MAPD and the ERP at each CL in a reverse rate-dependent manner. Amiodarone did not affect RA MAPD. Nifekalant did not affect IACT, whereas amiodarone increased IACT at each CL in a rate-dependent manner, and ibutilide increased IACT at CLs ≤ 350 ms. The atrial electrophysiologic effects of the class III antiarrhythmic drugs nifekalant, amiodarone, and ibutilide differ, depending on the degree of electrical and structural remodeling and the effects of the drugs on the depolarizing and repolarizing currents.

Safe and effective conversion of persistent atrial fibrillation to sinus rhythm by intravenous AZD7009

BACKGROUND

Acute drug conversion of persistent atrial fibrillation usually fails.

OBJECTIVES

The purpose of this study was to test the proarrhythmic potential, safety, and efficacy of the novel antiarrhythmic agent AZD7009 in patients with persistent atrial fibrillation (AF) or atrial flutter (mean duration 43 days) scheduled for direct current (DC) cardioversion.

METHODS

Patients were randomized to AZD7009 (3-hour intravenous infusion; n = 86) or placebo (n = 36). AZD7009 was given in doses intended to produce target pseudo-steady-state plasma levels of 0.25, 0.50, 0.75, 1.0, 1.5, 2.0, or 2.5 micromol/L after 30 minutes of infusion. DC cardioversion was performed if conversion to sinus rhythm (SR) did not occur within 2 hours of infusion.

RESULTS

AZD7009 in a concentration-dependent manner increased the rate of conversion of AF to SR and shortened the time to conversion. At the three highest target concentrations of AZD7009, 45%, 64%, and 70% of AF patients converted after a mean time of 62, 55, and 26 minutes, respectively, whereas no placebo-treated patients converted. SR was maintained for 24 hours in 21 of 22 patients with drug-associated conversion. AZD7009 treatment was associated with QT-interval prolongation; the increase in QT corrected according to Fridericia typically ranged from 40 to 80 ms at targeted pseudo-steady-state plasma concentrations >or=0.75 micromol/L, but a number of outliers with QT corrected according to Fridericia >550 ms were seen in the higher concentration groups, particularly after conversion to SR and prolonged infusion. None of the patients exhibited torsades de pointes according to predefined criteria; however, one patient exhibited a nonsustained, polymorphic ventricular tachycardia of eight beats with torsades de pointes-like features after AZD7009 infusion (asymptomatic and discovered only upon retrospective Holter tape analysis). Clinical adverse events (primarily dizziness, bradycardia, hypotension, and nausea) were significantly more common in the highest target concentration AZD7009 group vs placebo (P <.001).

CONCLUSION

AZD7009 exhibited dose-dependent effects in converting AF to SR in AF patients and appeared to be associated with a low risk of proarrhythmia despite continued administration during a period of heightened vulnerability.

Ibutilide – recent molecular insights and accumulating evidence for use in atrial flutter and fibrillation

Ibutilide is a 'pure' class III antiarrhythmic drug, used intravenously against atrial flutter and fibrillation. At a cellular level it exerts two main actions: induction of a persistent Na+ current sensitive to dihydropyridine Ca2+ channel blockers and potent inhibition of the cardiac rapid delayed rectifier K+ current, by binding within the channel pore cavity upon channel gating. Ibutilide has been shown to terminate atrial flutter and fibrillation in animal studies, with some risk of ventricular pro-arrhythmia. Experimental models of hypertrophy/heart failure show altered sensitivity to ibutilide, with increased dispersion of repolarisation and incidence of pro-arrhythmia. Patient trials show that ibutilide is effective at terminating atrial arrhythmias when given alone, and that it can increase effectiveness and reduce energy requirements of electrical cardioversion. The risk to patients of polymorphic ventricular tachycardia necessitates careful patient selection and monitoring during and after treatment. An ibutilide analogue, trecetilide, requires further investigation but may offer a less readily metabolised and pro-arrhythmic alternative to ibutilide.

Role of I Kur in Controlling Action Potential Shape and Contractility in the Human Atrium

Background— The ultrarapid outward current I Kur is a major repolarizing current in human atrium and a potential target for treating atrial arrhythmias. The effects of selective block of I Kur by low concentrations of 4-aminopyridine or the biphenyl derivative AVE 0118 were investigated on right atrial action potentials (APs) in trabeculae from patients in sinus rhythm (SR) or chronic atrial fibrillation (AF).

Methods and Results— AP duration at 90% repolarization (APD 90 ) was shorter in AF than in SR (300±16 ms, n=6, versus 414±10 ms, n=15), whereas APD 20 was longer (35±9 ms in AF versus 5±2 ms in SR, P <0.05). 4-Aminopyridine (5 μmol/L) elevated the plateau to more positive potentials from −21±3 to −6±3 mV in SR and 0±3 to +12±3 mV in AF. 4-Aminopyridine reversibly shortened APD 90 from 414±10 to 350±10 ms in SR but prolonged APD 90 from 300±16 to 320±13 ms in AF. Similar results were obtained with AVE 0118 (6 μmol/L). Computer simulations of I Kur block in human atrial APs predicted secondary increases in I Ca,L and in the outward rectifiers I Kr and I Ks , with smaller changes in AF than SR. The indirect increase in I Ca,L was supported by a positive inotropic effect of 4-aminopyridine without direct effects on I Ca,L in atrial but not ventricular preparations. In accordance with the model predictions, block of I Kr with E-4031 converted APD shortening effects of I Kur block in SR into AP prolongation.

Conclusions— Whether inhibition of I Kur prolongs or shortens APD depends on the disease status of the atria and is determined by the level of electrical remodeling.