Electrophysiological and antiarrhythmic effects of lidocaine in canine acute myocardial ischemia

Modernized Classification of Cardiac Antiarrhythmic Drugs

BACKGROUND

Among his major cardiac electrophysiological contributions, Miles Vaughan Williams (1918-2016) provided a classification of antiarrhythmic drugs that remains central to their clinical use.

METHODS

We survey implications of subsequent discoveries concerning sarcolemmal, sarcoplasmic reticular, and cytosolic biomolecules, developing an expanded but pragmatic classification that encompasses approved and potential antiarrhythmic drugs on this centenary of his birth.

RESULTS

We first consider the range of pharmacological targets, tracking these through to cellular electrophysiological effects. We retain the original Vaughan Williams Classes I through IV but subcategorize these divisions in light of more recent developments, including the existence of Na+ current components (for Class I), advances in autonomic (often G protein-mediated) signaling (for Class II), K+ channel subspecies (for Class III), and novel molecular targets related to Ca2+ homeostasis (for Class IV). We introduce new classes based on additional targets, including channels involved in automaticity, mechanically sensitive ion channels, connexins controlling electrotonic cell coupling, and molecules underlying longer-term signaling processes affecting structural remodeling. Inclusion of this widened range of targets and their physiological sequelae provides a framework for a modernized classification of established antiarrhythmic drugs based on their pharmacological targets. The revised classification allows for the existence of multiple drug targets/actions and for adverse, sometimes actually proarrhythmic, effects. The new scheme also aids classification of novel drugs under investigation.

CONCLUSIONS

We emerge with a modernized classification preserving the simplicity of the original Vaughan Williams framework while aiding our understanding and clinical management of cardiac arrhythmic events and facilitating future developments in this area.

Effects of lignocaine on dispersion of repolarisation and refractoriness in a working rabbit heart model of regional myocardial ischaemia

The aims of this study were to establish a working rabbit heart model of regional myocardial ischaemia in which electrophysiologic parameters and arrhythmogenesis could be correlated and to explore the mechanisms underlying the antiarrhythmic activity of lignocaine. Monophasic action-potential duration (MAPD90), effective refractory period (ERP), and conduction delay were measured at three ventricular sites in isolated hearts paced at 3.3 Hz. The hearts were treated before and throughout 30 min of ischaemia and 15 min of reperfusion with a vehicle or 20 microM lignocaine. In both groups, ischaemia produced a similar shortening in MAPD90. Lignocaine decreased ERP shortening during ischaemia from -56+/-4 to -32+/-6 ms. An ischaemia-induced increase in conduction delay was greater in the lignocaine than the control group (49+/-7 vs. 11+/-2 ms). Ischaemia-induced dispersion of repolarisation was reduced by lignocaine from 66+/-4 to 32+/-7 ms, and dispersion of refractoriness was decreased from 57+/-6 to 16+/-3 ms. Lignocaine decreased inducibility of ventricular fibrillation (VF) during ischaemia from 86 to 25%. We conclude that, in this model, the antiarrhythmic activity of lignocaine during regional ischaemia is associated with an increase in ischaemia-induced conduction delay and reduced dispersion of repolarisation and refractoriness.

Current concepts in cardiopulmonary resuscitation

The "chain of survival" is important in the resuscitation of a patient who has had a cardiac arrest. The provision of Basic Life Support (BLS) and Advanced Cardiac Life Support (ACLS) is essential in this "chain of survival." Both BLS and ACLS have undergone several revisions since their initial inception. This article reviews (1) the current established and investigational issues of cardiopulmonary resuscitation, (2) the incidence and outcomes of anesthesia-related cardiac arrest, (3) the use of cardiopulmonary bypass in resuscitation, and (4) cerebral protection during and after resuscitation.

Lack of selectivity for ventricular and ischaemic tissue limits the antiarrhythmic actions of lidocaine, quinidine and flecainide against ischaemia-induced arrhythmias

The antiarrhythmic effectiveness, electrocardiographic and haemodynamic properties of three representative class I antiarrhythmics have been investigated in anaesthetized rats. Quinidine, lidocaine and flecainide were chosen as representatives of class Ia, Ib and Ic, respectively. Lidocaine showed the greatest frequency and 'ischaemia' dependency and a high dose provided complete protection against ischaemic arrhythmias induced by coronary artery occlusion. Flecainide showed the least frequency and ischaemia dependency and the least antiarrhythmic effectiveness. Quinidine was only slightly more effective than flecainide. The three drugs were approximately equi-potent in lowering blood pressure which limited the maximum dose that could be tested. The highest dose of lidocaine also caused convulsions in conscious animals. Thus, while lidocaine had selectivity for ischaemic tissue, and for high frequencies, the central nervous system and cardiovascular toxicity limited its usefulness against ischaemia-induced arrhythmias. Quinidine and flecainide's lack of selectivity for ischaemia, and/or high frequencies, probably accounted for their limited antiarrhythmic actions against ischaemia-induced arrhythmias. This study emphasizes that class I drugs can only provide useful protection against ischaemia-induced arrhythmias if they have marked cardiac selectivity as well as selectivity for ischaemic cardiac tissue.

Electrophysiological evaluation of the sodium-channel blocker carbamazepine in healthy human subjects

Carbamazepine (CBZ) is a sodium-channel blocker used mainly for the treatment of epileptic seizures and neuralgias. It may impair the function of the cardiac conduction system in susceptible patients, but its electrophysiological effects have not been thoroughly assessed in the normal heart, which was the aim of the present study. Ten healthy volunteers, mean age 32 years, underwent two electrophysiological investigations at baseline and three at different dose levels of CBZ. The transesophageal atrial stimulation technique was used to evaluate sinus node function, refractoriness of the atrial myocardium, atrioventricular conduction, and ventricular depolarization and repolarization (as reflected by the QRS, JT, and QT intervals) at spontaneous rhythm and after atrial pacing. Atropine was administered to facilitate 1:1 conduction and assessment of rate-dependent effects. At the highest CBZ dose (800 mg/day), which gave plasma concentrations within the upper therapeutic range, the PQ interval was mildly prolonged (151 vs. 159 msec; p < 0.01). In addition, the shortening of the JT interval normally seen at higher pacing rates was counteracted by high-dose CBZ, as demonstrated by a lower mean slope of the regression line after atropine and CBZ than after atropine alone (0.17 vs. 0.20; p < 0.05). No other effects were detected. At therapeutic levels CBZ had minimal effects on the healthy conduction system, supporting its safe use in the absence of cardiac disease.

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.

Selective effects of tocainide in canine acute myocardial infarction

We examined the in vivo electrophysiologic effects of tocainide in canine acute myocardial infarction. We compared the effects of tocainide in infarcted and non-infarcted zones. The left anterior descending coronary artery of 8 dogs was ligated and bipolar ventricular electrograms were recorded from a needle electrode placed transmurally in the infarcted zone and from electrodes in the non-infarcted zone. Conduction intervals were measured from the onset of the limb lead QRS to the major deflection of the recorded electrograms in the infarcted and non-infarcted zones. Effective refractory periods were also determined. Measurements were made before, during and after intravenous infusion of tocainide in therapeutic doses 2 hours after infarct. Tocainide prolonged conduction intervals by 26-31% in the infarcted zone at peak (P less than 0.001), but by only 6% in the non-infarcted zone. Similarly, tocainide prolonged the effective refractory period by 27% (P less than 0.001) on the infarcted, but by 8% the non-infarcted zone. Tocainide had very slight effects on QRS duration. The present study shows that tocainide had selective effects in the infarcted zone on both conduction and effective refractory period. These selective effects may explain its antiarrhythmic effects in acute myocardial infarction.

Repolarization interactions between cardiac segments of varying action potential duration

We studied interactions between action potential duration (APD) disperse zones using a double compartment bath in which an APD lengthening solution (Ni++ 2 mmol/L, in Tyrode's solution) was added to one compartment (which contained a portion of the fiber labeled "segment A") followed by addition of an APD shortening solution (6 to 10 mmol/K+) to the other compartment ("segment B"). Standard microelectrode techniques were used in canine Purkinje fibers. With Ni++ in segment A, there was a dispersion in APD measured at both 50% (APD50) and 95% (APD95) of repolarization. After selective addition of K+ to segment B, APD50 dispersion remained constant while APD95 dispersion increased, which indicated a change in the slope of repolarization, a factor with possible arrhythmogenic potential. In addition, a characteristic transitional action potential was seen adjacent to the partition. This had a timely and normally sloping early repolarization followed by a much more gradually sloping shelf. Premature action potentials arising during this shelf had diminished upstrokes. Results of these experiments may be useful in evaluating APD dispersion in relation to arrhythmias.

Prolongation of refractoriness and activation time in normal canine ventricular myocardium following bolus administration of lidocaine

This study examines the electrophysiologic effects of an intravenous bolus of lidocaine in the normal canine ventricle. In 14 open-chest dogs, left ventricular effective refractory period, ventricular repolarization time, and activation time were measured continuously during ventricular pacing (150/min) before, during, and after intravenous lidocaine given over 4 minutes. Lidocaine doses of 1.25, 2.5, and 5.0 mg/kg transiently prolonged the ventricular effective refractory period from 177 +/- 4 (X +/- standard error of the mean) to 185 +/- 5 msec (p less than 0.05), 176 +/- 3 to 188 +/- 6 msec (p less than 0.05), and 176 +/- 5 to 197 +/- 5 msec, (p less than 0.05), respectively, at 6 +/- 1.5 minutes from the onset of the bolus. Serum lidocaine levels obtained at the peak of the effective refractory period prolongation averaged 4.3 +/- 1.0, 9.0 +/- 3.0, and 22.3 +/- 4.7 micrograms/ml, respectively. Ventricular repolarization time was not prolonged. Activation time also transiently increased from 43 to 45 msec (p less than 0.05) and from 47 to 52 msec with the 2.5 mg/kg and 5.0 mg/kg doses, respectively. Cardiac denervation and administration of propranolol, 1 mg/kg intravenously, did not influence these responses to lidocaine. We conclude that a bolus of lidocaine transiently prolongs the effective refractory period and activation time in the normal canine ventricle because of its direct membrane rather than autonomic effects.

Antifibrillatory drugs: the case for lidocaine and procainamide

Ventricular fibrillation (VF), in most instances, is sustained by multiple wavefronts. The most important prerequisites for development and maintenance of VF are inhomogeneity in activation and recovery and shortening of refractoriness. If drugs are to be effective in removing VF or facilitating its electrical removal, this should occur by alteration of these electrophysiologic mechanisms. Assessment of the antifibrillatory properties of many drugs has been compromised by at least two factors: 1) the questionable appropriateness of the model, and 2) the failure to distinguish between prevention of VF and removal, or facilitation of electrical removal, of VF. Most instances of VF encountered clinically are secondary to acute ischemia; therefore, the most applicable model is acute myocardial ischemia with spontaneous VF. In this setting little is known about the effectiveness of lidocaine (L) or procainamide (PA). While there is more information regarding the role of L in prevention of VF, and of both L and PA in elevating ventricular fibrillation threshold, there is a lack of data on the roles of either drug in removal of existing VF. Thus we are left to speculate on the actions of L and PA that may account for their usefulness in the treatment of VF. These actions are the basis for a projection of the role of L and PA in the treatment of VF. There is evidence in animal models of acute myocardial ischemia to support the use of at least lidocaine in VF.

Lidocaine block of cardiac sodium channels

Lidocaine block of cardiac sodium channels was studied in voltage-clamped rabbit purkinje fibers at drug concentrations ranging from 1 mM down to effective antiarrhythmic doses (5-20 muM). Dose-response curves indicated that lidocaine blocks the channel by binding one-to-one, with a voltage-dependent K(d). The half-blocking concentration varied from more than 300 muM, at a negative holding potential where inactivation was completely removed, to approximately 10 muM, at a depolarized holding potential where inactivation was nearly complete. Lidocaine block showed prominent use dependence with trains of depolarizing pulses from a negative holding potential. During the interval between pulses, repriming of I (Na) displayed two exponential components, a normally recovering component (tauless than 0.2 s), and a lidocaine-induced, slowly recovering fraction (tau approximately 1-2 s at pH 7.0). Raising the lidocaine concentration magnified the slowly recovering fraction without changing its time course; after a long depolarization, this fraction was one-half at approximately 10 muM lidocaine, just as expected if it corresponded to drug-bound, inactivated channels. At less than or equal to 20 muM lidocaine, the slowly recovering fraction grew exponentially to a steady level as the preceding depolarization was prolonged; the time course was the same for strong or weak depolarizations, that is, with or without significant activation of I(Na). This argues that use dependence at therapeutic levels reflects block of inactivated channels, rather than block of open channels. Overall, these results provide direct evidence for the "modulated-receptor hypothesis" of Hille (1977) and Hondeghem and Katzung (1977). Unlike tetrodotoxin, lidocaine shows similar interactions with Na channels of heart, nerve, and skeletal muscle.

Pharmacologic treatment of cardiac arrhythmias: 25 years of progress

This review of practical and theoretical advances in antiarrhythmic drug therapy consists of four parts. Part 1, on clinical applications, compares the approaches to treatment 25 years ago with those of today, examines the current status of antiarrhythmic drugs used 25 years ago, reports on drugs approved for clinical use during the past 25 years, reviews new experimental drugs and suggests an approach to classification of antiarrhythmic drugs. Part 2 summarizes the contributions of cellular electrophysiology to the understanding of drug action, with emphasis on the drug-induced block of the voltage- and time-dependent properties of the rapid sodium channel. The subsequent section contains a brief discussion of the impact made by the new knowledge and the new diagnostic technology on the contemporary practices. The main conclusions are 1) that the more rational approach to treatment has benefited proportionately more patients with supraventricular than with ventricular arrhythmias, and 2) that new advances have made it possible to design successful treatments for certain patients with problems that could not be resolved in the past.

Electrophysiologic effects of encainide on acutely ischemic rabbit myocardial cells

The electrophysiologic effects of encainide were determined in normal and acutely ischemic (30 min) rabbit ventricular muscle cells. Encainide (10(-6), 5 X 10(-6) and 10(-5) M) had no effect on resting potential (RP); 10(-6) M encainide reduced overshoot and action potential (AP) amplitude of cells in normal left ventricles and cells in normal areas of ischemic ventricles. Encainide, 5 X 10(-6) M and 10(-5) M, depressed Vmax and prolonged AP duration of normal cells. Surviving cells within ischemic areas displayed AP with reduced RP, overshoot, AP amplitude, Vmax and shortened AP duration. All encainide concentrations reduced overshoot, AP amplitude and Vmax of depressed AP. Encainide's lengthening of AP duration was greater in cells within ischemic areas than in surrounding normal cells. Encainide (10(-6) M) prolonged effective refractory period and often blocked AP in ischemic cells. Encainide also caused depression in membrane responsiveness. Encainide's differential effect upon AP may significantly contribute to its antiarrhythmic activity in ischemic heart disease.

Effects of lidocaine on regional intraventricular conduction in patients with coronary artery disease

Studies of isolated heart muscle and canine models of myocardial ischemia have demonstrated that lidocaine slows conduction in abnormal but not in normal tissues. To determine lidocaine's effects on intraventricular conduction (IVENT) in patients with coronary artery disease (CAD), we studied this agent in seven patients following left anterior descending coronary artery (LAD) bypass surgery. Epicardial electrodes were placed on the right atrium, left ventricle (LV) in the distribution of the LAD, and on the right ventricle (RV). On postoperative day 7, lidocaine was administered as 100 mg bolus followed by 4 mg/minute infusion for 2 hours. At constant atrially paced rate, bipolar electrograms were recorded from the LV and RV for the 2 hours of infusion and for 2 hours after discontinuation of infusion. Conduction intervals were measured from the earliest onset of QRS in three simultaneously recorded surface ECG leads to the major deflection of the electrogram from each ventricle. At peak effect, with mean lidocaine level of 2.7 +/- 0.5 mg/ml, lidocaine slowed LV conduction by a mean of 6 +/- 1 msec (14 +/- 2%) (p less than 0.001) and in the RV by 1 +/- 0.3 msec (4 +/- 1%) (p less than 0.01), QRS duration changed 1 +/- 1 msec) 1 +/- 1%) (NS). The values returned to baseline within 2 hours after discontinuation of lidocaine infusion. The difference in lidocaine's effect between the diseased LV and the normal RV was significant (p less than 0.001).

Effect of lidocaine on escape rate in patients with complete atrioventricular block: B. Proximal His bundle block

Lidocaine was administered intravenously (a loading dose of 1.5 mg/kg body weight followed by a 3 mg/min infusion) to 10 patients with complete atrioventricular (A-V) block proximal to the His bundle and A-V junctional escape rhythm. A-V block was not due to an acute myocardial infarction in seven patients (group I) and was due to an acute inferior wall infarction in three patients (group II). Lidocaine had either no or only a slight depressant effect on the rate of the escape pacemaker in patients in group I but caused severe bradycardia or asystole in two of three patients in group II. Lidocaine had no consistent effect on the atrial rate and did not change the QRS duration and H-V intervals in any patient. These observations are consistent with the results of animal studies that showed that lidocaine selectively depressed conduction in ischemic or depolarized myocardium. The findings also suggest that the use of lidocaine without prior insertion of a pacemaker is unsafe in patients with acute myocardial infarction and complete A-V block proximal to the His bundle.