Differences in rate dependence of transient outward current in rabbit and human atrium

Both human and rabbit atrial cells possess a large 4-aminopyridine-sensitive transient outward current (I(to1)). However, the slow reactivation of this current in rabbits suggests that its role may be limited to very slow heart rates. We used whole cell voltage-clamp recordings to evaluate the rate dependency of I(to1) in rabbit and human atrial myocytes. Our results show that at physiological temperatures in human atrium, I(to1) is rate independent at rates between 0.1 and 4.0 Hz. Peak I(to1) at 4.0 Hz in rabbit was 3.4 +/- 1.4% (mean +/- SE) of current at 0.1 Hz (P < 0.001, n = 8), whereas in humans, peak I(to1) at 4.0 Hz averaged 88.8 +/- 6.1% of the current at 0.1 Hz (P > 0.05, n = 7). These differences were due to marked discrepancies in reactivation time course, which was biexponential with time constants that averaged 650 +/- 159 ms and 8.4 +/- 1.1 s in rabbit (n = 8) compared with a single exponential time constant of 33.6 +/- 6.8 ms (n = 8) in human atrium (both at 30 degrees C). These findings suggest that I(to1) can contribute importantly to atrial repolarization at all physiological heart rates in humans. Furthermore, these results emphasize that there are important interspecies variations in the rate dependence of I(to1), which need to be considered in understanding the physiological and pharmacological regulation of atrial repolarization.

Sustained outward current observed after I(to1) inactivation in rabbit atrial myocytes is a novel Cl- current

In rabbit atrial myocytes, depolarization of the membrane results in a rapidly activating transient outward current (I(to)) that then decays to a sustained level. The sustained current (Isus) remains constant for at least 5 s during continued depolarization. The present study was designed to identify the ionic mechanism underlying Isus with the use of whole cell voltage-clamp techniques. After exposure to 2 mM 4-aminopyridine (4-AP), the 4-AP-sensitive transient outward current (I(to1)) was abolished, but Isus was unaffected. Isus was not blocked by the K+ channel blockers tetraethylammonium chloride and Ba2+, was not changed by increasing superfusate K+ concentration, and was still present when K+ was replaced by Cs+ in both the superfusate and the pipette. Isus was significantly reduced by the Cl- transport blockers 4-acetamido-4'-isothiocyanatostilbene-2.2'-disulfonic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The current-voltage relations of Isus showed outward rectification, and the reversal potential of Isus shifted with changes in the transmembrane Cl- gradient in the fashion expected for a Cl- current. We conclude that Isus in rabbit atrium is due to a noninactivating Cl- current which, unlike previously described cardiac Cl- currents, is manifest in the absence of exogenous stimulators of adenosine 3',5'-cyclic monophosphate formation, cytosolic Ca2+ transients, or cell swelling.

Rate-dependent effects of diltiazem on human atrioventricular nodal properties

BACKGROUND

Tachycardia enhances the channel-blocking effects of antiarrhythmic drugs. In contrast to the extensive data regarding the rate-dependent effects of sodium channel blockers in humans, little is known about the frequency-dependent effects of calcium channel blockers on human atrioventricular (AV) nodal properties. Accordingly, the purpose of this study was to evaluate the importance of heart rate in modulating the electrophysiological effects of diltiazem in humans.

METHODS AND RESULTS

Electrophysiological studies were performed in 25 patients. Sinus node, atrial, and AV nodal function were evaluated at multiple atrial rates under control conditions and after administration of one of three intravenous doses of diltiazem designed to produce low, intermediate, and high stable plasma concentrations (designated doses 1, 2, and 3, respectively). Results were analyzed in terms of the longest and shortest cycle lengths obtainable in each patient under control and drug conditions. Plasma concentrations of diltiazem were stable and averaged 43 +/- 4, 73 +/- 6, and 136 +/- 11 ng/ml for doses 1, 2, and 3, respectively. Sinus node recovery time, intra-atrial conduction time, atrial effective refractory period, and HV interval were unaffected by diltiazem infusion. Effects of diltiazem were limited to changes in AV nodal parameters. Stable, dose-dependent increases in Wenckebach cycle length were observed after all three doses of diltiazem (increases of 54 +/- 13, 84 +/- 18, and 174 +/- 33 msec for doses 1, 2, and 3, respectively). Small nonsignificant increases in AH interval and atrioventricular effective refractory period (AVERP) were observed after dose 1 of diltiazem. At long cycle lengths, diltiazem caused modest increases in AH interval (3 +/- 4 and 25 +/- 8 msec for doses 2 and 3, respectively) and AVERP (36 +/- 12 and 70 +/- 25 msec). Drug effects were far greater at short cycle lengths (45 +/- 17 msec, 58 +/- 12 msec for AH interval and 80 +/- 24 msec, 163 +/- 41 msec for AVERP; p less than 0.05 versus values at long cycle lengths). At rapid rates, effects of diltiazem on AVERP substantially exceeded those on AV conduction, a result that could account for the beneficial effects of diltiazem during paroxysmal AV reentrant tachycardia by decreasing the excitable gap.

CONCLUSIONS

Depressant effects of diltiazem on human AV nodal function are highly dependent on atrial rate; the rate-dependent actions on AV nodal refractoriness probably contribute to beneficial effects of diltiazem in patients with supraventricular arrhythmias.

Mechanism of flecainide's antiarrhythmic action in experimental atrial fibrillation

Class Ic antiarrhythmic drugs are effective in the treatment of atrial fibrillation, but their mechanism of action is unknown. In previous work, we have found that flecainide causes tachycardia-dependent increases in atrial action potential duration (APD) and effective refractory period (ERP) by reducing APD accommodation to heart rate. The present study was designed to evaluate the efficacy and mechanisms of action of flecainide in an experimental model of sustained atrial fibrillation (AF). AF was produced by a brief burst of atrial pacing in the presence of vagal stimulation and persisted spontaneously until vagal stimulation was stopped. The actions of flecainide at two dose levels were compared with those of isotonic glucose placebo in each dog, with a randomized order of blinded drug administration. Flecainide terminated AF in all 16 dogs, while glucose was effective in none (p less than 0.0001). Flecainide increased atrial ERP and reduced conduction velocity in a tachycardia-dependent manner. Doses of flecainide that converted AF resulted in larger changes in ERP than in conduction velocity, increasing the minimum path-length capable of supporting reentry (wavelength). In addition, flecainide reduced regional heterogeneity in ERP and wavelength, an action opposite that of vagal stimulation. Atrial epicardial mapping with a 112-electrode atrial array was used to study the mechanism of flecainide action on AF. Under control conditions, multiple small zones of reentry coexisted. Flecainide progressively increased the size of reentry circuits, decreased their number, and slowed the frequency of atrial activation until the arrhythmia finally terminated; all changes were compatible with an increase in wavelength. We conclude that flecainide terminates atrial fibrillation in this experimental model by causing tachycardia-dependent increases in atrial ERP, which increase the wavelength at the rapid rates characteristic of AF to the point that the arrhythmia can no longer sustain itself.

Preoperative and long-term cardiac risk assessment. Predictive value of 23 clinical descriptors, 7 multivariate scoring systems, and quantitative dipyridamole imaging in 360 patients

A total of 360 patients underwent preoperative cardiac risk assessment using 23 clinical parameters, seven multivariate clinical scoring systems, and quantitative dipyridamole-thallium imaging to predict postoperative and long-term myocardial infarction and cardiac death after noncardiac surgery. There were 30 postoperative and an additional 13 cumulative long-term cardiac events after an average follow-up of 15 months. Clinical descriptors were not useful in predicting the outcome of individual patients. The postoperative and long-term cardiac event rates were 1% and 3.5%, respectively, in patients with normal scans or fixed perfusion defects, and 17.5% and 22% in patients with reversible defects. Using quantitative indices reflecting the amount of jeopardized myocardium, patients could be stratified by dipyridamole imaging into multiple scintigraphic subsets, with corresponding postoperative and 1-year coronary morbidity and mortality rates ranging from 0.5% to 100% (p = 0.0001). Thus, postoperative and long-term cardiac events cannot be predicted clinically, whereas quantitative dipyridamole imaging accurately identifies high-risk patients who require preoperative coronary angiography.

A quantitative analysis of use-dependent ventricular conduction slowing by procainamide in anesthetized dogs

BACKGROUND

Use-dependent effects of antiarrhythmic drugs on phase 0 sodium current result in rate-dependent conduction slowing with important potential clinical consequences. The purpose of the present study was to determine whether state-dependent interactions of procainamide with sodium channels can be analyzed based on conduction changes in vivo.

METHODS AND RESULTS

Procainamide infusions were used to produce stable drug concentrations causing greater than or equal to 25% conduction slowing at a basic cycle length (BCL) of 300 msec in morphine/chloralose-anesthetized dogs with formalin-induced atrioventricular block. Computer-based epicardial activation mapping was applied to assess the time course and pattern of conduction over a wide range of BCLs before and after drug administration. Action potential duration was measured from recordings of monophasic action potentials. The onset and steady-state values of fractional sodium channel block estimated from conduction changes were fitted to equations obtained from a stepwise exponential analysis. The rate constant for the onset of block (lambda *) decreased, as predicted, with decreasing cycle length. The slope of the relation between lambda * and recovery time at each BCL averaged 0.29 +/- 0.03 sec-1, resulting in a calculated recovery time constant (3.4 seconds) similar to values previously obtained by direct measurement. Estimates of binding and unbinding rate constants for the sodium channel during the action potential plateau and after repolarization were of the same order as previous results obtained using microelectrode methods in vitro.

CONCLUSIONS

Use-dependent conduction changes produced by procainamide in vivo closely follow the predictions of mathematical models of drug-channel interactions, and underlying kinetic interactions with the sodium channel inferred from conduction changes agree with previous, more direct observations. These results support the relevance of basic concepts about antiarrhythmic drug actions on sodium channels for understanding drug effects on conduction in vivo and advance analytical tools that can be used to explore the latter in humans.

Effects of beta-adrenergic receptor stimulation and blockade on rate-dependent atrioventricular nodal properties

Recent work has shown that alterations in the dynamic atrioventricular (AV) nodal response to changes in heart rate can significantly modify AV nodal function. The present study was designed to evaluate the nature and potential importance of sympathetic regulation of the rate-dependent properties of the AV node. Selective stimulation protocols and mathematical formulations were used to independently quantify AV nodal recovery, facilitation, and fatigue in 12 morphine-chloralose-anesthetized dogs. Vagal effects were prevented by bilateral vagal transection and intravenous atropine, and the sinus node was crushed to allow a broader range of pacing cycle lengths. In seven dogs with sympathetic nerves intact, beta-adrenergic receptor blockade increased the recovery time constant (tau rec) for the conduction of premature test beats from 47 +/- 2 (mean +/- SEM) msec (control) to 62 +/- 1 msec (p less than 0.001), whereas isoproterenol decreased tau rec to 38 +/- 1 msec (p less than 0.001). In addition, beta-blockade increased the maximum amount of rate-dependent AV nodal fatigue from 7 +/- 1 msec (at a cycle length of 198 +/- 9 msec [control]) to 17 +/- 2 msec (p less than 0.001). In five dogs with decentralized stellate ganglia, tau rec was decreased from 71 +/- 3 msec (control) to 57 +/- 4 msec and 48 +/- 2 msec (p less than 0.001 for each) by left stellate ganglion stimulation at 5 and 10 Hz, respectively. Maximum fatigue was similarly reduced from 16 +/- 1 msec (control) to 12 +/- 2 msec (p = NS) and 8 +/- 1 msec (p less than 0.01), respectively. Stellate ganglion stimulation, isoproterenol, and beta-blockade did not alter AV nodal facilitation. A mathematical model incorporating quantitative indexes of AV nodal function accurately accounted for tachycardia-dependent increases in the atrial-His activation interval, which were enhanced by beta-adrenergic receptor blockade and reduced by isoproterenol. Furthermore, this model showed that beta-adrenergic effects were increased by increasing heart rate, with the majority of the rate-dependent action being due to changes in the time course of AV nodal recovery. We conclude that beta-adrenergic receptor stimulation alters functional properties that govern the AV nodal response to changes in heart rate. These changes in functional properties alter the ability of the AV node to conduct impulses during tachycardia and, as such, could play a major role in the ability of sympathetic stimulation to promote and beta-adrenergic receptor blockade to prevent the occurrence of AV nodal reentrant arrhythmias.

Effects of flecainide on the rate dependence of atrial refractoriness, atrial repolarization and atrioventricular node conduction in anesthetized dogs

Flecainide is effective against certain supraventricular arrhythmias (atrial fibrillation and atrioventricular [AV] node reentrant tachycardia), but its mechanisms of action are unknown. Previous in vitro work suggests that flecainide attenuates rate-dependent action potential duration shortening, producing tachycardia-dependent prolongation of the refractory period. This study was designed to assess whether similar changes occur in vivo and whether the effects of flecainide on AV node conduction depend on heart rate and on direction of propagation (anterograde vs. retrograde). The effects of flecainide at three clinically relevant concentrations were assessed in open chest, morphine-chloralose-anesthetized dogs. Flecainide increased atrial refractory period in a concentration- and rate-related fashion (e.g., dose 3 increased the atrial effective refractory period by 9 +/- 4% at a cycle length of 1,000 ms but by 36 +/- 5% and 55 +/- 10% at a basic cycle length of 400 and 300 ms, respectively; p less than 0.001 for each). Flecainide attenuated the action potential duration accommodation (measured by monophasic action potentials) to heart rate, causing tachycardia-dependent action potential duration prolongation and accounting for most of the rate-dependent atrial effective refractory period changes. Flecainide increased Wenckebach cycle length, but the concentration-response curve was much steeper in the retrograde (slope 41 +/- 7 ms/mumol.liter-1) than in the anterograde direction (17 +/- 4 ms/mumol.liter-1; p less than 0.01), indicating more potent effects on retrograde conduction. The depressant action of the drug on the AV node was also rate dependent, with an effect on the AH interval at a basic cycle length of 400 ms that averaged 1.8, 1.5 and 2 times that at a basic cycle length of 1,000 ms for doses 1 (p less than 0.05), 2 (p less than 0.01) and 3 (p less than 0.001), respectively.

CONCLUSIONS

1) Flecainide suppresses atrial action potential duration accommodation to heart rate changes in vivo, leading to rate-dependent atrial effective refractory period prolongation, which may be important in suppressing atrial fibrillation. 2) The drug has frequency- and direction-dependent effects on AV node conduction, which may lead to selective antiarrhythmic actions during AV node reentry.

Actions of intravenous magnesium on ventricular arrhythmias caused by acute myocardial infarction

Although the antiarrhythmic properties of magnesium are well recognized, its mechanisms of antiarrhythmic action are poorly understood. This study was designed to characterize the effects of magnesium on ventricular tachyarrhythmias related to acute myocardial infarction (MI) in dogs. When the circumflex coronary artery was occluded repeatedly for 10 min at 30-min intervals, ventricular fibrillation (VF) occurred in 30, 35 and 33% of dogs during occlusions 1, 2 and 3, respectively. Magnesium pretreatment reduced the incidence of VF to 14% during occlusion 3 (P less than .05 compared to occlusions without magnesium pretreatment). Neither the prevalence of ventricular ectopic complexes 24 h after MI nor arrhythmia inducibility 4 days after infarction were significantly altered by i.v. magnesium. Magnesium significantly attenuated the ST segment elevation (an index of ischemic injury) and ventricular conduction slowing caused by MI. Because magnesium has been reported to reverse the effects of hyperkalemia, we evaluated the role of this action by infusing potassium directly into a coronary artery (to mimic ischemia-induced hyperkalemia) and administered i.v. magnesium. Potassium infusion markedly slowed intraventricular conduction, an effect fully reversed by discontinuing potassium administration but unaffected by i.v. magnesium. We conclude that magnesium has antiarrhythmic actions only during the early phases of an experimental MI, and that these actions are associated with attenuation of indices of ischemic injury and conduction slowing. These properties of magnesium are similar to those of calcium antagonists, and suggest that magnesium's calcium antagonist properties may be important in its antiarrhythmic actions.

Quantitation of dynamic AV nodal properties and application to predict rate-dependent AV conduction

A number of functional properties of the atrioventricular (AV) node have been described in response to changes in the atrial input rate. The purpose of this study was 1) to develop quantitative descriptors of these properties, and 2) to determine whether they can account for rate-dependent changes in AV nodal conduction. The delay in AV nodal conduction of single premature beats (recovery) was found to be an exponential function of coupling interval with a time constant of 66 +/- 2 (+/- SE) ms. A single abbreviated (facilitation) cycle did not alter the time constant of recovery or basal conduction for a subsequent beat but shifted its recovery curve to the left to an extent exponentially related to the facilitation cycle length. The induction of a tachycardia with HA interval fixed so as to control the recovery and facilitation variables resulted in a first-order onset of AV conduction slowing (fatigue). The fatigue process had a time constant in the range of 70 beats and a magnitude that was a decaying exponential function of HA interval. An equation incorporating quantitative descriptors of recovery, facilitation, and fatigue accurately predicted rate-dependent changes in AH interval. We conclude that 1) the AV nodal properties of recovery, facilitation, and fatigue are amenable to quantitative characterization, and 2) rate-dependent changes in AV nodal conduction time can be well described in terms of these underlying properties.

Multivariate clinical models and quantitative dipyridamole-thallium imaging to predict cardiac morbidity and death after vascular reconstruction

Patients with peripheral vascular disease have a high prevalence of coronary artery disease and are at increased risk for cardiac morbidity and death after vascular reconstruction. The present study was undertaken to assess the value of 18 clinical parameters, of 7 clinical scoring systems, and of quantitative dipyridamole-thallium imaging for predicting the occurrence of postoperative myocardial infarction or cardiac death. Vascular surgery was performed in 125 patients. Thirteen postoperative cardiac events occurred, including 10 cardiac deaths and 3 nonfatal infarctions. Clinical parameters were not useful in predicting postoperative outcome. All 63 patients with normal scan results or fixed perfusion defects underwent surgery uneventfully, whereas 21% (13/62) of patients with reversible defects had a postoperative cardiac complication. By use of quantitative scintigraphic indexes we found that patients with reversible defects could be stratified into intermediate and high-risk subgroups with postoperative event rates of 5% (2/47) and 85% (11/13), respectively, despite intensive postoperative monitoring and antianginal medication. Thus in patients unable to complete a standard exercise stress test, postoperative outcome cannot be predicted clinically, whereas dipyridamole-thallium imaging successfully identified all patients who had a postoperative cardiac event. By use of quantification we found that patients with reversible defects can be stratified into an intermediate risk subgroup that can undergo surgery with minimal complication rate and a high-risk subgroup that requires coronary angiography.

Kinetics of use-dependent ventricular conduction slowing by antiarrhythmic drugs in humans

BACKGROUND

Rate-dependent conduction slowing by class I antiarrhythmic agents has clinically important consequences. Class I drugs are known to produce use-dependent sodium channel blockade. If rate-dependent conduction slowing by class I agents is due to sodium channel blocking actions, the kinetics of conduction slowing should be similar to those of depression of sodium current indexes in vitro. The purpose of the present investigation was to study the onset time course of ventricular conduction slowing caused by a variety of class I agents in humans.

METHODS AND RESULTS

Twenty-seven patients undergoing electrophysiological evaluation for antiarrhythmic therapy were studied. Changes in QRS duration at initiation of ventricular pacing at cycle lengths of 400 and 500 msec were used to evaluate the kinetics of drug action. Mean time constants for each drug were similar to values for Vmax depression reported in vitro studies: flecainide, 24.9 +/- 11.6 beats in eight patients (versus 34.5 beats reported for Vmax block); propafenone, 17.8 +/- 6.9 beats in five patients (versus 8.4-20.8 beats); quinidine, 7.0 +/- 2.4 beats in six patients (versus 5.6-6.2 beats); and amiodarone, 3.6 +/- 2.0 beats for eight patients (versus 3.0 beats). Time constants were significantly different among the various drugs tested (p = 0.0002 at a cycle length of 400 msec; p = 0.002 at 500 msec), and there was a strong correlation (r = 0.89, p less than 0.0001) between values obtained at a cycle length of 400 msec and those at a cycle length of 500 msec. No rate-dependent changes in QRS duration were seen at onset of ventricular pacing among eight age- and disease-matched control patients not taking class I antiarrhythmic drugs, including three patients subsequently showing such changes during type I antiarrhythmic drug therapy.

CONCLUSIONS

We conclude that class I agents produce use-dependent QRS prolongation in humans with characteristic kinetics for each agent that are similar to the kinetics of Vmax depression in vitro. These results suggest that rate-dependent ventricular conduction slowing by antiarrhythmic drugs in humans is due to use-dependent sodium channel blockade.

A unified model of atrioventricular nodal conduction predicts dynamic changes in Wenckebach periodicity

The atrioventricular (AV) node responds in a complex fashion to changes in activation rate. A variety of approaches have been used to explain these dynamic AV nodal responses, but none has been able to account fully for AV nodal behavior. Three specific rate-dependent properties of the AV node have been described: 1) time-dependent recovery after excitation, 2) an effect of short cycles to advance recovery ("facilitation"), and 3) a gradual slowing of conduction in response to sustained, high-frequency activation ("fatigue"). We hypothesized that a model incorporating quantitative descriptors of all three processes might be able to account for a wide variety of AV nodal behaviors. Quantitative descriptors of AV nodal recovery, facilitation, and fatigue were developed based on AV nodal conduction changes during selective pacing protocols in seven autonomically blocked dogs. These descriptors were incorporated into a set of mathematical equations that define AV nodal conduction of any beat based on activation history. The equations were then applied to predict pacing-induced Wenckebach periodicity in each dog. Experimental data were obtained after nine to 19 step decreases in atrial cycle length into the Wenckebach zone in each animal. Observed behaviors included complex patterns of block, a progressive increase in the level of block over 5 minutes of rapid pacing, and periods of alternating patterns of block. The model accurately predicted the onset of AV block at each cycle length, the relation between conduction ratio and cycle length as a function of time, and the changing patterns of Wenckebach periodicity during sustained atrial pacing. All three terms of the model equation (describing recovery, facilitation, and fatigue) were essential to account fully for the observed behaviors. Elimination of AV nodal fatigue from the model resulted in failure to account for time-dependent changes in Wenckebach patterns, whereas exclusion of facilitation led to consistent overestimation of the degree of AV block at each cycle length. We conclude that a mathematical model incorporating terms to describe recovery, facilitation, and fatigue accurately predicts a wide range of Wenckebach-type behavior and that complex conduction patterns of the AV node can be fully accounted for by simple functional AV nodal properties.

Antiarrhythmic drug classifications. A critical appraisal of their history, present status, and clinical relevance

Classifications of antiarrhythmic drugs have developed because of a need to organize the large number of agents available according to pharmacological properties of clinical relevance. The current classification is a hybrid of classification systems developed in the early 1970s. It subdivides drugs according to 4 major pharmacological actions: (a) depression of phase 0 sodium current; (b) antagonism of adrenergic effects on the heart; (c) prolongation of of action potential duration; and (d) calcium channel blockade. Further subdivision of sodium channel blockers is based on the kinetics of sodium channel blockade and drug effects on action potential duration. A critical analysis of selected aspects of the clinical actions of antiarrhythmic drugs indicates the value of the current classification, as well as some limitations in its ability to separate drugs into distinct groups with characteristic clinical properties. The strengths of the current classification are due to the clinical importance of the pharmacological properties on which it is based. These results in electrophysiological actions, indications, and adverse effects that are typical for each group of drugs. The limitations of the current system relate to the propensity of individual drugs to have actions of more than one class simultaneously, the way that the various actions of a given drug are dependent on concentration, rate, and tissue type, and to problems in subclass definition. Some of these shortcomings could be alleviated by returning to the concept, originally put forward by Singh and Vaughan Williams, of classes of drug action rather than classes of drug per se. This approach would be pharmacologically more realistic than trying to assign each antiarrhythmic agent to a single unique class, would be better able to incorporate the complexities of drug action, and would potentially be more flexible. The wide use of antiarrhythmic drug classifications attests to their value, and suggests that they are likely to continue to be important in the future.

Repolarization differences between guinea pig atrial endocardium and epicardium: evidence for a role of Ito

It has long been known that ventricular epicardial action potential duration (APD) is shorter than endocardial, and recent evidence suggests that a larger transient outward current (Ito) in epicardium is responsible for the difference. To evaluate possible corresponding regional variations in atrial tissue, we studied guinea pig atrial epicardial and endocardial action potentials using standard microelectrode techniques. Epicardial APD was consistently shorter than endocardial, but the difference was greatly diminished by rapid pacing or early premature activation, situations in which Ito availability should be limited. 4-Aminopyridine (4-AP), at concentrations (0.5 mM) producing specific Ito blockade, increased APD significantly in atrial epicardium without affecting endocardium. The effect of 4-AP on APD was most marked at slow rates, at which Ito would be greatest, and was negligible at rapid rates or during premature activation, during which Ito would be largely inactivated. At larger concentrations (5 mM) 4-AP caused an equalization of epicardial and endocardial APD. The equimolar substitution of strontium for calcium did not affect APD at slow rates and increased APD (particularly in endocardium) at rapid rates, suggesting that the Ito underlying endocardial-epicardial differences was unlikely to be calcium dependent. We conclude that epicardial-endocardial differences in APD, well documented in ventricular tissue, can also occur in atrial tissue and that the underlying ionic mechanisms appear to be similar.

Diltiazem at reperfusion reduces neutrophil accumulation and infarct size in dogs with ischaemic myocardium

STUDY OBJECTIVE

The aim was to demonstrate the ability of diltiazem to protect the ischaemic myocardium in the course of coronary reperfusion, and to establish if an interaction with neutrophils is implied.

DESIGN

Ischaemia was induced by occluding the left anterior descending coronary artery for 90 min followed by 6 h of reperfusion with a residual critical stenosis left in place. Three groups were studied: group 1 (control) received a saline perfusion; group 2 was given a bolus injection of 400 micrograms.kg-1 of diltiazem 10 min before reperfusion, followed by 4 micrograms.kg-1.min-1 perfusion until termination of experiment; group 3 was made neutropenic by injecting a neutrophil antiserum produced in rabbits and was then treated with diltiazem, as in the second group.

SUBJECTS

60 mongrel dogs of either sex were allocated at random into one of the three groups the day before the experiment.

MEASUREMENTS AND MAIN RESULTS

Diltiazem plasma concentrations ranged from 68.6(SEM 10.0) to 102.5(15.2) micrograms.litre-1 during the study. Transmural collateral blood flow, measured with 153Gd microspheres 15 min after occlusion, and area at risk, evaluated by Evans blue perfusion, did not differ among the three groups. Infarct size, estimated by triphenyltetrazolium staining of heart slices and expressed as a percentage of area at risk, was less (p less than 0.05) in the diltiazem [20.5(5.2)%] and diltiazem plus neutropenia [17.6(5.4)%] groups compared to controls [39.8(6.9)%] but neutropenia added no significant benefit to diltiazem alone. The animals treated with diltiazem alone had lower serum creatine kinase levels than controls, at 5719(891) v 14,333(2885) IU.litre-1, p less than 0.05. The neutrophilia seen in controls was virtually absent in diltiazem dogs. Myocardial neutrophil accumulation estimated by scintigraphy of 111In labelled autologous neutrophils was much less in diltiazem than in control dogs, at 3948(1228) v 11,021(2081) 111In-neutrophil.g-1 of infarct, p less than 0.02.

CONCLUSIONS

Diltiazem given during reperfusion reduces infarct size by a mechanism that includes an inhibition of neutrophil accumulation in the post-ischaemic myocardium.

Electrophysiologic evaluation and outcome of patients with syncope of unknown origin

Electrophysiologic studies were performed in 134 patients (87 males, mean age 59 years) with unexplained syncope. Seventy-one patients had organic heart disease (ischaemic in 50). Electrophysiologic studies revealed conduction abnormalities and tachyarrhythmias that could account for syncope in 40 patients (30%). Thirty-seven (93%) of these patients received pacing or antiarrhythmic therapy compared with 23 (24%) of the remaining 94 patients who had a negative study and received empiric therapy (P less than 0.0001). Risk of having an abnormal electrophysiologic study was greater in patients with underlying heart disease (P less than 0.05). During a mean follow-up of 22 +/- 17 months, 26 patients (19%) either had recurrent syncope (22 patients) or died (four patients) suddenly. Men had a higher incidence of recurrent syncope than women (26% vs 6%, P less than 0.005). Other clinical characteristics, electrophysiologic findings, final diagnosis and therapy at discharge were not predictive of outcome. We conclude that (1) 19% of patients investigated for syncope will have a recurrent event, (2) female gender may be an independent predictor of favourable outcome.

The Treatment of PVCs and Prevention of Sudden Cardiac Death: New findings from the CAST study

Premature ventricular complexes (PVCs) have traditionally been suppressed using antiarrhythmic drugs. Recent studies have failed to show that reducing the number of PVCs can prevent sudden death; moreover, treatment with some antiarrhythmic agents can increase the risk. There is a close link between the severity of ischemic heart disease and sudden death.

Diltiazem increases late-onset congestive heart failure in postinfarction patients with early reduction in ejection fraction. The Adverse Experience Committee; and the Multicenter Diltiazem Postinfarction Research Group

The Multicenter Diltiazem Postinfarction Trial (MDPIT) reported no consistent diltiazem effect on new or worsened congestive heart failure (CHF) during 12-52 months' follow-up after acute myocardial infarction. This was puzzling in light of the observation that patients with findings suggesting left ventricular dysfunction (LVD) at baseline on diltiazem had more cardiac events (cardiac mortality or recurrent nonfatal infarction) than such patients on placebo. We hypothesized that diltiazem increased the frequency of late CHF as well as of cardiac events, but only in patients predisposed by LVD. Using the same characterizing variables as the primary MDPIT analysis, we found that patients with pulmonary congestion, anterolateral Q wave infarction, or reduced ejection fraction (EF) at baseline were more likely to have CHF during follow-up than those without these markers of LVD. CHF was particularly frequent in the patients with LVD who were randomized to diltiazem. Among those with a baseline EF of less than 0.40, late CHF appeared in 12% (39/326) receiving placebo and 21% (61/297) receiving diltiazem (p = 0.004). Life table analysis in patients with an EF of less than 0.40 confirmed more frequent late CHF in those taking diltiazem (p = 0.0017). In addition, the diltiazem-associated rise in the frequency of late CHF was progressively greater with increasingly severe decrements in baseline EF. This diltiazem effect was absent in patients with pulmonary congestion at baseline but an EF of 0.40 or more, suggesting a unique association between diltiazem-related late CHF and systolic LVD.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants and significance of diltiazem plasma concentrations after acute myocardial infarction. The Multicenter Diltiazem Postinfarction Trial Research Group

A total of 1,975 plasma diltiazem concentrations were obtained from 1,067 patients enrolled in a multicenter secondary intervention study of diltiazem after acute myocardial infarction. To evaluate the determinants and significance of diltiazem concentrations in this patient population, we related drug concentrations to a variety of clinical variables recorded on the case history forms. Multiple linear regression analysis showed that (1) time from the last drug dose, (2) drug dose taken, (3) patient height (an index of lean body weight), and (4) patient age were important determinants of plasma concentration. For an equivalent dose, plasma diltiazem concentrations in a 75-year-old patient were about double those of a 25-year-old patient. Total weight and drug dose prescribed did not significantly affect plasma concentrations. Whereas drug concentrations were higher (p = 0.01) among patients with left-sided heart failure, they were not altered by renal dysfunction, hepatic disease or beta blockers. Diltiazem concentrations were a significant determinant of diastolic arterial pressure (p less than 10(-9), but neither systolic pressure nor heart rate were significantly related to diltiazem concentration. The overall incidence of adverse experiences was not related to drug concentrations, but the occurrence of second- and third-degree atrioventricular block in the coronary care unit and the need for a temporary pacemaker were substantially higher among patients with a drug concentration greater than 150 ng/ml (7.4 and 1.9%, respectively) than among patients with lower concentrations (2.6% for atrioventricular block, 0.3% for pacemaker; p = 0.02 for each). The risk of atrioventricular block was particularly increased by high diltiazem concentrations in the face of acute inferior infarction. These results suggest that diltiazem's pharmacologic and clinical effects in a large population are concentration-related, and that the consideration of patient size, age, and left ventricular function in selecting a diltiazem dose may allow for effective drug therapy with a reduced likelihood of adverse effects.

Direct electrophysiological actions of pentobarbital at concentrations achieved during general anesthesia

Pentobarbital and alpha-chloralose are widely used for experimental anesthesia, but their direct electrophysiological actions at anesthetic concentrations are unknown. Trough and peak concentrations measured by high-performance liquid chromatography averaged 27 +/- 3 and 45 +/- 13 mg/l (means +/- SD) for pentobarbital and 41 +/- 15 and 103 +/- 13 mg/l for alpha-chloralose in dogs receiving them for general anesthesia. The direct effects of each agent on papillary muscle action potentials obtained from guinea pigs killed by decapitation were studied in vitro. Pentobarbital increased action potential duration to 95% by 24 +/- 6 and 33 +/- 4% at 25 and 50 mg/l (P less than 0.001 for each), respectively, and caused corresponding increases in effective refractory period. Furthermore, pentobarbital reduced maximum rate of voltage change (Vmax) of phase 0 in a voltage-, rate-, and concentration-dependent fashion, suggesting use-dependent sodium channel blocking actions. The voltage dependence of Vmax was shifted by 3.7 +/- 1.7 (P less than 0.01) and 6.5 +/- 1.8 mV (P less than 0.001) by 25 and 50 mg/l pentobarbital, respectively. In canine ventricular muscle, pentobarbital caused rate- and concentration-dependent decreases in Vmax and increases in action potential duration and refractory period over a concentration range of 5-100 mg/l. alpha-Chloralose was devoid of direct electrophysiological effects in both species. We conclude that pentobarbital has potentially important electrophysiological actions on ventricular tissues at concentrations required for general anesthesia and may confound the results of in vivo studies.

Vagal modulation of the rate-dependent properties of the atrioventricular node

Vagal effects on atrioventricular (AV) nodal conduction are accentuated by increases in heart rate. To establish the mechanism of these rate-dependent negative dromotropic actions, we studied the properties governing AV nodal adaptation to changes in heart rate in chloralose-anesthetized dogs in the absence and presence of bilateral cervical vagal nerve stimulation (20 Hz, 0.2 msec). Stimulation protocols were applied to evaluate the contributions of changes in AV nodal recovery, facilitation, and fatigue independently of each other. Vagal stimulation slowed AV nodal recovery in a voltage-dependent way, increasing the time constant of recovery (tau r) from 80 +/- 7 to 194 +/- 16 msec (mean +/- SEM, p less than 0.01) at the highest voltage studied. The facilitating effect of a premature (A2) beat was manifested by a leftward shift of the recovery curve (A3H3 versus H2A3) of a subsequent A3 beat. The magnitude of shift depended on the A1A2 coupling interval and was reduced by vagal stimulation at all A1A2 intervals (maximum shift: control, 63 +/- 12 msec; vagus, 24 +/- 11 msec; p less than 0.01). When recovery and facilitation were kept constant, abrupt increases in AV nodal activation rate caused a slow (tau = 75 beats) increase in AH interval (fatigue). Vagal stimulation increased the magnitude of this process (maximum: control, 11 +/- 2 msec; vagus, 27 +/- 3 msec; p less than 0.001), without altering its time course. At activation rates comparable to sinus rhythm in humans, vagal stimulation at an intermediate voltage increased the AH interval by 25 msec. As heart rate increased, vagally induced changes in dynamic processes amplified AH prolongation up to fivefold at maximum rate. The role of vagal changes in individual functional properties depended on heart rate, but slowing of recovery was the single most important factor, constituting over 50% of overall vagal action at rapid rates. We conclude that vagal stimulation alters the ways in which the AV node responds to changes in activation rate and that at rapid rates most of the negative dromotropic action of the vagus is due to changes in the AV nodal response to tachycardia. Alterations in rate-dependent AV nodal properties are a novel and potentially important mechanism through which interventions may affect AV nodal conduction.

Effects of flecainide and quinidine on human atrial action potentials. Role of rate-dependence and comparison with guinea pig, rabbit, and dog tissues

Flecainide and other class IC antiarrhythmic drugs are effective in the prevention and termination of atrial fibrillation, but the mechanism of this action is unknown. To gain insights into potential cellular mechanisms, we evaluated the response of human atrial action potentials to equimolar therapeutic concentrations of flecainide and quinidine and compared this response to that of guinea pig, rabbit, and dog atria. Both compounds reduced Vmax more as activation rate increased, but flecainide was more potent than quinidine and had slower kinetics. The rate-dependence of Vmax reduction was similar for all species, but human tissue was more sensitive to the drugs tested. In contrast to changes in Vmax, drug-induced alterations in action potential duration showed opposite rate-dependence for the two drugs. Quinidine increased action potential duration to 95% repolarization (APD95) in human atria by 33 +/- 7% (mean +/- SD) at a cycle length of 1,000 msec, but this effect was reduced as cycle length decreased, to 12 +/- 4% (p less than 0.001) at a cycle length of 300 msec. Flecainide increased APD95 (by 6 +/- 3%) much less than quinidine at a cycle length of 1,000 msec, but its effect was increased by faster pacing, to 27 +/- 12% at a cycle length of 300 msec and 35 +/- 8% (p less than 0.001) at the shortest 1:1 cycle length. The rate-dependent response of APD to drugs was qualitatively similar but quantitatively different among species. Human tissue showed the greatest frequency-dependent drug effects on repolarization, followed by tissue from dogs and rabbits. Guinea pig atria showed the least (and statistically nonsignificant) rate-dependence of drug effect on APD. Drug-induced changes in refractoriness paralleled those in APD. We conclude that: 1) flecainide and quinidine both increase APD in human atrial tissue but with opposite rate-dependence, 2) the effects of flecainide to increase atrial APD and refractoriness are enhanced by the rapid rates typical of atrial fibrillation, and 3) animal tissues may differ importantly from human in both their sensitivity and rate-dependent response to antiarrhythmic drugs. The salutary response of atrial fibrillation to flecainide may be due to enhancement of drug action by the rapid atrial activation rates characteristic of this arrhythmia.