Effect of altering potassium concentration on the action of lidocaine and diphenylhydantoin on rabbit atrial and ventricular muscle

Lidocaine as an anti-arrhythmic drug: Are there any indications left?

Lidocaine is classified as a class Ib anti-arrhythmic that blocks voltage- and pH-dependent sodium channels. It exhibits well investigated anti-arrhythmic effects and has been the anti-arrhythmic of choice for the treatment of ventricular arrhythmias for several decades. Lidocaine binds primarily to inactivated sodium channels, decreases the action potential duration, and increases the refractory period. It increases the ventricular fibrillatory threshold and can interrupt life-threatening tachycardias caused by re-entrant mechanisms, especially in ischemic tissue. Its use was pushed into the background in the era of amiodarone and modern electric device therapy. Recently, lidocaine has come back into focus for the treatment of acute sustained ventricular tachyarrhythmias. In this brief overview, we review the clinical pharmacology including possible side effects, the historical course, possible indications, and current Guideline recommendations for the use of lidocaine.

Immediate and Delayed Response of Simulated Human Atrial Myocytes to Clinically-Relevant Hypokalemia

Although plasma electrolyte levels are quickly and precisely regulated in the mammalian cardiovascular system, even small transient changes in K⁺, Na⁺, Ca²⁺, and/or Mg²⁺ can significantly alter physiological responses in the heart, blood vessels, and intrinsic (intracardiac) autonomic nervous system. We have used mathematical models of the human atrial action potential (AP) to explore the electrophysiological mechanisms that underlie changes in resting potential (V_r) and the AP following decreases in plasma K⁺, [K⁺]_o, that were selected to mimic clinical hypokalemia. Such changes may be associated with arrhythmias and are commonly encountered in patients (i) in therapy for hypertension and heart failure; (ii) undergoing renal dialysis; (iii) with any disease with acid-base imbalance; or (iv) post-operatively. Our study emphasizes clinically-relevant hypokalemic conditions, corresponding to [K⁺]_o reductions of approximately 1.5 mM from the normal value of 4 to 4.5 mM. We show how the resulting electrophysiological responses in human atrial myocytes progress within two distinct time frames: (i) Immediately after [K⁺]_o is reduced, the K⁺-sensing mechanism of the background inward rectifier current (I_K1) responds. Specifically, its highly non-linear current-voltage relationship changes significantly as judged by the voltage dependence of its region of outward current. This rapidly alters, and sometimes even depolarizes, V_r and can also markedly prolong the final repolarization phase of the AP, thus modulating excitability and refractoriness. (ii) A second much slower electrophysiological response (developing 5-10 minutes after [K⁺]_o is reduced) results from alterations in the intracellular electrolyte balance. A progressive shift in intracellular [Na⁺]_i causes a change in the outward electrogenic current generated by the Na⁺/K⁺ pump, thereby modifying V_r and AP repolarization and changing the human atrial electrophysiological substrate. In this study, these two effects were investigated quantitatively, using seven published models of the human atrial AP. This highlighted the important role of I_K1 rectification when analyzing both the mechanisms by which [K⁺]_o regulates V_r and how the AP waveform may contribute to "trigger" mechanisms within the proarrhythmic substrate. Our simulations complement and extend previous studies aimed at understanding key factors by which decreases in [K⁺]_o can produce effects that are known to promote atrial arrhythmias in human hearts.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Antiarrhythmic and Proarrhythmic Properties of QT-Prolonging Antianginal Drugs

In recent years there has been a major reorientation of drug therapy for cardiac arrhythmias, its changing role, and above all, a radical change in the class of arrhythmia drugs because of their impact on mortality. The decline in the use of sodium-channel blockers has led to an expanding use of β-blockers and simple or complex class III agents for controlling cardiac arrhythmias. Success with these agents in the context of their side effects has spurred the development of compounds with simpler ion-channel blocking properties that have less complex adverse reactions. The resulting so-called pure class III agents, such as dofetilide or ibutilide, were found to have antifibrillatory effects in atrial fibrillation and flutter and in ventricular tachyarrhythmias. Such agents are effective and have diversity, but they have come into therapeutics with a price: the sometimes-fatal torsades de pointes. The drug amiodarone, a complex compound that was synthesized as an antianginal agent, has been an exception in this regard. Its therapeutic use is associated with a negligibly low incidence of torsades de pointes, even though the drug produces significant bradycardia and QT lengthening to 500 to 700 msec. Recent electrophysiologic studies suggest that this paradox is likely due to the differential block of ion channels in endocardium, epicardium, midmyocardial (M) cells, and Purkinje fibers in the ventricular myocardium. There is also clinical evidence suggesting that amiodarone reduces the “torsadogenic” effects of pure class III agents. Ranolazine was also synthesized for the development of antianginal properties that stem from a partial inhibition of fatty acid oxidation; it too has been found to have electrophysioloigic properties. These are somewhat similar to those of amiodarone on ion channels in endocardium, epicardium, M cells, and Purkinje fibers in the ventricular myocardium, but the drug does not prolong the QT interval to the same extent as amiodarone does. Thus, the drug produces modest increases in repolarization as judged by its effects on the action potential duration (APD) without the potential for the development of torsades de pointes. By virtue of its suppressant action on early afterdepolarizations and triggered activity in Purkinje fibers and M cells, the drug appears to have a powerful potential for reducing the torsadogenic proclivity of conventional class III antiarrhythmic compounds. The rationale for the therapeutic niche for amiodarone, and especially in the case of ranolazine, in the prevention of drug-induced torsades de pointes is discussed.

β-Adrenergic Blockers as Antiarrhythmic and Antifibrillatory Compounds: An Overview

β-Adrenergic blockers have a wide spectrum of action for controlling cardiac arrhythmias that is larger than initially thought. Data from the past several decades indicate that, as an antiarrhythmic class, β-blockers remain among the very few pharmacologic agents that reduce the incidence of sudden cardiac death, prolong survival, and ameliorate symptoms caused by arrhythmias in patients with cardiac disease. As a class of compounds, β-blockers have a fundamental pharmacologic property that attenuates the effects of competitive adrenergic receptors. However, the net clinical effects of the different β-receptor blockers may vary quantitatively because of variations in associated intrinsic sympathomimetic agonism and in their intrinsic potency for binding to β-receptors. These individual compounds also differ in their selectivity for β1- and β2-receptors. Metoprolol is a β1-selective blocker, whereas carvedilol is a nonselective β1- and β2-blocker, an antioxidant, and has a propensity to inhibit α1-receptors and endothelin. Evolving data from controlled and uncontrolled clinical trials suggest that there are clinically significant differences among this class of drugs. Recent evidence also suggests that the antiarrhythmic actions of certain β-receptor blockers such as carvedilol and metoprolol extend beyond the ventricular tissue to encompass atrial cells and help maintain sinus rhythm in patients with atrial fibrillation, especially in combination with potent antifibrillatory agents such as amiodarone. This introduction provides a current perspective on these newer developments in the understanding of the antiarrhythmic and antifibrillatory actions of β-blockers.

Asystole Associated with Lidocaine Use in a Hyperkalemic Patient during Advanced Cardiac Life Support

A case report of fatal asystole associated with use of lidocaine in a hyperkalemic patient is presented. The patient was a 61–year-old man with a rapidly increasing serum potassium level related to acute renal failure. Ventricular tachycardia with a pulse developed twice, for which lidocaine was administered according to the American Heart Association's ACLS protocol. Both episodes were immediately followed by asystole, the second of which was terminal. Available information suggests that this phenomenon can be explained by a synergistic effect on membrane responsiveness and conduction velocity. Thus, extreme caution should be exercised in the use of lidocaine when ventricular tachycardia complicates severe hyperkalemia.

Insulin Facilitates the Recovery of Myocardial Contractility and Conduction during Cardiac Compression in Rabbits with Bupivacaine-Induced Cardiovascular Collapse

Bupivacaine inhibits cardiac conduction and contractility. Insulin enhances cardiac repolarization and myocardial contractility. We hypothesizes that insulin therapy would be effective in resuscitating bupivacaine-induced cardiac toxicity in rabbits. Twelve rabbits were tracheally intubated and midline sternotomy was performed under general anesthesia. Cardiovascular collapse (CVC) was induced by an IV bolus injection of bupivacaine 10 mg/kg. The rabbits were treated with either saline (control) or insulin injection, administered as a 2 U/kg bolus. Internal cardiac massage was performed until the return of spontaneous circulation (ROSC) and the time to the return of sinus rhythm (ROSR) was also noted in both groups. Arterial blood pressure, and electrocardiography were continuously monitored for 30 min and plasma bupivacaine concentrations at every 5 min. The ROSC, ROSR and normalization of QRS duration were attained faster in the insulin-treated group than in the control group. At the ROSC, there was a significant difference in bupivacaine concentration between two groups. Insulin facilitates the return of myocardial contractility and conduction from bupivacaine-induced CVC in rabbits. However, recovery of cardiac conduction is dependent mainly on the change of plasma bupivacaine concentrations.

Mechanisms of hypokalemia-induced ventricular arrhythmogenicity

Hypokalemia is a common biochemical finding in cardiac patients and may represent a side effect of diuretic therapy or result from endogenous activation of renin-angiotensin system and high adrenergic tone. Hypokalemia is independent risk factor contributing to reduced survival of cardiac patients and increased incidence of arrhythmic death. Animal studies demonstrate that hypokalemia-induced arrhythmogenicity is attributed to prolonged ventricular repolarization, slowed conduction, and abnormal pacemaker activity. The prolongation of ventricular repolarization in hypokalemic setting is caused by inhibition of outward potassium currents and often associated with increased propensity for early afterdepolarizations. Slowed conduction is attributed to membrane hyperpolarization and increased excitation threshold. Abnormal pacemaker activity is attributed to increased slope of diastolic depolarization in Purkinje fibers, as well as delayed afterdepolarizations caused by Ca2+ overload secondary to inhibition of Na+--K+ pump and stimulation of the reverse mode of the Na+--Ca2+ exchange. Hypokalemia effect on repolarization is not uniform at distinct ventricular sites thereby contributing to amplified spatial repolarization gradients which promote unidirectional conduction block. In hypokalemic heart preparations, the prolongation of action potential may be associated with shortening of effective refractory period, thus increasing the propensity for ventricular re-excitation over late phase of repolarization. Shortened refractoriness and slowed conduction contribute to reduced excitation wavelength thereby facilitating re-entry. The interplay of triggering factors (early and delayed afterdepolarizations, oscillatory prepotentials in Purkinje fibers) and a favorable electrophysiological substrate (unidirectional conduction block, reduced excitation wavelength, increased critical interval for ventricular re-excitation) may account for the mechanism of life-threatening tachyarrhythmias in hypokalemic patients.

Predictive value of electrical restitution in hypokalemia-induced ventricular arrhythmogenicity

The ventricular action potential (AP) shortens exponentially upon a progressive reduction of the preceding diastolic interval. Steep electrical restitution slopes have been shown to promote wavebreaks, thus contributing to electrical instability. The present study was designed to assess the predictive value of electrical restitution in hypokalemia-induced arrhythmogenicity. We recorded monophasic APs and measured effective refractory periods (ERP) at distinct ventricular epicardial and endocardial sites and monitored volume-conducted ECG at baseline and after hypokalemic perfusion (2.5 mM K+ for 30 min) in isolated guinea pig heart preparations. The restitution of AP duration measured at 90% repolarization (APD90) was assessed after premature extrastimulus application at variable coupling stimulation intervals, and ERP restitution was assessed by measuring refractoriness over a wide range of pacing rates. Hypokalemia increased the amplitude of stimulation-evoked repolarization alternans and the inducibility of tachyarrhythmias and reduced ventricular fibrillation threshold. Nevertheless, these changes were associated with flattened rather than steepened APD90 restitution slopes and slowed restitution kinetics. In contrast, ERP restitution slopes were significantly increased in hypokalemic hearts. Although epicardial APD90 measured during steady-state pacing (S1-S1 = 250 ms) was prolonged in hypokalemic hearts, the left ventricular ERP was shortened. Consistently, the epicardial ERP measured at the shortest diastolic interval achieved upon a progressive increase in pacing rate was reduced in the hypokalemic left ventricle. In conclusion, this study highlights the superiority of ERP restitution at predicting increased arrhythmogenicity in the hypokalemic myocardium. The lack of predictive value of APD90 restitution is presumably related to different mode of changes in ventricular repolarization and refractoriness in a hypokalemic setting, whereby APD90 prolongation may be associated with shortened ERP.

Inactivation of sodium channels underlies reversible neuropathy during critical illness in rats

Neuropathy and myopathy can cause weakness during critical illness. To determine whether reduced excitability of peripheral nerves, rather than degeneration, is the mechanism underlying acute neuropathy in critically ill patients, we prospectively followed patients during the acute phase of critical illness and early recovery and assessed nerve conduction. During the period of early recovery from critical illness, patients recovered from neuropathy within days. This rapidly reversible neuropathy has not to our knowledge been previously described in critically ill patients and may be a novel type of neuropathy. In vivo intracellular recordings from dorsal root axons in septic rats revealed reduced action potential amplitude, demonstrating that reduced excitability of nerve was the mechanism underlying neuropathy. When action potentials were triggered by hyperpolarizing pulses, their amplitudes largely recovered, indicating that inactivation of sodium channels was an important contributor to reduced excitability. There was no depolarization of axon resting potential in septic rats, which ruled out a contribution of resting potential to the increased inactivation of sodium channels. Our data suggest that a hyperpolarized shift in the voltage dependence of sodium channel inactivation causes increased sodium inactivation and reduced excitability. Acquired sodium channelopathy may be the mechanism underlying acute neuropathy in critically ill patients.

Atrial fibrillation: epidemiologic considerations and rationale for conversion and maintenance of sinus rhythm

Atrial fibrillation is now the most common cardiac arrhythmia for which a patient is hospitalized. Clinically, it presents in a form that is paroxysmal, persistent, or permanent and may be symptomatic or asymptomatic, occurring in the setting of either no cardiac disease ("lone atrial fibrillation") or, most often, in association with an underlying disease. Atrial fibrillation is associated with a 2-fold increase in mortality and, in the United States alone, causes over 75,000 cases of stroke per year. The annual prevalence of stroke is 5% to 7%, but the use of adequate anticoagulation can reduce this to less than 1%. Atrial fibrillation is a disorder of the elderly, with almost equal prevalence in men and women. In the United States, 80% of atrial fibrillation occurs in patients over the age of 65 years, and its prevalence tracks that of heart failure, which may be the cause, as well as the result, of the arrhythmia. Both conditions are increasing in epidemic proportions in the aging population. The most common causes of atrial fibrillation are hypertensive heart disease, coronary artery disease, and heart failure with a miscellany of lesser conditions, with about 10% lacking structural heart disease. Unlike other supraventricular arrhythmias, cure by the use of catheter ablation and surgical techniques has not been a reality except in a relatively small number of cases. However, restoration and maintenance of sinus rhythm remain the initial goal of therapy for most patients. Pharmacologic approaches remain the mainstay of therapy for rate control and anticoagulation as well as for maintenance of sinus rhythm following pharmacological or electrical conversion. The changing epidemiology of atrial fibrillation is highlighted, with the focus on its conversion by the use of newer and novel antifibrillatory agents relative to the mechanisms of the arrhythmia, to restore the stability of sinus rhythm.

Mechanisms of antiarrhythmic drug actions and their clinical relevance for controlling disorders of cardiac rhythm

This review on antiarrhythmic drugs traces the evolution of the fundamental mechanisms of action of drugs that have been used to control disorders of cardiac rhythm. It describes the very earliest data from experimental studies that dealt with the effects of acute and chronic administration of drugs in whole animals combined with the measurements of the action potential duration and the effective refractory period in isolated tissues. Antiarrhythmic drugs were found to have properties consistent with the block of fast sodium channel conduction, adrenergic blockade, repolarization block, and the block of slow-channel mediated conduction especially in the atrioventricular node. Over the past 15 years, the attention has focused on atrial tissue with atrial fibrillation emerging as the most common arrhythmia in clinical practice. Drug-induced increases in refractoriness as a function rate and in wavelength (product of refractoriness and conduction velocity), and a reduction in numbers of wavelets have been found to be critical in the conversion of atrial fibrillation and maintenance of sinus rhythm. The continued development of newer pharmacologic agents is likely to lead to the resolution of the controversy regarding rhythm versus rate control in various clinical subsets of the arrhythmia by controlled clinical trials.

Insulin reverses bupivacaine-induced cardiac depression in dogs

We tested the hypothesis that an insulin infusion would effectively treat bupivacaine-induced cardiac depression in dogs. In 24 mongrel dogs anesthetized with pentobarbital (5 mgkg(-1)h(-1), IV), 0.5% bupivacaine was administrated at a rate of 0.5 mgkg(-1)min(-1) until the mixed venous oxygen saturation decreased to 60% or less. The bupivacaine infusion induced a decrease in mean arterial pressure, cardiac output, and heart rate. The dogs were randomly assigned to one of four groups after the end of bupivacaine infusion. The Control (C, n = 6) and Glucose (G, n = 6) groups received an IV infusion of normal saline (2 mL/kg) and glucose (2 mL/kg of 50% dextrose in water) for 15 min, respectively. The Insulin-Glucose (IG, n = 6) group received an IV bolus of regular insulin (1 U/kg), plus a glucose infusion (2 mL/kg of 50% dextrose in water) for 15 min. The Insulin-Glucose-Potassium (IGK, n = 6) group received the same dose of insulin and glucose as the IG group, plus potassium (1-3 mEqkg(-1)h(-1)). Mean arterial pressure, cardiac output, heart rate, and mixed venous oxygen saturation recovered toward baseline level more rapidly in the IG and IGK groups than in the C group (within 5 min versus more than 20 min). These results suggest that the infusion of insulin and glucose might reverse bupivacaine-induced cardiac depression in dogs.

IMPLICATIONS

We found that insulin and glucose rapidly reversed hemodynamic abnormality in dogs with bupivacaine-induced cardiac depression. This study implies a possible clinical application of insulin treatment for bupivacaine-induced cardiac depression.

Presynaptic inhibitory actions of lignocaine in canine isolated, blood-perfused atrial preparations

1. Cardiac effects of lignocaine on sinoatrial nodal pacemaker activity and atrial contractility were investigated in five canine isolated, blood-perfused right atria that were perfused with heparinized blood from support dogs. The effects of lignocaine on responses to intracardiac nerve stimulation and administered acetylcholine and noradrenaline were also examined. 2. Lignocaine was injected into the support dog intravenously or administered selectively to the sinus node artery of the isolated atrium. At doses that did not produce significant depressor action (0.3, 1.0 and 3.0 mg/kg), lignocaine produced no significant changes in heart rate. A large dose of 10 mg/kg lignocaine caused significant depressor effects and slight bradycardia. Direct administration of lignocaine (0.3, 1.0, 3.0, 10.0 and 30.0 mumol) into the sinus node artery of the isolated atrium consistently caused slight negative chronotropic and rather marked negative inotropic effects. 3. After treatment with a relatively large dose of lignocaine, electrical stimulation-induced negative chronotropic and inotropic responses were significantly inhibited in a dose-related manner, but positive chronotropic and inotropic responses were slightly depressed only at an extremely high dose of lignocaine (10.0 mumol). 4. Noradrenaline-induced positive chronotropic and inotropic effects were not modified by any doses of lignocaine used (0.3, 1.0, 3.0 and 10.0 mumol). Acetylcholine-induced negative chronotropic and inotropic effects were slightly, but significantly, depressed by 10 mumol lignocaine. 5. These results suggest that a relatively large dose of lignocaine has a dominant presynaptic inhibitory action, particularly on the parasympathetic component.

Evolution, mechanisms, and classification of antiarrhythmic drugs: focus on class III actions

Since the use of cinchona bark to treat heart palpitations in the 1700s, antiarrhythmic drug therapy has developed with the discovery of new compounds and the identification of ionic, cellular, and tissue mechanisms of action. Classifications have been developed that organize the large amount of information available about antiarrhythmic drugs around groups of compounds with common mechanisms of action. Despite important and well-recognized limitations, antiarrhythmic drug classification is still widely used. In particularly broad use is the system developed by Singh and Vaughan Williams in the early 1970s and subsequently modified by Singh and Hauswirth and by Harrison. This classification divides drug actions into class I for sodium-channel blockade (with subclasses IA, IB and IC), class II for adrenergic antagonism, class III for action-potential prolongation, and class IV for calcium-channel blockade. The development of class I drugs was curtailed when studies showed that potent sodium-channel blockers (particularly IC agents) can increase mortality in patients with active coronary artery disease. The emphasis in drug development shifted to class III agents, but their use has been limited by the risk of ventricular tachyarrhythmia induction associated with QT prolongation. Current research focuses on the development of new class III drugs that may have improved safety by virtue of greater selectivity of action at faster rates (like those of arrhythmia) or for atrial tissue. Alternative approaches include the modification of existing molecules (like amiodarone) to maintain positive properties while removing undesirable ones, and treatments that target development of the arrhythmia substrate instead of the final electrical product.

The effects of lignocaine and tetrodotoxin on the action potentials and contractions of left ventricles from normo- and hypertensive rats

The objective was to test the hypothesis that the effects of the sodium channel blockers lignocaine and tetrodotoxin are modified in the presence of hypertension-induced hypertrophy. We describe the effects of lignocaine and tetrodotoxin on the action potentials and contractions of left ventricles isolated from 6-month-old Wistar Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). The upstroke velocity, amplitude, and overshoot of the action potential were reduced; action potentials were prolonged; and the contractions were reduced on the hypertrophied left ventricles of the SHRs. Lignocaine and tetrodotoxin reduced the upstroke velocity, amplitude, and overshoot and prolonged the left ventricular action potentials. These effects of lignocaine and tetrodotoxin on the SHR were less than those on the WKY left ventricle, possibly because the action potential was already modified by hypertrophy. Lignocaine also reduced the left ventricular contractions and the concentrations producing this reduction were lower for the hypertrophied than those for the normal left ventricle. Tetrodotoxin at 3 x 10(-6)-10(-5) M caused similar attenuation of the WKY and SHR left ventricle contractions. Our study shows that the effects of lignocaine on contraction are enhanced in the hypertrophied left ventricle of the SHR, which suggests that the binding is increased or the access of lignocaine to the receptor is enhanced in hypertrophy. In contrast, the effects of tetrodotoxin on contractions are similar, and thus the binding or access of tetrodotoxin to the receptor is not altered in the hypertrophied left ventricle of the SHR.

Antiarrhythmic drugs: a reorientation in light of recent developments in the control of disorders of rhythm

Numerous developments in our knowledge of arrhythmias during the past decade or so have had a major influence on antiarrhythmic drug therapy. It has become increasingly evident that arrhythmias merit treatment not only for the relief of symptoms, with improvement in quality of life, but also for the prolongation of survival by decreasing arrhythmic deaths. No longer can mere suppression of arrhythmias, symptomatic or asymptomatic, be equated with prolonged survival. We now know that antiarrhythmic drugs that act by blocking sodium channels can increase mortality and that the most important determinants of arrhythmia mortality are the degree and nature of ventricular dysfunction. To these considerations must be added the advances in nonpharmacologic approaches to controlling cardiac arrhythmias. There has been a shift to the use of implantable devices and of drugs with alternative modes of action, such as beta blockers and class III drugs (e.g., sotalol, amiodarone). However, the side-effect profiles of these 2 classes of compounds have led to the synthesis and characterization of agents that act simply by blocking > or = 1 membrane ion channels. The isolated block of the rapid component of the delayed rectifier potassium current (IKr) has been associated with potent antifibrillatory activity in the atria, with a neutral (e.g., with dofetilide) or deleterious (with d-sotalol) effect on mortality in postinfarct survivors. Therefore, the focus now is on compounds that can block > 1 ion channel (e.g., tedisamil and azimilide). Azimilide is the first of the class III agents that blocks both components of the delayed rectifier potassium current. The drug's overall action is associated with a spectrum of electrophysiologic properties that hold promise in the control of atrial and ventricular arrhythmias, with potential for improving survival in patients at risk for cardiac arrest.

Controlling cardiac arrhythmias: an overview with a historical perspective

During the past decade, several developments in our knowledge of antiarrhythmic drugs have had a major influence on our approach to their use. These developments may be summarized as follows: (1) it has become clear that arrhythmias merit treatment only for the relief of symptoms, with improved quality of life, and for prolongation of survival by reducing arrhythmic deaths; (2) suppression of arrhythmias--symptomatic or asymptomatic--may not necessarily decrease mortality, the net impact on mortality being agent-specific; (3) antiarrhythmic drugs have the propensity to decrease as well as to increase cardiac arrhythmias (producing proarrhythmias); (4) the most important determinant of arrhythmia mortality is the degree and nature of ventricular dysfunction; and (5) only controlled trials have the potential to establish the effect of treatment on mortality in patients with cardiac arrhythmias. To these considerations must be added the advances in nonpharmacologic approaches to controlling cardiac arrhythmias. These include catheter ablation of cardiac arrhythmias, certain surgical techniques that in selected patients offer prospects of cure, and the development of implantable ventricular and atrial cardioverter defibrillators, which allow the evaluation of drugs versus placebo against the background of the defibrillator. This is particularly germane in the case of life-threatening symptomatic ventricular arrhythmias such as sustained ventricular tachycardia and ventricular fibrillation. Antiarrhythmic drugs and implantable devices in the control of arrhythmias cannot be considered in isolation. Their role in mortality reduction needs to be defined alone as well as in combination by controlled clinical trials.

Mechanisms of action of antiarrhythmic drugs: from ion channel blockage to arrhythmia termination

Over the past decade, the strategies for arrhythmia management have been in transition. Physical methods to treat arrhythmias, such as ICDs and RF ablation, have undergone considerable refinement and wider application. Ischemic heart disease and congestive heart failure have been identified as clinical situations in which antiarrhythmic drugs have a significant proarrhythmic potential. However, drugs retain an important role in arrhythmia management. Strategies to mitigate the structural and functional changes that occur in hypertrophy, ischemia, and infarction have not been thoroughly explored. Membrane ion channels and receptors are the targets for the action of currently available drugs. The cloning and sequencing of these ion channels and receptors should improve the efficacy and specificity of drug design. Cardiac Na+, Ca2+, K+, and nonspecific cation channels have a clearly defined role in the generation of the normal action potential. Their specific roles in the various clinical arrhythmias is less certain. There are sufficient data to associate specific ionic channels with normal and abnormal automaticity and with reentry occurring in specific regions of the heart. A rational choice of antiarrthymic drugs can be made when an arrhythmogenic mechanism and the putative underlying membrane currents can be identified based on the clinical characteristics of the arrhythmia. For a majority of clinical arrhythmias, this ideal has not been achieved. When a particular drug is used to treat an arrhythmia, the full complement of its actions will depend on which multiple ion channels or receptors are blocked and the kinetics of drug interaction with these sites.

The effects of lidocaine on cardiac parasympathetic control in normal subjects and in subjects after myocardial infarction

It has been widely accepted that lidocaine has little or no effect on the autonomic nervous system. However, we have previously shown that in dogs with vagally induced atrial fibrillation, lidocaine has a pronounced parasympatholytic effect. To study the possible effect of lidocaine on autonomic cardiac control in humans, we performed spectral analysis of heart rate fluctuations in 19 healthy volunteers, who received an i.v. bolus of lidocaine (1.4 mg/kg), as well as in 13 patients suffering from acute inferior myocardial infarction (IMI) and 13 patients suffering from acute anterior myocardial infarction (AMI), who received therapeutic doses of i.v. lidocaine infusion (4 mg/min). Heart rate variability and respiratory pattern were monitored according to a predetermined protocol, with and without lidocaine. Computing the heart rate power spectrum and integrating over predetermined frequency bands, we focused mainly on the respiratory frequency band, known to predominantly reflect parasympathetic control. The administration of lidocaine resulted in a significant overall increase in mean heart rate: for the healthy control group an increase of 5.5 +/- 2.2% (mean +/- SE), for the IMI group an increase of 9.4 +/- 3.5%, and for the AMI group an increase of 8.1 +/- 2.9% (p < 0.01 for all). Simultaneously, following the administration of lidocaine, there was a decrease in the power of respiratory fluctuations: for the healthy control group a decrease of 38.4 +/- 12.5%, for the IMI group a decrease of 46.3 +/- 32.9%, and for the AMI group a decrease of 33.9 +/- 16.2% (p < 0.01 for all). These findings indicate that lidocaine has a consistent and significant parasympatholytic effect on the human heart, in healthy volunteers as well as in patients in the acute phase of myocardial infarction.

The coming of age of the class III antiarrhythmic principle: retrospective and future trends

Antiarrhythmic drug therapy is in a state of continuous flux. In the last decade or so, numerous experimental and clinical studies have revealed that drugs that act by delaying conduction, while markedly suppressing ventricular arrhythmias, have the proclivity to increase mortality in subsets of patients with significant cardiac disease. The adverse impact on mortality was confirmed by placebo-controlled randomized trials as well as meta-analysis of smaller randomized clinical trials. The latter indicated that beta blockers exert a beneficial effect on mortality. Benefit from drugs that lengthen repolarization, especially drugs that have the additional property of blocking sympathetic excitation, was also seen in relatively small numbers of patients. Sotalol and amiodarone fell into this category of antiarrhythmic drugs. There were 2 major consequences that stemmed from the results of these trials. First, the endpoint of clinical trials shifted to total mortality from surrogates such as defined degree of suppression of ventricular arrhythmias. Second, concern regarding increases in mortality produced by class I drugs engendered a shift in favor of drugs that prolong repolarization. Such a shift was bolstered by the growing body of data that established the efficacy of sotalol and amiodarone as potent agents for the control of life-threatening ventricular arrhythmias. They were both found to be superior to class I agents. The perception that the critical factor that mediates their efficacy is the homogeneous prolongation of repolarization has led to the synthesis and characterization of so-called pure class III agents, which include d-sotalol and other lKr blockers such as dofetilide, sematilide, E-4031, and almokalant, among numerous others. The increase in mortality produced by d-sotalol in patients with myocardial infarction and lowered ejection fraction and in patients with and without heart failure has led researchers to question how to design future antiarrhythmic molecules. In the search for an ideal antifibrillatory agent, should emphasis be placed on simple molecules such as pure class III agents or on those with more complex profiles, such as sotalol and amiodarone, which exhibit antiadrenergic actions and the ability to prolong cardiac repolarization? The available data are in favor of the latter approach.

The effects of veratridine and BDF 9148 on the action potentials and contractility of the rat right ventricle

1. The effects of veratridine, BDF 9148 and lignocaine on the action potentials and contractile force of the electrically-driven rat right ventricle have been determined. 2. Veratridine at 10(-7)-10(-6) M and BDF 9148 at 10(-7)-10(-5) M had no effect on the threshold potential or amplitude but prolonged the ventricular action potentials. 3. In contractility studies, veratridine at 10(-7)-10(-6) M augmented the cardiac stimulation responses and the augmenting effects with 3 x 10(-7) and 10(-6) M were greater at 2 than 4 Hz. In the presence of veratridine at 3 x 10(-6) M, the ventricle would not pace. 4. BDF 9148 at 10(-7)-10(-5) M augmented the cardiac stimulation responses and the augmenting effects with 10(-7) and 3 x 10(-7) M were greater at 2 than 4 Hz and the effect was maximal at 3 x 10(-7) M and submaximal at 10(-5) M. The effects of BDF 9148 at 10(-5) were not readily reversible. 5. Lignocaine at 10(-4) M had no effect on the ventricular action potential duration but decreased the threshold potential and amplitude and also reduced the cardiac stimulation force responses. In the presence of lignocaine, the augmenting effects of veratridine and BDF 9148 on ventricular force were reduced. 6. In summary this study has shown that BDF 9148 prolongs the action potential and augments the contractile force responses of the rat right ventricle by a lignocaine-sensitive mechanism. BDF 9148 or similar drugs may have potential as positive inotropes in the treatment of heart failure.

Modulation of procainamide's effect on cardiac conduction in dogs by extracellular potassium concentration. A quantitative analysis

BACKGROUND

Antiarrhythmic drugs are known to have state-dependent interactions with cardiac sodium channels, and these have potentially important implications for drug effects on cardiac conduction, particularly in situations of changed resting potential and heart rate. Recent advances in theoretical approaches permit beat-to-beat changes in sodium channel block to be inferred from conduction changes in vivo and allow for an analysis of state-dependent drug action from conduction changes occurring on the onset of pacing at different rates. The purpose of the present study was to use this method to analyze the interaction between hyperkalemia and procainamide's sodium channel-blocking action in terms of resulting changes in left ventricular conduction.

METHODS AND RESULTS

Epicardial mapping with a 56-electrode array was used to assess ventricular conduction in open chest, anesthetized mongrel dogs with Formalin-induced atrioventricular block. Procainamide was infused as a series of loading and maintenance infusions until at least 20% conduction slowing was obtained at the shortest basic cycle length (300 milliseconds). Results in a control set of normokalemic dogs were compared with results in dogs with moderate hyperkalemia produced by a loading and maintenance infusion of potassium chloride. Plasma procainamide concentration was measured by high-performance liquid chromatography, and the constancy of serum potassium concentration was verified with ion-sensitive electrode measurement. Although hyperkalemia itself (mean +/- SEM potassium concentration, 6.64 +/- 0.66 mmol/L) did not alter conduction, it resulted in substantially increased conduction slowing by procainamide despite substantially lower plasma drug concentrations (102 +/- 10 mumol/L) compared with normokalemic dogs (potassium concentration, 3.87 +/- 0.24 mmol/L; procainamide concentration, 277 +/- 16 mumol/L). The onset of conduction slowing and block followed basic molecular theory, with an exponential time constant that was faster at longer cycle lengths and total block that increased as cycle length decreased. Piecewise exponential analysis of block during the rested and depolarized phases of the action potential showed that the enhancement of procainamide's action by hyperkalemia was due almost exclusively to increased rested-phase block. Hyperkalemia produced a bradycardia-dependent and slight reduction in action potential duration and antagonized the action potential-prolonging effect of procainamide, particularly at shorter cycle lengths.

CONCLUSIONS

Hyperkalemia strongly enhances procainamide-induced conduction slowing by increasing the interaction between the drug and sodium channels during the rested phase of the cardiac cycle. These results indicate the applicability of basic molecular theories of antiarrhythmic drug action to understanding drug-induced changes in conduction velocity in vivo and highlight the potential importance of heterogeneous magnification of sodium channel-blocking drug action by the spatially variable hyperkalemia that occurs with acute myocardial ischemia. The latter could play an important role in the known proarrhythmic potential of sodium channel-blocking drugs in patients with coronary artery disease.

Propranolol and lidocaine inhibit neural norepinephrine release in hearts with increased extracellular potassium and ischemia

BACKGROUND

Propranolol and lidocaine are effective antiarrhythmic drugs in myocardial ischemia and infarction. As sympathetic nerve activation and norepinephrine release in ischemic hearts are arrhythmogenic, we tested the possibility that both agents inhibit neural norepinephrine release following sympathetic activation in the ischemic environment.

METHODS AND RESULTS

The model used was an in situ perfused innervated rat heart. Norepinephrine release was induced by electrical stimulation of the left cervicothoracic stellate ganglion and analyzed using radioenzymatic assay or high-performance liquid chromatography. In normoxically perfused hearts, evoked norepinephrine release was not affected by either of the two agents at doses of 1 to 10 mumol/L when extracellular K+ concentration was 4 mmol/L but dose-dependently reduced at 10 mmol/L K+ (D,L-propranolol: -53 +/- 4% at 1 mumol/L and -64 +/- 6% at 10 mumol/L K+, lidocaine: -37 +/- 11% at 0.1 mumol/L, -67 +/- 5% at 1 mumol/L, and -75 +/- 6% at 10 mumol/L). At 10 mmol/L K+, norepinephrine release was not affected by timolol or atenolol (both 10 mumol/L but was equally inhibited by D- or L-propranolol at 10 mumol/L (-56 +/- 5% and -53 +/- 9%, respectively), indicating a beta-blocking-independent mechanism. In hearts with metabolic acidosis (pH 6.85) at K+ of 4 mmol/L, neural norepinephrine release was also reduced by propranolol at 10 mumol/L (-37%). Finally, in hearts perfused with 4 mmol/L K+ and subjected to 6-minute periods of ischemia, neural norepinephrine release was similarly suppressed by D,L-propranolol (-38 +/- 6% at 0.1 mumol/L, -44 +/- 5% at 1 mumol/L, and -78 +/- 3% at 10 mumol/L) or lidocaine (-39 +/- 7% at 0.1 mumol/L, -58 +/- 9% at 1 mumol/L, and -91 +/- 3% at 10 mumol/L).

CONCLUSIONS

These data indicate that propranolol and lidocaine inhibit neural norepinephrine release via a Na+ channel-blocking mechanism that is synergistic with changes induced by ischemia, primarily raised extracellular K+. This mechanism may contribute to the anti-ischemic and antiarrhythmic properties of both agents in acute myocardial ischemia, which induces increased extracellular K+ and sympathetic activation.

An undocumented effect of lidocaine revealed by computerized electrocardiography

The influence of an intravenous push of lidocaine 1 mg/kg followed by 2 mg/min infusion on total QRS amplitude and R-to-S duration in leads V2-V4 of the electrocardiogram was studied in 11 patients who were hospitalized in the coronary care unit. A computerized system sampled the amplitude and timing of peak R and S waves of 1100 QRS complexes of leads V2-V4, starting two minutes before the first push of lidocaine. Curves of amplitude and R-to-S duration versus time were obtained in all patients. QRS amplitude in V2 and V3 increased significantly in 10 of 11 patients (p < 0.005). The mean voltage increase for all patients was 0.1 +/- 0.07 millivolt. The authors also observed a mild prolongation of R-to-S duration (0.8 +/- 1.0 millisecond p < 0.02). This new finding, obtained by a simple computerized system, may possibly have potential future practical clinical application as a feedback parameter for closed-loop control systems for the administration of lidocaine.

Aggravation of arrhythmia: a complication of antiarrhythmic drugs

Aggravation of arrhythmia, defined as worsening of a preexisting arrhythmia or the occurrence of a new arrhythmia, is a common complication of antiarrhythmic drug therapy. Although it is largely an unpredictable event, patients at greatest risk are those with a history of congestive heart failure due to systolic dysfunction who present with a sustained ventricular tachyarrhythmia. As a rule, aggravation of arrhythmia is an early event, occurring within the first few days of initiating therapy. However, in the Cardiac Arrhythmia Suppression Trial (CAST), the increased sudden death mortality due to drug therapy, which was a result of arrhythmia aggravation, occurred throughout the entire duration of the trial, suggesting that arrhythmia aggravation can also be a late complication of therapy. Also disturbing was the fact that patients in CAST were low risk and did not have congestive heart failure or a serious ventricular tachyarrhythmia. This suggests that another important risk factor is myocardial ischemia and its potentially dangerous interaction with antiarrhythmic drugs. In patients with heart disease, especially those with coronary artery disease, antiarrhythmic drugs must therefore be used cautiously. Close and continuous follow-up is mandatory.

Effects of ibutilide fumarate, a novel antiarrhythmic agent, and its enantiomers on isolated rabbit myocardium

Ibutilide fumarate is currently in Phase II clinical trials for the treatment of life-threatening cardiac arrhythmias. The cardiovascular effects of ibutilide and its d- and l-stereoisomers, U82208E and U82209E were tested in an isolated rabbit myocardium system. In a series of repeated measures experiments, threshold, effective refractory period, force of contraction, conduction time and rate were measured at various pacing frequencies in isolated papillary muscles, ventricular muscle strips and right atria exposed to 10(-7), 10(-6) and 10(-5) M drug. Although there were occasional instances where one form had a greater or lesser effect on a given parameter, overall there was little pharmacological difference between the racemic mixture and its constituent forms. At the highest dose, effective refractory periods at 1 and 3 Hz increased by 18-32 ms, conduction times measured at 3 Hz increased by 27-30% and atrial rate decreased by 19-32%, while threshold and force of contraction were generally unaffected. In this study there were no clear cut pharmacologic differences between the three forms of this class III antiarrhythmic agent. Parallel studies to determine pA2 values of ibutilide and sotalol demonstrated that ibutilide possesses weak beta-adrenoceptor blocking properties.

Subclassification of class I antiarrhythmic drugs: enhanced relevance after CAST

Class I antiarrhythmic drugs are traditionally divided into three subclasses--Ia, Ib, and Ic--on the grounds of differences in kinetics of interaction with the sodium channel and different effects on the duration of the action potential. The CAST study has highlighted our growing awareness of the proarrhythmic potential of this group of agents, particularly the Class Ic subgroup. Class I drugs can cause arrhythmias either by slowing conduction to critical levels, thus enhancing the possibility of reentrant arrhythmias, or in some cases by prolonging action potential duration, leading to early afterdepolarizations, which probably underlie triggered automaticity. Evidence is presented that the Class Ic compounds may be inherently more proarrhythmic than the Ib compounds, because of their lesser ability to depress ischemic myocardium selectively. Arguments are advanced for the continued use of a slightly modified subclassification of Class I antiarrhythmic drugs.

Selective prolongation of QRS late potentials by sodium channel blocking antiarrhythmic drugs: relation to slowing of ventricular tachycardia. Electrophysiologic Study Versus Electrocardiographic Monitoring Trial (ESVEM) Investigators

Sodium channel blocking antiarrhythmic drugs have preferential effects on diseased, slowly conducting myocardium, and slowing of tachycardia caused by these drugs may result primarily from further prolongation of conduction time in slowly conducting tissue. In patients with sustained ventricular tachycardia, late potentials detected by signal-averaged electrocardiography (ECG) are thought to arise from slowly conducting ventricular myocardium. This study tested the hypothesis that sodium channel blocking drugs selectively prolong the late potential, or terminal low amplitude signal, portion of the signal-averaged QRS complex and that prolongation of the late potential would correlate with slowing of ventricular tachycardia. Fifty-six drug trials in 32 patients with spontaneous and inducible ventricular arrhythmias were studied. Prolongation of the late potential (11 +/- 15 ms) was significantly greater than prolongation of the initial portion of the QRS complex (4 +/- 9 ms) (p = 0.01). Selective prolongation of the late potential by drugs resulted in significantly greater QRS prolongation detectable by signal-averaged ECG than by standard ECG (p less than 0.0001). In 40 trials in which ventricular tachycardia remained inducible during drug therapy, the increase in induced tachycardia cycle length correlated strongly with the increase in late potential duration (p = 0.005) but not with change in the initial portion of the QRS complex. These data suggest that in patients with ventricular tachycardia, sodium channel blocking antiarrhythmic drugs have preferential effects on slowly conducting tissue and that drug effect on slowly conducting tissue contributes to prolongation of ventricular tachycardia cycle length.

Thoracic epidural anaesthesia combined with enflurane anaesthesia reduces atrioventricular conduction in dogs

Cardiac electrophysiological variables during thoracic epidural lidocaine (TEL) were compared with those during continuous intravenous lidocaine (IVL) infusion in 14 mongrel dogs anaesthetized with enflurane in order to investigate the combined effects of thoracic epidural anaesthesia (TEA) and enflurane anaesthesia on intracardiac conduction. Thoracic epidural lidocaine suppressed intracardiac conduction. Sinus cycle length (SCL) and Atrium-His (AH) interval increased by 9 and 11 per cent respectively (P less than 0.05), 30 min after TEL. Intravenous lidocaine did not increase either SCL or AH. The functional refractory period of the atrioventricular node increased five per cent above the control value 15 min after TEL (P less than 0.05), while it was unchanged in the IVL group. The mean plasma concentrations of lidocaine ranged from 0.48 +/- 0.07 to 1.00 +/- 0.14 micrograms.ml-1 in the TEL group and from 0.98 +/- 0.13) to 1.21 +/- 0.15 micrograms.ml-1 in the IVL group. There were no significant differences in plasma concentrations of lidocaine in both groups during the observation period. Therefore, it is concluded that the depressant effects of TEA on intracardiac conduction were caused by blocking of the sympathetic efferent activity. Caution may be advised in administering TEA when cardiac conduction is already compromised.

Do antiarrhythmic drugs work? Some reflections on the implications of the Cardiac Arrhythmia Suppression Trial

Despite major advances in our understanding of the mechanisms of cardiac arrhythmias and how antiarrhythmic drugs appear to work, there remains much doubt whether these agents reduce arrhythmic mortality except in certain subsets of patients. The results of the Cardiac Arrhythmia Suppression Trial (CAST) have indicated that certain antiarrhythmic drugs not only "fail to work" but may substantially increase mortality. The effects of Class Ic agents in CAST and the meta-analysis of randomized antiarrhythmic trials in the survivors of acute infarction suggest that drugs that act primarily by delaying conduction are particularly deleterious in the survivors of acute infarction. Whether these data have a wide applicability in terms of all ventricular arrhythmias is unclear, but beta-blockers remain the only class of agents that in control trials have been shown to reduce sudden death. The effect appears to be related to beta-blockade and not to suppression of premature ventricular contractions (PVCs). Beta blockers appear to act by preventing ventricular fibrillation. It is reasonable to assume that PVC suppression per se is unlikely to produce a reduction in sudden death. Uncontrolled data with amiodarone suggests that it has the potential to prolong survival by controlling arrhythmias. The effects of amiodarone and beta blockers, both significantly attenuating adrenergic stimulation, provide pharmacologic probes to define the crucial determinants of efficacy of a compound for mortality reduction in high risk survivors of myocardial infarction. The focus must now shift from antiectopic and antiarrhythmic agents that delay conduction to those that exert antifibrillatory actions by sympathetic antagonism and those that exhibit the added property of lengthening myocardial refractoriness.

Pharmacological analysis of established ventricular fibrillation

1. The effects of anti-arrhythmic drugs on the power spectrum of established ventricular fibrillation induced by endocardial electrical stimulation, have been studied in greyhounds anaesthetized with sodium pentobarbitone (35 mg kg-1, i.v.). 2. In dogs receiving no drug, initial recording of ventricular fibrillation showed a dominant frequency of 9.9 +/- 0.7 Hz (lead II) and 10.0 +/- 0.6 Hz (endocardium). After 3.3 min the frequency had fallen to 4.0 +/- 0.4 Hz in lead II, but remained high in the endocardium (10.7 +/- 0.5 Hz). 3. Lignocaine significantly reduced the dominant frequency for fibrillation recorded from lead II at (0-80 s), and for endocardial fibrillation at (0-200 s). 4. Pretreatment with propranolol or bretylium had little effect on the time course of the dominant frequency of fibrillation in lead II or the endocardium. 5. Verapamil prevented the fall in frequency seen in lead II after 80 s in the no drug group. A significantly higher frequency was maintained in both lead II (14.7 +/- 0.9 Hz) and the endocardium (14.8 +/- 0.9 Hz) for 3.3 min, compared with the no drug group (P less than 0.01). 6. Activation of fast sodium channels may determine the rapid frequency of the initial stages of ventricular fibrillation. The rapid fall in dominant frequency in lead II after fibrillation for 80 s can be prevented by calcium channel blockade and may be due to intracellular accumulation of calcium.

Parasympathetically modulated antiarrhythmic action of lidocaine in atrial fibrillation

Clinical experience has shown that the antiarrhythmic effect of lidocaine on atrial arrhythmias, and specifically for the conversion of atrial fibrillation to normal sinus rhythm, is minimal. This study summarizes our experience in 30 dogs in which atrial fibrillation was initiated and sustained (greater than or equal to 15 minutes) under increased vagal tone achieved by either alpha-chloralose anesthesia (26 dogs) or pentobarbital sodium anesthesia combined with direct external electrical vagal stimulation (four dogs). Under increased vagal tone (regardless of the procedure), an intravenous bolus of lidocaine (2 to 3 mg/kg) was 100% effective (101 of 101 episodes) in pharmacologically converting atrial fibrillation to normal sinus rhythm. This was associated with marked slowing of intra-atrial electrical activity, as shown by fast Fourier analysis of intra-atrial electrograms. Over a period of 3 to 5 minutes, lidocaine progressively shifted the peak frequency content from 84 +/- 18 mV2/Hz in the 10 to 20 Hz frequency band during the pre-lidocaine phase to 110 +/- 34 mV2/Hz in the 0 to 10 Hz frequency band immediately prior to conversion to normal sinus rhythm. When atropine was administered or electrical vagal stimulation was discontinued, the conversion of atrial fibrillation to normal sinus rhythm followed a similar electrophysiologic pattern. When isoproterenol was infused, it was difficult to induce atrial fibrillation; when the arrhythmia was initiated, it could not be sustained even with concomitant electrical vagal stimulation. Thus in this model of parasympathetically sustained atrial fibrillation, lidocaine was 100% effective in converting atrial fibrillation to normal sinus rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium and ventricular arrhythmias

Potassium is a major determinant of the electrophysiologic properties of the myocardial membrane, and it plays an important role in the occurrence of arrhythmia. Hypokalemia has been associated with an increased frequency of ventricular premature complexes (VPCs) in some studies of hypertensive patients treated with diuretics, but other studies have failed to confirm any association. Studies involving patients with an acute myocardial infarction have also provided conflicting data about the association between hypokalemia and VPCs. Whereas the role of potassium in the genesis of simple VPCs remains uncertain, animal and clinical studies have demonstrated a strong relation between hypokalemia and the occurrence of sustained ventricular tachycardia and ventricular fibrillation during acute ischemic states. Hypokalemia may also affect the action of antiarrhythmic drugs by altering the electrophysiologic properties of the myocardium, potentially negating some of the antiarrhythmic activity of these agents. Although diuretic use is the most frequent cause of hypokalemia, epinephrine can also lower serum potassium as a result of stimulation of the beta 2 adrenoreceptor. This mechanism may, in part, explain the ability of beta blockers to prevent sudden death in patients with a recent myocardial infarction.

Preferential effect of procainamide on the reentrant circuit of ventricular tachycardia

Transient entrainment was used to test the hypotheses that 1) procainamide prolongs the cycle length of ventricular tachycardia in patients with coronary artery disease because it has a preferential effect on the reentrant tachycardia circuit, and 2) regions of slow conduction in the reentrant circuit are more susceptible to the effect of procainamide than are other areas of the ventricles. In five patients with prior myocardial infarction, sustained ventricular tachycardia with identical QRS configuration was inducible before and after intravenous infusion of procainamide. Transient entrainment of ventricular tachycardia was demonstrated at two or more cycle lengths by rapid pacing in the baseline state and after procainamide. Rapid pacing was performed from the same site during sinus rhythm at the cycle lengths that demonstrated transient entrainment of ventricular tachycardia. The conduction interval to the transiently entrained site during ventricular tachycardia (orthodromic interval) was compared with the conduction interval to the same site during pacing in sinus rhythm (antidromic interval). The mean tachycardia cycle length increased by 27% after procainamide administration (p = 0.002). The antidromic conduction intervals were prolonged by 9% (p = 0.06) compared with a 28% increase in the mean orthodromic conduction interval (p = 0.002). The difference between the orthodromic and antidromic conduction intervals increased by 40% (p = 0.003). Prolongation of the tachycardia cycle length after procainamide administration correlated positively with increases in the orthodromic conduction intervals (r = 0.94, p = 0.02) but not with changes in the antidromic intervals (r = -0.08, p = NS). The effect of procainamide on the difference between correlated strongly with changes in the cycle length of ventricular tachycardia (r = 0.97, p = 0.006).(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium channels from human brain RNA expressed in Xenopus oocytes. Basic electrophysiologic characteristics and their modification by diphenylhydantoin

We describe the expression and characterization of sodium channels from human brain RNA in the Xenopus oocyte. The expressed channel, studied by whole-cell voltage clamp, reveals characteristic selectivity for sodium as the permeant ion, voltage-dependent gating, and block by nanomolar concentrations of tetrodotoxin. Such channels are not seen in control oocytes injected with solvent only. The anticonvulsant diphenylhydantoin (DPH) inhibits the expressed channel in a voltage- and use-dependent manner, much like the effect seen in primary mammalian neuronal preparations. The inhibition of the expressed human sodium channel by DPH can be described by models previously developed to explain block of Na channels by local anesthetics. The preferential block of Na channels during depolarization helps explain the selectivity of DPH for neurons involved in seizure activity.