Slow sodium channel inactivation and use-dependent block modulated by the same domain IV S6 residue.

Voltage- and/or conformation-dependent association and dissociation of local anesthetic-class drugs from a putative receptor site in domain IV S6 of the sodium channel and slow conformation transitions of the drug-associated channel have been proposed as mechanisms of use- and frequency-dependent reduction in sodium current. To distinguish these possibilities, we have explored the reactivity to covalent modification by thiols and block of the mutations F1760C and F1760A at the putative receptor site of the cardiac sodium channel expressed as stable cell lines in HEK-293 cells. Both mutations decreased steady-state fast inactivation, shifting V1/2h from -86 +/- 1.3 mV (WT) to -72.3 +/- 1.4 mV (F1760C) and -67.7 +/- 1 mV (F1760A). In the absence of drug, the F1760C mutant channel displayed use-dependent current reduction during pulse-train stimulation, and faster onset of slow inactivation. This mutant also retained some sensitivity to lidocaine. In contrast, the F1760A mutant showed no use-dependent current reduction or sensitivity to lidocaine. The covalent-modifying agent MTS-ET enhanced use-dependent current reduction of the F1760C mutant channel only. The use-dependent reduction in current of the covalently modified channel completely recovered with rest. Lidocaine produced no additional block during exposure to MTS-ET-treated cells (MTS-ET 43 +/- 2.7%: MTS-ET lidocaine 47 +/- 4.5%), implying interaction at a common binding site. The data suggest that use-dependent binding at the F1760 site results in enhanced slow inactivation rather than alteration of drug association and dissociation from that site and may be a general mechanism of action of sodium-channel blocking agents.

[Quantitative analysis of variability of electrocardiograms typical for polymorphic arrhythmias]. / Kolichestvennyi analiz variabel'nosti élektrokardiogramm, tipichnykh dlia polimorfnykh aritmii.

A new approach to the analysis of variability of electrocardiograms (ECGs) typical of polymorphic arrhythmias is developed. In these ECGs, separate QRS complexes can be often hardly identified. As a result, the mathematical methods that have been elaborated hitherto are not suitable for such arrhythmias. The approach presented here is based on the quantitative estimation of the variability of neighboring parts of the ECG. In this case, the necessity of the identification of separate QRS complexes ceases to be significant. Based on this approach, the analysis of normalized ECG variability is developed in the framework of which two indices that characterize the oscillation variability and its changes in time are related to a part of the ECG and/or the ECG as a whole. Variations of these indices allow both the polymorphism of a separate ECG to be estimated and different ECGs to be compared with each other. The method presented may be useful in studies of the mechanisms and in the diagnosis of polymorphic arrhythmias.

A theoretical analysis of acute ischemia and infarction using ECG reconstruction on a 2-D model of myocardium.

We developed a two-dimensional ventricular tissue model in order to probe the determinants of electrocardiographic (ECG) morphology during acute and chronic ischemia. Hyperkalemia was simulated by step changes in [K+]out, while acidosis was induced by reducing Na+ and Ca2+ conductances. Hypoxia was introduced by its effect on potassium activity. During the initial moments of ischemia, ECG changes were characterized by increases in QRS amplitude and ST segment shortening, followed in the advanced phase by ST baseline elevation, T conformation changes, widening of the QRS and significant decreases in QRS amplitude in spite of an enlarged Q. During each phase, potential proarrhythmic mechanisms were investigated. The presence of unexcitable regions of simulated myocardial infarction led to polymorphic ECG. We also observed a nonuniform deflection of the ST segment from beat to beat. We used similar protocols to explore the responses of infarcted myocardium after impairment resolving. We found that despite irreversible uncoupling of the necrotic region, the restored normal ionic concentrations produced an isopotential ST segment and monomorphic ECG complexes, while an enlarged Q wave was still visible. In summary, these numerical experiments indicate the possibility to track in the ECG pathologic changes following the altered electrophysiology of the ischemic heart.

Block of wild-type and inactivation-deficient cardiac sodium channels IFM/QQQ stably expressed in mammalian cells.

The role of inactivation as a central mechanism in blockade of the cardiac Na(+) channel by antiarrhythmic drugs remains uncertain. We have used whole-cell and single channel recordings to examine the block of wild-type and inactivation-deficient mutant cardiac Na(+) channels, IFM/QQQ, stably expressed in HEK-293 cells. We studied the open-channel blockers disopyramide and flecainide, and the lidocaine derivative RAD-243. All three drugs blocked the wild-type Na(+) channel in a use-dependent manner. There was no use-dependent block of IFM/QQQ mutant channels with trains of 20 40-ms pulses at 150-ms interpulse intervals during disopyramide exposure. Flecainide and RAD-243 retained their use-dependent blocking action and accelerated macroscopic current relaxation. All three drugs reduced the mean open time of single channels and increased the probability of their failure to open. From the abbreviation of the mean open times, we estimated association rates of approximately 10(6)/M/s for the three drugs. Reducing the burst duration contributed to the acceleration of macroscopic current relaxation during exposure to flecainide and RAD-243. The qualitative differences in use-dependent block appear to be the result of differences in drug dissociation rate. The inactivation gate may play a trapping role during exposure to some sodium channel blocking drugs.

beta-Adrenergic action on wild-type and KPQ mutant human cardiac Na+ channels: shift in gating but no change in Ca2+:Na+ selectivity.

OBJECTIVE: Prior studies of the modulation of the Na+ current by sympathetic stimulation have yielded controversial results. Separation of the Na+ and Ca2+ currents poses a problem in myocyte preparations. The gating of cloned Na+ channels is different in oocytes compared with mammalian expression systems. We have examined the sympathetic modulation of the alpha-subunit of the wild-type human cardiac Na+ channel (hH1) and the long QT-associated mutant, delta KPQ, expressed in human embryonic kidney cells. METHODS: Stable cell lines of hH1 and delta KPQ were established in human embryonic kidney cells. Whole-cell and single-channel currents were measured with the patch-clamp technique. Sympathetic stimulation was effected by exposure to isoproterenol or 8-bromo-cAMP. Na+ channel activation and inactivation were determined using standard voltage clamp protocols. Ca2+:Na+ permeability ratio was determined under bi-ionic conditions. RESULTS: We observed a qualitatively different effect of sympathetic stimulation on the cardiac Na+ current from that reported in frog oocytes: activation and inactivation kinetics were shifted to more negative potentials. This shift was similar for both hH1 and delta KPQ. [delta V0.5 for inactivation: 8.3 +/- 1.7 mV, p < 0.001 (hH1); 6.8 +/- 0.9 mV, p < 0.001 (delta KPQ)]. Increased rate of closed-state inactivation contributed to the shifting of the inactivation-voltage relationship. Open-state inactivation was not affected as mean open times were unchanged. Reversal potential measurement in hH1 suggested a low Ca2+:Na+ permeability ratio of 0.017, uninfluenced by sympathetic stimulation. In delta KPQ, the size of the persistent relative to the peak current was increased with 8-bromo-cAMP from 3.0 +/- 0.7% to 4.3 +/- 0.6% (p = 0.056). CONCLUSIONS: Sympathetic stimulation exerts multiple effects on the gating of hH1. Similar effects are also seen in delta KPQ which may increase arrhythmia susceptibility in long QT syndrome by modifying the Na+ channel contribution to the action potential.

Multiple effects of KPQ deletion mutation on gating of human cardiac Na+ channels expressed in mammalian cells.

Several aspects of the effect of the KPQ deletion mutation on Na+ channel gating remain unresolved. We have analyzed the kinetics of the early and late currents by recording whole cell and single-channel currents in a human embryonic kidney (HEK) cell line (HEK293) expressing wild-type and KPQ deletion mutation in cardiac Na+ channels. The rate of inactivation increased three- to fivefold between -40 and -80 mV in the mutant channel. The rate of recovery from inactivation was increased twofold. Two modes of gating accounted for the late current: 1) isolated brief openings with open times that were weakly voltage dependent and the same as the initial transient and 2) bursts of opening with highly voltage-dependent prolonged open times. Latency to first opening was accelerated, suggesting an acceleration of the rate of activation. The delta KPQ mutation has multiple effects on activation and inactivation. The aggregate effects may account for the increased susceptibility to arrhythmias.

[Slow excitation waves and mechanisms of polymorphic ventricular tachycardia in an experimental model: isolated walls of the right ventricle from rabbit and squirrel]. / Medlennye volny vozbuzhdeniia i mekhanizmy polimorfnoi zheludochkovoi takhikardii v éksperimental'noi modeli: izolirovannye stenki pravogozheludochka krolika i suslika.

The mechanism of polymorphic disturbances of the heart rhythm is studied on an experimental model, isolated ventricular preparations of ground squirrel and rabbit. Polymorphic arrhythmias are identified from habitus of the isolated preparation pseudoECGs mathematically derived from electrograms registered simultaneously at 32 endocardial and 32 epicardial points. The same electrograms allow one to visualize the excitation wave propagation along each of the preparation surfaces. The comparison of excitation wave pictures and corresponding pseudoECGs enabled us to reveal the conditions necessary and sufficient for polymorphism in heart rhythm disturbances. Polymorphic arrhythmias are due to changes in wave pictures in the regions of retarded excitation propagation.

[A system for computer visualization of excitation wave propagation in the myocardium]. / Sistema komp'iuternoi vizualizatsii rasprostraneniia voln vozbuzhdeniia v miokarde.

A method of computer-aided visualization of autowave vortices on the cardiac tissue surface is developed. The software for research into autowave vortex evolution is designed, which allowed an adequate presentation of excitation propagation along complex trajectories and the detection of the most essential features of excitation source behavior.

The cardiac vulnerable period and reentrant arrhythmias: targets of anti- and proarrhythmic processes.

Because sudden cardiac death is usually preceded by a reentrant arrhythmia, the precipitating arrhythmia must be multicellular in origin. Therefore clinicians seeking to reduce the incidence of reentrant arrhythmias in their patients with antiarrhythmic drugs that alter propagation may reasonably question the applicability of drug classification schemes (e.g. Sicilian Gambit) that are based on measurements in single cells. This raises a major question: are measures of a drug's anti- and proarrhythmic potential in single cells predictive of its anti- and proarrhythmic properties in tissue? The problem is as follows. From single cell measurements, one expects Class I drugs to reduce excitability, thereby attenuating the occurrence of PVCs. Similarly, one expects Class III drugs to prolong refractoriness and reduce the occurrence of PVCs. We have found in simple models of cardiac tissue that sodium channel blockade (the target of Class I drugs) extends the vulnerable period of a propagating excitation wave, whereas potassium channel blockade (the target of Class III drugs) destabilizes the reentrant path in a manner that amplifies the likelihood of polymorphic tachyarrhythmias. Using analytical, numerical, and experimental studies, we determined that sodium channel blockade was proarrhythmic. In fact, we found that any intervention that slowed conduction was proarrhythmic because slowed conduction increases the vulnerable period and reduces the spatial requirements for sustained reentry. We also found that when obstacles were placed in the path of a propagating wave, reentry occurred when the conduction velocity was less than a critical value. Once reentry was established, we observed that the ECG displayed monomorphic QRS complexes when the reentrant path did not vary from cycle to cycle. Moreover, when the reentry path did vary from cycle to cycle, the ECG displayed polymorphic QRS complexes. The cycle-to-cycle variation in QRS morphology was caused by the spatial variability of the reentry path. The variability of reentry paths (and hence the degree of polymorphic variation in QRS complexes) was amplified by Class III agents. The results presented here show that multicellular proarrhythmic effects are derived from the same mechanisms that exhibit antiarrhythmic properties in single cells.

The role of inactivation in open-channel block of the sodium channel: studies with inactivation-deficient mutant channels.

Inactivation has been implicated as an important determinant of the block of Na+ channel by local anesthetic-class drugs. This proposition has been difficult to examine because agents used to modify inactivation change other channel properties and both inactivated and blocked channels do not conduct. We used site-directed mutagenesis of Phe1304 to glutamine in the linker between the third and fourth domains of the mu-1 Na+ channel to slow inactivation. Wild-type and mutant channels were expressed in frog oocytes. Macropatch and single-channel currents were recorded in cell-attached membrane patches. The F1304Q mutation increased mean open time (1.7 fold at -20 mV) and reduced the probability that the channel would fail to open. Closed times were best fit by a double-exponential function, suggesting that the inactivated state transitions were no longer absorbing. In wild-type channels, 100 microM disopyramide decreased mean open time from 1.64 +/- 0.08 to 0.34 +/- 0.04 msec. Total open time per trial was decreased 2-fold. There also was a marked increase in the fraction of null sweeps. In the inactivation-deficient mutant channel, mean and total open times were also reduced. These data indicate that even when inactivation is slowed by a localized specific mutation, open-channel block by disopyramide persists. Inactivation may not be a necessary requirement for open-channel block.

Wavelet formation in excitable cardiac tissue: the role of wavefront-obstacle interactions in initiating high-frequency fibrillatory-like arrhythmias.

High-frequency arrhythmias leading to fibrillation are often associated with the presence of inhomogeneities (obstacles) in cardiac tissue and reduced excitability of cardiac cells. Studies of antiarrhythmic drugs in patients surviving myocardial infarction revealed an increased rate of sudden cardiac death compared with untreated patients. These drugs block the cardiac sodium channel, thereby reducing excitability, which may alter wavefront-obstacle interactions. In diseased atrial tissue, excitability is reduced by diminished sodium channel availability secondary to depolarized rest potentials and cellular decoupling secondary to intercellular fibrosis. Excitability can also be reduced by incomplete recovery between successive excitations. In all of these cases, wavefront-obstacle interactions in a poorly excitable medium may reflect an arrhythmogenic process that permits formation of reentrant wavelets leading to flutter, fibrillation, and sudden cardiac death. To probe the relationship between excitability and arrhythmogenesis, we explored conditions for new wavelet formation after collision of a plane wave with an obstacle in an otherwise homogeneous excitable medium. Formulating our approach in terms of the balance between charge available in the wavefront and the excitation charge requirements of adjacent medium, we found analytically the critical medium parameters that defined conditions for wavefront-obstacle separation. Under these conditions, when a parent wavefront collided with a primitive obstacle, the resultant fragments separated from the obstacle boundaries, subsequently curled, and spawned new "daughter" wavelets. We identified spatial arrangements of obstacles such that wavefront-obstacle collisions leading to spawning of new wavelets could produce high-frequency wavelet trains similar to fibrillation-like arrhythmias.

Cardiac transient outward potassium current: a pulse chemistry model of frequency-dependent properties.

Recent voltage-clamp studies of isolated myocytes have demonstrated widespread occurrence of a transient outward current (I(to)) carried by potassium ions. In the canine ventricle, this current is well developed in epicardial cells but not in endocardial cells. The resultant spatial dispersion of refractoriness is potentially proarrhythmic and may be amplified by channel blockade. The inactivation and recovery time constants of this channel are in excess of several hundred milliseconds, and consequently channel availability is frequency dependent at physiological stimulation rates. When the time constants associated with transitions between different channel conformations are rapid relative to drug binding kinetics, the interactions between drugs and an ion channel can be approximated by a sequence of first-order reactions, in which binding occurs in pulses in response to pulse train stimulation (pulse chemistry). When channel conformation transition time constants do not meet this constraint, analytical characterizations of the drug-channel interaction must then be modified to reflect the channel time-dependent properties. Here we report that the rate and steady-state amount of frequency-dependent inactivation of I(to) are consistent with a generalization of the channel blockade model: channel availability is reduced in a pulsatile exponential pattern as the stimulation frequency is increased, and the rate of reduction is a linear function of the pulse train depolarizing and recovery intervals. I(to) was reduced in the presence of quinidine. After accounting for the use-dependent availability of I(to) channels, we found little evidence of an additional use-dependent component of block after exposure to quinidine, suggesting that quinidine reacts with both open and closed I(to) channels as though the binding site is continuously accessible. The model provides a useful tool for assessing drug-channel interactions when the reaction cannot be continuously monitored.

Proarrhythmic response to potassium channel blockade. Numerical studies of polymorphic tachyarrhythmias.

BACKGROUND: Prompted by the results of CAST results, attention has shifted from class I agents that primarily block sodium channels to class III agents that primarily block potassium channels for pharmacological management of certain cardiac arrhythmias. Recent studies demonstrated that sodium channel blockade, while antiarrhythmic at the cellular level, was inherently proarrhythmic in the setting of a propagating wave front as a result of prolongation of the vulnerable period during which premature stimulation can initiate reentrant activation. From a theoretical perspective, sodium (depolarizing) and potassium (repolarizing) currents are complementary so that if antiarrhythmic and proarrhythmic properties are coupled to modulation of sodium currents, then antiarrhythmic and proarrhythmic properties might similarly be coupled to modulation of potassium currents. The purpose of the present study was to explore the role of repolarization currents during reentrant excitation. METHODS AND RESULTS: To assess the generic role of repolarizing currents during reentry, we studied the responses of a two-dimensional array of identical excitable cells based on the FitzHugh-Nagumo model, consisting of a single excitation (sodium-like) current and a single recovery (potassium-like) current. Spiral wave reentry was initiated by use of S1S2 stimulation, with the delay timed to occur within the vulnerable period (VP). While holding the sodium conductance constant, the potassium conductance (gK) was reduced from 1.13 to 0.70 (arbitrary units), producing a prolongation of the action potential duration (APD). When gK was 1.13, the tip of the spiral wave rotated around a small, stationary, unexcited region and the computed ECG was monomorphic. As gK was reduced, the APD was prolonged and the unexcited region became mobile (nonstationary), such that the tip of the spiral wave inscribed an outline similar to a multipetaled flower; concomitantly, the computed ECG became progressively more polymorphic. The degree of polymorphism was related to the APD and the configuration of the nonstationary spiral core. CONCLUSIONS: Torsadelike (polymorphic) ECGs can be derived from spiral wave reentry in a medium of identical cells. Under normal conditions, the spiral core around which a reentrant wave front rotates is stationary. As the balance of repolarizing currents becomes less outward (eg, secondary to potassium channel blockade), the APD is prolonged. When the wavelength (APD.velocity) exceeds the perimeter of the stationary unexcited core, the core will become unstable, causing spiral core drift. Large repolarizing currents shorten the APD and result in a monomorphic reentrant process (stationary core), whereas smaller currents prolong the APD and amplify spiral core instability, resulting in a polymorphic process. We conclude that, similar to sodium channel blockade, the proarrhythmic potential of potassium channel blockade in the setting of propagation may be directly linked to its cellular antiarrhythmic potential, ie, arrhythmia suppression resulting from a prolonged APD may, on initiation of a reentrant wave front, destabilize the core of a rotating spiral, resulting in complex motion (precession) of the spiral tip around a nonstationary region of unexcited cells. In tissue with inhomogeneities, core instability alters the activation sequence from one reentry cycle to the next and can lead to spiral wave fractination as the wave front collides with inhomogeneous regions. Depending on the nature of the inhomogeneities, wave front fragments may annihilate one another, producing a nonsustained arrhythmia, or may spawn new spirals (multiple wavelets), producing fibrillation and sudden cardiac death.

Potassium channel blockade amplifies cardiac instability numerical studies of torsades de pointes.

Suppression of responses to premature stimulation has been the guiding principle in managing many cardiac arrhythmias. Recent clinical trails revealed that sodium channel blockade increased the incidence of re-entrant cardiac arrhythmias resulting in sudden cardiac death, although the physiologic mechanism remains uncertain. Potassium channel blockade offers an alternative mechanism for suppressing responses to premature stimuli. We have developed a simple model of a 2D sheet of excitable cells. We can initiate re-entrant activation with stimuli timed to occur within a period of vulnerability (VP). Reducing the Na conductance increases the VP while reducing the K conductance increases the collective instability of the array, and arrhythmias similar to torsades de pointes seen in patients subjected to K channel blocked can be readily initiated. Thus, while K channel blockade may suppress excitability by prolonging the action potential duration, it appears to simultaneously exhibit proarrhythmic properties that result in complex re-entrant arrhythmias.

Voltage-independent effects of extracellular K+ on the Na+ current and phase 0 of the action potential in isolated cardiac myocytes.

A rise in [K+]o, by depolarizing the resting membrane potential and partially inactivating the inward Na+ current (INa), is believed to play a critical role in slowing conduction during myocardial ischemia. In multicellular ventricular preparations, elevation of [K+]o has been suggested to decrease Vmax to a greater extent than expected from membrane depolarization alone. The mechanism of this voltage-independent effect of [K+]o is currently unknown, and its significance in single cardiac cells has not been determined. We have examined the voltage-independent effects of elevated [K+]o on INa and the action potential upstroke in isolated rabbit atrial and ventricular myocytes under voltage- and current-clamp conditions. Superfusate [K+] was varied from 5 mmol/L to 14 or 24 mmol/L, whereas [Na+] was maintained at 150 mmol/L. In cultured atrial cells and excised outside-out patches from freshly isolated atrial and ventricular cells, the amplitude and kinetics of INa were unchanged by elevation of [K+]o. In atrial cells, action potentials elicited from a holding potential of -70 mV had a similar Vmax (114.9 +/- 5.7 versus 112.2 +/- 4.8 V/s, mean +/- SEM, n = 6) and action potential amplitude (115.0 +/- 2.4 versus 113.4 +/- 3.9 mV) in 5 and 24 mmol/L [K+]o. In contrast, in ventricular cells at a holding potential of -70 mV, increasing [K+]o fro 5 to 14 mmol/L decreased Vmax from 161.8 +/- 18.0 to 55.3 +/- 5.0 V/s (n = 7, P < .001) and action potential amplitude from 128.1 +/- 1.3 to 86.6 +/- 5.4 mV (P < .001). This voltage-independent decrease in Vmax and action potential amplitude induced by elevated [K+]o was abolished in the presence of 1 mmol/L Ba2+, suggesting that it is attributable to an increased background K+ conductance. We conclude that elevation of [K+]o to levels expected during ischemia causes a marked voltage-independent depression of Vmax in ventricular cells, which may, in turn, contribute to the slowing of myocardial conduction characteristic of early ischemia.

Late Na channels in cardiac cells: the physiological role of background Na channels.

Two types of the late Na channels, burst and background, were studied in Purkinje and ventricular cells. In the whole-cell configuration, steady-state Na currents were recorded at potentials (-70 to -80 mV) close to the normal cell resting potential. The question of the contribution of late Na channels to this background Na conductance was investigated. During depolarization, burst Na channels were active for periods (up to approximately 5 s), which exceeded the action potential duration. However, they eventually closed without reopening, indicating the presence of slow and complete inactivation. When, at the moment of burst channel opening, the potential was switched to -80 mV, the channel closed quickly without reopening. We conclude that the burst Na channels cannot contribute significantly to the background Na conductance. Background Na channels undergo incomplete inactivation. After a step depolarization, their activity decreased in time, approaching a steady-state level. Background Na channel openings could be recorded at constant potentials in the range from -120 to 0 mV. After step depolarizations to potentials near -70 mV and more negative, a significant fraction of Na current was carried by the background Na channels. Analysis of the background channel behavior revealed that their gating properties are qualitatively different from those of the early Na channels. We suggest that background Na channels represent a special type of Na channel that can play an important role in the initiation of cardiac action potential and in the TTX-sensitive background Na conductance.

Open Na+ channel blockade: multiple rest states revealed by channel interactions with disopyramide and quinidine.

In voltage-clamp studies of atrial myocytes exposed to disopyramide or quinidine, pulse-train stimulation revealed use-dependent block that increased with increased pulse amplitude. Use-dependent block also became negligible at hyperpolarized holding potentials (< -150 mV), consistent with either rapid unbinding at the holding potential or trapping of the drug in a drug-complexed rest conformation followed by rapid unbinding during the next channel opening event. To explore the unbinding properties of hypothetically different rest-blocked conformations, we exposed cells to a postdepolarization "conditioning" potential after channels had become fully inactivated so as to vary the transition to different hypothetical rest-blocked channels. Pulse-train stimulation from -130 to -30 mV generated only a small amount of use-dependent block. Inserting a 120-ms subthreshold (e.g., -100 mV) postdepolarization conditioning potential before return to -130 mV increased use-dependent block. The fraction of steady-state block exhibited a bell-shaped dependence on the conditioning potential. These results are consistent with the existence of a mixture of rest-blocked channel conformations, each having direct access to the blocked-inactivated state. These intermediate rest conformations display radically different drug unbinding rates.

Combination ethacizin and ethmozin treatment of resistant ventricular ectopy: theoretical, experimental, and clinical study.

Ethmozin (Moricizine HCl) and ethacizin are two class I antiarrhythmic drugs with different rate constants of interaction with the sodium channel. Computer simulation using the "guarded-receptor" model predicted that the combination of ethacizin and ethmozin should exert a greater decrease in excitability and conduction at short coupling intervals, but little effect at normal heart rates (HR). To test this prediction, we measured intraventricular conduction delay in canine hearts in vivo. In agreement with the model, the combination more potently prolonged the delay only at intervals < 600 ms as compared with ethacizin alone. Combination therapy was tested in 6 patients with idiopathic ventricular ectopic depolarizations (VEDs). Three patients were resistant to either ethmozin or ethacizin monotherapy, and three could not tolerate effective doses because of side effects. Quantitative continuous ECG monitoring showed that total VEDs in the resistant group decreased 0 and 17 +/- 13% for 400 and 800 mg/day ethmozin and 18 +/- 12 and 55 +/- 12% for 100 and 200 mg/day ethacizin, respectively. Combined therapy with ethmozin (400 mg/day) and ethacizin (100 mg/day) reduced the number of VEDs by 78 +/- 2% in these patients without side effects. In the "nonresistant" but intolerant group of patients, use of the combination allowed relief of symptomatic ectopy without side effects. A theoretical model correctly predicted an effective combination of class I antiarrhythmic drugs, one with "fast-off" and one with "slow-off" kinetics, which may provide a general rationale for choosing drug combinations.