Membrane factors in arrhythmogenesis: concepts and definitions

Flecainide and the electrophysiologic matrix: the effects of flecainide acetate on the determinants of cardiac excitability in sheep Purkinje fibers

Flecainide was associated with excess mortality distributed virtually equally throughout the period of the Cardiac Arrhythmia Suppression Trial, suggesting the intersection of two events, drug effect and perhaps ischemia. Flecainide's effect on active properties has been studied extensively, but nothing is known of its effects on passive properties or on the balance among active and passive cellular properties that determines cardiac excitability. The multiple microelectrode method of intracellular current application and transmembrane voltage recording was used in sheep Purkinje fibers to determines strength- and charge-duration as well as constant current-voltage relationships and to estimate active properties, liminal length, and cable properties at a normal [K+]o and in a setting of hyperkalemia analogous to that of ischemia. A computer tracked in time the alterations in the active and passive properties relevant to excitability. Flecainide slightly decreased excitability at a normal [K+]o, primarily by depressing the sodium system with some contributory effect of passive properties. At high [K+]o, flecainide caused a frequency-dependent decrease in excitability and conduction, the latter best interpreted as a failure of the fiber to attain the liminal length requirements to produce a local action potential due primarily to an effect on sodium conductance. Together, the observations suggest that the action potential is the local phenomenon and that the propagated event is the sequential fulfillment of liminal length requirements. The data were interpreted in terms of the electrophysiologic matrix first proposed in detail in this Journal, which indicated that the electrophysiologic universe moved as a system in response to the drug and a change in [K+]o, the presumed antiarrhythmic and proarrhythmic electrophysiologic matrices for flecainide were quite similar, and the matrical configuration shared characteristics with the matrices of other drugs with known proarrhythmic potential.

The role of catecholamines on intercellular coupling, myocardial cell synchronization and self ventricular defibrillation

Ventricular fibrillation (VF) is one of the most life threatening events. Although in humans VF is generally sustained (SVF) requiring artificial defibrillation, in various mammals and in some cases in humans VF terminates by itself, reverting spontaneously into sinus rhythm. Since VF is one of the main causes of sudden death, one of the important clinical problems today is if and how we can transform the fatal SVF into a self limited transient one (TVF). From electrophysiological studies carried out on anaesthetized open chest animals, we have found that TVF requires a high degree of intercellular coupling and synchronization. Cardiac myocytes are electrically coupled with adjacent cells. The intercellular coupling is a focus of low electrical resistance which allows rapid transmission of electrical impulses between cells. Any decrease in intercellular coupling decreases the ability of the heart for self defibrillation. The cell-to-cell coupling decreases with age, ischemia, VF and variations in physiological conditions probably due to an increase in intercellular resistance (Ri), widening in the internexal gaps, decrease in electrotonic space constant (lambda) etc. All of these factors are known to be affected by intracellular concentration of free Ca++ ([Ca++]). On the basis of studies carried out on various mammals at different ages, we hypothesized that the ability of the heart to defibrillate depends on the cardiac catecholamine level [CA], during VF. This hypothesis is supported by the facts, known from the literature, that increase in [CA] decreases intracellular free Ca++ concentration, decreases Ri and increases lambda.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac excitability and antiarrhythmic drugs: a different perspective

A matrix of active and passive cellular properties determines net cardiac excitability. The hypothesis of altered excitability suggests that for cardiac arrhythmias to arise, the normal matrix must be perturbed by arrhythmogenic influences to produce a proarrhythmic matrical configuration to permit rhythm disturbances caused by abnormalities of propagation, abnormal automaticity, or altered excitability. Antiarrhythmic drugs may act with one or more components of the normal or proarrhythmic matrix to normalize or to create new antiarrhythmic or, perhaps, proarrhythmic matrices. Traditionally, antiarrhythmic drug classifications have been based on predominant drug actions. These classifications have clinical and some experimental utility but fail to consider the complicated effects that pathophysiologic influences and pharmacologic actions may have on active and passive cellular properties. Cluster analysis may allow the development of new classifications of arrhythmogenesis and antiarrhythmic drugs. The matrical concept has important clinical implications and suggest strategies for treating patients with cardiac rhythm disturbances.

Activation during ventricular defibrillation in open-chest dogs. Evidence of complete cessation and regeneration of ventricular fibrillation after unsuccessful shocks

To test the hypothesis that a defibrillation shock is unsuccessful because it fails to annihilate activation fronts within a critical mass of myocardium, we recorded epicardial and transmural activation in 11 open-chest dogs during electrically induced ventricular fibrillation (VF). Shocks of 1-30 J were delivered through defibrillation electrodes on the left ventricular apex and right atrium. Simultaneous recordings were made from septal, intramural, and epicardial electrodes in various combinations. Immediately after all 104 unsuccessful and 116 successful defibrillation shocks, an isoelectric interval much longer than that observed during preshock VF occurred. During this time no epicardial, septal, or intramural activations were observed. This isoelectric window averaged 64 +/- 22 ms after unsuccessful defibrillation and 339 +/- 292 ms after successful defibrillation (P less than 0.02). After the isoelectric window of unsuccessful shocks, earliest activation was recorded from the base of the ventricles, which was the area farthest from the apical defibrillation electrode. Activation was synchronized for one or two cycles following unsuccessful shocks, after which VF regenerated. Thus, after both successful and unsuccessful defibrillation with epicardial shocks of greater than or equal to 1 J, an isoelectric window occurs during which no activation fronts are present; the postshock isoelectric window is shorter for unsuccessful than for successful defibrillation; unsuccessful shocks transiently synchronize activation before fibrillation regenerates; activation leading to the regeneration of VF after the isoelectric window for unsuccessful shocks originates in areas away from the defibrillation electrodes. The isoelectric window does not support the hypothesis that defibrillation fails solely because activation fronts are not halted within a critical mass of myocardium. Rather, unsuccessful epicardial shocks of greater than or equal to 1 J halt all activation fronts after which VF regenerates.

Basic understanding of the electrophysiologic actions of antiarrhythmic drugs. Sources, sinks, and matrices of information

The author creates an intellectual framework consisting of key electrophysiologic principles, basic mechanisms of arrhythmogenesis, and important drug reactions that will allow the rational use of antiarrhythmic drugs. Basic principles have been emphasized because current understanding requires it.

Decreased intercellular coupling after prolonged rapid stimulation in rabbit atrial muscle

Driving rabbit atrial trabeculae at a rapid rate for 15 minutes resulted in a decrease in the space constant for electrotonic decay from an average of 670 to 440 micrometers. Input resistance, Rin, as measured by use of a double-barrelled microelectrode, increased from a mean value of 380 kOhms to one of 600 kOhms. The time to return to control values after the end of rapid driving was 20-60 minutes. Similar effects of rapid driving were observed in the presence of atropine, propranolol, and atropine plus propranolol and phentolamine. According to the theory of current spread in a three-dimensional syncytium, a rise of input resistance should be interpreted mainly as an increase of cell-to-cell resistance. We advance the hypothesis that, when driven at their maximal possible rate (or when fibrillating), cardiac cells gain Na+ ad Ca2+, and that this results in partial but reversible uncoupling.

Effects of verapamil on supraventricular tachycardia in patients with overt and concealed Wolff-Parkinson-White syndrome

Verapamil (0.15 mg/kg) intravenously, was administered to 19 patients with recurrent supraventricular tachycardia (SVT) undergoing electrophysiological evaluation. Twelve patients had overt Wolff-Parkinson-White (WPW) syndrome and seven patients had concealed accessory pathways conducting in the retrograde direction only. Verapamil had a significant effect in delaying conduction and prolonging refractoriness in the atrioventricular (AV) node, but no significant actions on any of the other cardiac tissues that formed the tachycardia circuit in these patients. In particular, it had no significant effects on anterograde or retrograde bypass conduction or refractoriness. Sustained SVT was initiated in 15 patients, and was terminated within 60 to 105 seconds of a 30-second injection of verapamil in 13 patients. Cycle length alternation during SVT was seen in six patients prior to reversion, and spontaneous ventricular complexes (VPCs) were observed following verapamil administration in five patients. Two patients with apparently normal sinus node function showed prolongation of their sinus node recovery times immediately following reversion of SVT by verapamil. Echo zones were assessed before and after verapamil, and sustained or self-terminating SVT could still be induced after the drug in 13 of the 15 patients who had sustained SVT beforehand. It was concluded that intravenous verapamil was effective in terminating sustained SVT in the majority of patients with overt or concealed WPW and that, despite a potential for sinus node depression and the initiation of VPCs, it had no clinically significant side effects. The ability to reinitiate SVT following its administration suggests the need for immediate follow-up with maintenance drug therapy.

Human ventricular refractoriness: effects of increasing current

The ventricular effective refractory period is commonly employed as a measurement of ventricular excitability. Because the current strength used to make this determination varies among laboratories, the relation of refractoriness and current was examined over a range of current strengths from 0.1 to 10 mA. Sixty determinations of refractoriness at variable current strengths were made in 40 patients using the extrastimulus technique with a rectangular pulse of 1 ms duration. These data were obtained by measuring the effective refractory period at threshold current and at 0.25 to 0.50 mA increments from threshold up to 10 mA. In these studies the drive stimulus (S1) and extrastimulus (S2) were kept at the same amplitude. In all patients the ventricular effective refractory period decreased as the current increased. The total decrease ranged from 8 to 100 ms (mean +/- standard deviation 36.9 +/- 17.1). The current strength at which the ventricular effective refractory period became fixed (that is, less than 2 ms change in ventricular effective refractory period with further increase in current strength) varied among the patients, but in all instances equaled or exceeded 1.8 mA, which in all but three patients was greater than three times threshold. The curves relating current strength and refractoriness were shifted to the left at shorter cycle lengths with no change in threshold. These data suggest that (1) current strength-effective refractory period curves more completely characterize ventricular excitability than does a ventricular effective refractory period at single current strength; and (2) studies of drug effects, alterations of autonomic tone, or reentrant arrhythmias, which may affect or are affected by ventricular refractoriness, may be enhanced by more complete measurements of refractoriness afforded by the current strength-effective refractoriness curves.

Cardiac muscle: excitability and passive electrical properties

The field of cellular cardiac electrophysiology has made excellent progress in the last decade in its effort to understand the electrical properties of the heart. It has profited from progress in membrane electrophysiology that has increased our understanding of the basic ionic mechanisms. It has developed quantitative methods for study of these mechanisms, in spite of the geometrical complexity of the heart. Indeed this complexity has added a richness and challenge to this area. We have shared with skeletal muscle the problem of electromechanical coupling. This review has discussed the present state of our knowledge of basic membrane mechanisms of importance in understanding normal and pathological function of heart muscle. It is intended to lay the groundwork for the following articles dealing with special aspects of cardiac electrophysiology. There is a continual interchange of ideas and concepts between applied and basic electrophysiology, and, as a result of this interchange, we can expect both areas to grow greatly in the coming years.