Spontaneous and induced cardiac arrhythmias in subendocardial Purkinje fibers surviving extensive myocardial infarction in dogs

Intracellular K+ activity, intracellular Na+ activity and maximum diastolic potential of canine subendocardial Purkinje cells from one-day-old infarcts

The basis for the reduced maximum diastolic potential of canine cardiac subendocardial Purkinje fibers surviving one day after extensive transmural infarction was investigated, using double-barrel potassium and sodium ion-sensitive microelectrodes. The maximum diastolic potential of Purkinje fibers in infarct preparations from the left ventricular apex measured during the first hour of superfusion in a tissue bath was -50.1 +/- 13.7 mV, a value markedly reduced from the value in control Purkinje fibers from noninfarcted preparations (-85.0 +/- 4.5 mV). The intracellular potassium ion activity was reduced by 50.4 mM during this time (intracellular potassium ion activity equals 61.6 +/- 16.1 mM, as compared to control intracellular potassium ion activity of 112 +/- 19.8 mM). The potassium equilibrium potential was reduced by 16.0 mV (from -97.2 +/- 4.7 mV in controls to -81.2 +/- 6.9 mV), thus accounting for about one half of the reduction in the maximum diastolic potential. After 6 hours of superfusion, the maximum diastolic potential increased to -78.9 +/- 8.7 mV (still significantly less than control). The potassium equilibrium potential had largely recovered (-93.8 +/- 5.9 mV). The intracellular sodium ion activity of Purkinje fibers in the infarcts (15.6 +/- 6.9 mM) was elevated during the first hour of superfusion by 6.2 mM compared to control (9.4 +/- 2.6 mM), and this was only 12% as much as the initial intracellular potassium ion activity decrease. Sodium ion activity after 3-6 hours of superfusion was not significantly different than normal (12.1 +/- 4.9 mM). In conclusion, only a portion of the maximum diastolic potential changes can be explained by a reduction of the potassium equilibrium potential. It is likely that change(s) in the cell membrane sodium-potassium pump's function and in the membrane conductance are also involved. Furthermore, the lack of a compensatory increase in intracellular sodium ion activity accompanying the large reduction of intracellular potassium ion activity may be a consequence of the cellular acidosis, which is known to occur during myocardial ischemia.

Effects of tocainide on Purkinje fibers from normal and infarcted ventricular tissues

The left anterior descending coronary artery was occluded in anesthetized dogs. Dogs were sacrificed after 24 hours and the experimental preparations, which included both normal and infarcted tissues, were dissected from the left ventricles. Effects of tocainide in concentrations of 15-40 mg/l on action potentials of Purkinje fibers from normal and infarcted zones were studied using conventional microelectrode techniques. In the normal zone cells, tocainide superfusion produced a significant decrease in maximum diastolic potential, action potential amplitude, action potential duration to 50% and 90% repolarization and the rate of phase O depolarization, and no significant change in effective and functional refractory periods. In the infarct zone cells, it produced a significant decrease in action potential amplitude and the rate of phase O depolarization, a significant increase in effective and functional refractory periods, and no significant changes in the other parameters. The unequal actions of tocainide resulted in selective depression of maximum diastolic potential and action potential duration to 50% repolarization in the normal cells only, reducing the disparity in these parameters between normal and infarcted tissues. Tocainide increased the refractoriness (ratio of effective refractory period to action potential duration) in both cell types but this change was greater in the infarct zone. This decreased disparity of membrane potential and repolarization combined with increased refractoriness may help to block the arrhythmias observed in infarcted preparations following closely coupled stimuli.

Contribution of depolarized foci with variable conduction impairment to arrhythmogenesis in 1 day old infarcted canine cardiac tissue: an in vitro study

To assess the roles of entrance and exit block after canine myocardial infarction, single stage coronary artery ligations of canine circumflex coronary arteries were performed. After 1 day, atria and ventricles were paced using single stimuli and trains. After isolation, simultaneous microelectrode impalements were made in infarcted and uninfarcted tissue. Spontaneous foci, when identifiable, were always located in infarcted tissue. They could frequently be triggered by one or more driven beats, and their activity could often be terminated ("annihilated") by a properly timed beat. Foci with varying combinations of entrance and exit conduction impairment were observed. Variations in conduction characteristics altered the manifest arrhythmic pattern. With partial entrance block and intact exit conduction, foci could be electrotonically modulated and entrained into regular patterns. Activity that emerged from a focus with sufficient conduction delay could modulate the focus, and entrain it to discharge at a slower rate ("autoentrainment"). The results suggest that modulated parasystole may contribute to arrhythmogenesis after canine myocardial infarction and that variations in entrance and exit characteristics of depolarized foci may result in variable and complex arrhythmic patterns.

Serial production of controlled periods of temporary heart block used to unmask and assess latent ventricular automaticity during experimental acute myocardial ischemia

This study examined the onset, time course of development and response to overdrive stimulation of ventricular tachycardia in 10 dogs that underwent a Harris two-stage ligation of the left anterior descending coronary artery. Transient (12 +/- 3 minutes) complete atrioventricular (AV) block was produced 2, 3, 4, 5, 8, 12, 16, 20 and 24 hours after onset of infarction through selective injection of physostigmine salicylate into the AV node artery. Seven of the 10 dogs had early transient arrhythmic episodes that occurred within 20 to 40 minutes after coronary occlusion but none of the dogs had any spontaneous ventricular tachycardia in the ensuing 2 hours. Two hours after left anterior descending coronary artery ligation, complete AV block unmasked in every dog a slow (37 +/- 9 beats/min) AV junctional rhythm readily suppressed by overdrive. Three hours after coronary ligation, AV block revealed a monomorphic ventricular tachycardia (106 +/- 10 beats/min) in 3 of the 10 dogs. Four and five hours after coronary ligation, five and eight dogs, respectively, had ventricular tachycardia during AV block and in three the tachycardia was polymorphic. The two remaining dogs did not develop ventricular tachycardia during the 24 hours of observation. Ventricular tachycardia always began abruptly, first with brief and then longer bursts. Soon after onset the rate of tachycardia began to increase to reach a plateau 2 to 3 hours later at frequencies 21 +/- 9% greater than the initial tachycardia rate. Concomitant with this increase in rate there was a steady decline of overdrive suppressibility and during the plateau phase there was little or no overdrive suppression.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological effects of tocainide on canine subendocardial Purkinje fibers surviving infarction

The effects of 10 and 20 mg/l of tocainide on transmembrane action potential characteristics were examined in Purkinje fibers surviving infarction. Infarcted tissue was obtained from canine hearts 24 h after coronary artery ligation. The preparation was stimulated at basic cycle lengths (BCL) of 1000-300 ms. Tocainide reduced the overshoot and amplitude of Purkinje fibers surviving infarction. The maximum upstroke velocity (Vmax) was decreased by tocainide in a dose dependent manner. This effect was more prominent at the shorter BCL. Statistical analysis revealed a significant interaction of the BCL with the drug effect on overshoot, amplitude, Vmax and action potential durations (APD50% and APD90%). Tocainide reduced the effective refractory period (ERP) at the BCL of 1000 ms, but had no significant effect at the BCL of 300 ms. Membrane responsiveness and steady state characteristics of Vmax were shifted significantly to more negative membrane potentials by tocainide. Investigation of the recovery kinetics of Vmax in the presence of tocainide showed an exponential recovery of Vmax with a time constant of 514 ms. These results support the finding that the effect of tocainide on Vmax and conductions is enhanced at faster rates of stimulation. Thus tocainide may be able to depress conduction to produce bidirectional block in the termination of ventricular tachycardia caused by reentry, while having minimal effect on conduction at normal heart rates.

Effects of collateral circulation on electrophysiological properties during the acute phase of canine myocardial infarction

To investigate the effect of collateral circulation on the electrophysiological properties of the acutely ischemic myocardium, acute myocardial infarction was produced in the canine heart by coronary artery occlusion alone, and by coronary artery occlusion plus embolization with vinyl latex. Multiple bipolar electrodes for stimulation or for recording electrograms were placed on the subepicardial layer and the subendocardial layer to examine the time course of changes in excitability threshold, effective refractory period, and conduction time. Soon after coronary occlusion plus embolization, electrophysiological properties of the ischemic subepicardium became severely and almost uniformly damaged and showed no recognizable recovery of electrical activities, whereas transient deterioration and subsequent recovery of the electrophysiological properties were observed after coronary occlusion alone. On the other hand, the subendocardium was much less affected electrophysiologically by either coronary occlusion alone or coronary occlusion plus embolization. These results indicate that collateral circulation plays an important role in the recovery from electrophysiological abnormalities in the ischemic subepicardium caused by acute myocardial ischemia, but has little effect on the electrophysiological properties of the ischemic subendocardium.

Autonomic control of ventricular tachycardia: sympathetic neural influence on spontaneous tachycardia 24 hours after coronary occlusion

This study was performed to determine whether sympathetic nerves influence the rate of ventricular tachycardia occurring spontaneously in dogs 24 hr after occlusion of the anterior descending coronary artery. Seventeen chloralose-anesthetized dogs underwent activation mapping during spontaneous ventricular tachycardia with QRS morphologies similar to those recorded in the conscious state. Bilateral stellate ganglionectomy (n = 8) decreased mean arterial pressure from 71 +/- 4 (mean +/- SE) to 52 +/- 5 mm Hg (p less than .001) and heart rate from 121 +/- 9 to 79 +/- 15 beats/min (p less than .025) by decreasing the number of complexes of ventricular tachycardia from 120 +/- 9 to 49 +/- 15 per minute (p less than .001). Subsequent unilateral sympathetic nerve stimulation (n = 4) was shown to accelerate ventricular tachycardia foci originating from the ipsilateral aspect of the infarction. Regional sympathetic denervation (n = 7) was performed by application of phenol to the epicardium surrounding an electrode at the site of origin of at least one morphology of ventricular tachycardia. Mean arterial pressure did not change, but total heart rate decreased from 122 +/- 9 to 106 +/- 9 beats/min (p less than .01) and the number of complexes of ventricular tachycardia with a morphology arising from the phenol-treated area fell from 68 +/- 12 to 28 +/- 9 (p less than .001). Evidence for regional denervation was documented by prolongation of duration of electrograms and local repolarization times limited to the phenol-treated area. We conclude that sympathetic nerves directly control rate of spontaneous ventricular tachycardia 24 hr after myocardial infarction in the dog.

An evaluation of automaticity and triggered activity in the canine heart one to four days after myocardial infarction

Both abnormal automaticity and triggered activity induced by delayed afterdepolarizations have been proposed as the primary mechanism for ventricular tachycardia (VT) occurring in dogs 24 hr after ligation of the left anterior descending coronary artery. Because of this controversy, we studied the effects of ventricular pacing and therapeutic concentrations of lidocaine and ethmozin on sustained rhythmic activity of isolated subendocardial preparations excised from the infarct, and on VT in conscious dogs. There were differences in the sustained rhythmic activity cycle length of isolated preparations and the VT cycle length that were attributable to the absence of sympathetic input in the former and its presence in the latter. In isolated tissues, pacing for 1 or 10 beats reset the sustained rhythmic activity and pacing for 1 min induced overdrive suppression. Lidocaine (5 micrograms/ml) had no effect on sustained rhythmic activity but ethmozin (2 micrograms/ml) suppressed it. Delayed afterdepolarizations occurred but appeared to be induced by pacing or by the hyperpolarization associated with recovery. Although delayed afterdepolarizations were infrequent at 24 hr, their frequency increased with the hyperpolarization of the membrane that occurred at 48 to 96 hr after infarction. Delayed afterdepolarizations also occurred more readily when superfusate temperature was lowered. In conscious dogs, pacing the VT for 1 or 10 beats or 1 min had no effect. Lidocaine (2 to 10 micrograms/ml) did not affect the VT but ethmozin (2 to 5 micrograms/ml) increased VT cycle length significantly. Pacing for 1 min in the presence of ethmozin, but not lidocaine, converted VT to sinus rhythm. Our results suggest that although delayed afterdepolarizations occur at 24 hr after infarction in the standard Harris preparation, they are most readily seen as an accompaniment of hyperpolarization, pacing, or lowering of bath temperature. The predominant rhythm at 24 hr appears to be automatic.

Ectopic ventricular tachycardia sensitive to calcium antagonists in acute myocardial infarction in dogs

The effects of antiarrhythmic agents on automatic ventricular tachycardia (VT), which emerged in the early stage of acute myocardial infarction (AMI), were examined in 30 closed-chest mongrel dogs. Antiarrhythmic agents were administered intravenously when the rate of VT became almost equal to sinus rate (5.6 +/- 1.4 hours). VT was slowed significantly by verapamil (0.15 or 0.3 mg/kg), diltiazem (0.2 or 0.4 mg/kg), propranolol (0.1 mg/kg) and amiodarone (5 mg/kg), but not by procainamide (20 mg/kg), lidocaine (2 or 4 mg/kg), nifedipine (0.01 mg/kg) and nicorandil (0.03 mg/kg). The number of ventricular premature complexes was reduced most effectively by verapamil. The significant suppressive effects of calcium antagonist drugs (verapamil and diltiazem) and propranolol indicate that an inward calcium current during diastole may play a critical role in the abnormal enhancement of ventricular automaticity in the early stage (4 to 8 hours) of AMI.

Structural and electrophysiological changes in the epicardial border zone of canine myocardial infarcts during infarct healing

Structural and electrophysiological properties of the epicardial muscle which survives on the surface of transmural infarcts of the canine heart (epicardial border zone) were studied at different times after occlusion of the left anterior coronary artery (LAD). Isolated preparations were superfused in vitro, transmembrane potentials recorded, and impulse propagation mapped. In preparations from subacute infarcts (1 and 5 days), resting potential, action potential amplitude, upstroke velocity, and duration were all significantly reduced. Well-defined directional differences in propagation occurred. Propagation was more rapid in the direction perpendicular to the left anterior coronary artery than in the direction perpendicular to the base of the heart, because of the uniform anisotropic structure of the surviving muscle fibers which were arranged in tightly packed bundles oriented perpendicular to the left anterior coronary artery. The only ultrastructural abnormalities found in these muscle fibers was an accumulation of large amounts of lipid droplets. As the infarcts healed, resting potential, action potential amplitude, and upstroke velocity returned to normal by 2 weeks, although action potential duration decreased further. Lipid droplets had disappeared, and connective tissue had invaded the epicardial border zone, separating the muscle bundles. By 2 months, action potentials were normal, but the muscle fibers were widely separated and disoriented by the connective tissue (parallel bundles no longer were found). In these regions with a nonuniform anisotropic structure, the well-defined directional differences in impulse propagation were lost. However, activation was very slow, perhaps because of diminished connections between cells. The persistence of slow conduction in healed infarcts may contribute to the occurrence of chronic arrhythmias.

Spontaneous ventricular tachycardia associated with isolated right ventricular infarction, one day after right coronary artery occlusion in the dog: studies on the site of origin and mechanism

The electrophysiologic and arrhythmic properties of isolated infarcted right ventricle (RV) were studied in 17 dogs during the first 24 hours after complete occlusion of the right coronary artery (RCA). During the 16-to-20-hour post occlusion period, spontaneously occurring sustained monomorphic ventricular tachycardia (VT) was present in all 17 dogs. Overdrive ventricular pacing (cycle lengths 200 to 250 msec) caused significant suppression of the VT when the rate of the VT was slower than 150 bpm (range 120 to 145 bpm) (n = 9), but had negligible effect when VT rate was higher than 150 bpm (range 160 to 245 bpm (n = 8). Overdrive pacing could not terminate either the slow or the fast type of VT. Bipolar intramural electrograms have showed electrical activity in the infarcted RV zone to precede Q wave of the VT by 15.4 +/- 5.8 msec regardless of VT rate. Microelectrode studies on isolated RV endocardial infarcted tissues 24 hours after RCA occlusions have shown the presence of spontaneous repetitive activity at a rate of 87 +/- 47 bpm, which was overdrive suppressed in dogs with slow VT, and spontaneous activity at a rate of 115.2 +/- 36 bpm (p less than 0.05) which was not overdrive suppressed in dogs with fast VT. Maximum diastolic potential, action potential amplitude, and Vmax of surviving subendocardial Purkinje fibers (SEPF) in the infarct zone were slightly but significantly depressed (p less than 0.05), and they manifested enhanced phase 4 depolarization, giving rise to automatic impulse initiation. Although action potential duration of these fibers was somewhat prolonged (p less than 0.05), no conduction delay occurred. Histopathologic examinations have shown necrosis of the basal two thirds of the RV, with no left ventricular involvement. Electron microscopy revealed lipid accumulation in the surviving SEPF as the sole abnormality. We conclude (1) that occlusion of the RCA in the dog is associated with high survival rate despite extensive necrosis involving exclusively the RV and (2) that VT seen during the 20 to 24 hours after occlusion arise in the infarcted zone of the RV, by an enhanced automatic mechanism in the surviving SEPF, possibly caused by cytoplasmic lipid accumulation. This model, by virtue of its high survival rate and frequency of late VTs, should be useful in providing clues to determine factors involved in the genesis of early VT/VF and for the evaluation of new pharmacologic agents during the 20- to 24-hour VT period.

Physiopathology of ventricular tachyarrhythmias

Ventricular arrhythmias may be the result of three mechanisms: abnormal automaticity, triggered activity complicating early or late after-depolarizations, and reentry by circular pathway or by reflection. These three fundamental mechanisms have been observed in the intact heart in experimental models of myocardial ischemia and digitalis intoxication. In man, the arguments in favor of a given mechanism are indirect and may be determined by their response to stimulation. It may be possible to state the following conclusions: (1) reentry is at the origin of ventricular fibrillation, certain ventricular tachycardias of bundle branch reentry and probably most chronic sustained ventricular tachycardias that are easily inducible; (2) the mechanism of certain other ventricular arrhythmias sustained remains unknown; (3) even when arrhythmias are associated with reentry, the triggering extrasystole can arise from a focal origin.

Sustained ventricular tachycardia in patients with idiopathic dilated cardiomyopathy: electrophysiologic testing and lack of response to antiarrhythmic drug therapy

Eleven consecutive patients with idiopathic dilated cardiomyopathy and spontaneous, sustained ventricular tachycardia (VT) of uniform morphology underwent programmed ventricular stimulation and serial antiarrhythmic drug testing. The mean ejection fraction was 30 +/- 6.4%. Sustained VT was induced by programmed electrical stimulation in all 11 patients. A mean of 3.7 +/- 2.4 antiarrhythmic drugs were evaluated by programmed stimulation, including at least one experimental agent in eight patients. In nine of 11 patients VT remained inducible on all drug therapy. During a mean follow-up period of 21 +/- 14 months there were four sudden deaths and two patients with recurrences of VT. In all six patients with sudden death or recurrence of VT, the arrhythmia remained inducible on drug therapy. Three patients who died suddenly had a hemodynamically stable, induced tachycardia on antiarrhythmic therapy. Of eight patients treated with amiodarone, only two were successfully treated. We conclude that in patients with sustained VT and idiopathic dilated cardiomyopathy, VT can be induced by programmed electrical stimulation. VT will usually remain inducible on antiarrhythmic therapy, and sudden death can occur despite slowing and improved tolerance of the induced arrhythmia. Amiodarone may have limited efficacy, and more aggressive therapy, such as surgery or implantation of an automatic internal defibrillator, should be considered in this patient population.

Rationale of therapy in the patient with acute myocardial infarction and life-threatening arrhythmias: a focus on bretylium

Experimental evidence suggests a number of pathologic and electrophysiologic mechanisms that may help initiate ventricular arrhythmias accompanying myocardial ischemia and infarction. Early and late phase events are associated with reentry or an enhancement of focal mechanisms, or both. These can initiate ventricular tachycardia (VT) or ventricular fibrillation (VF), or both. The presence of distinct mechanisms that may initiate and maintain life-threatening dysrhythmias early in myocardial ischemia suggest different pharmacologic approaches for their prevention or suppression. Another consideration concerns patients subjected to coronary artery angioplasty or thrombolytic therapy and the development of arrhythmias associated with reperfusion of the once ischemic myocardium. The electrophysiologic mechanisms associated with reperfusion arrhythmias are unknown, and little is known about appropriate therapy for each episode of cardiac dysrhythmia. Ventricular extrasystoles or VT usually precedes VF. These premonitory arrhythmias are poor criteria for the institution of antiarrhythmic drug therapy, because VF develops within 1 to 10 minutes after the appearance of the rhythmic disturbances. Some authorities suggest that all patients with acute myocardial infarction should receive prophylactic antiarrhythmic therapy, because warning arrhythmias either do not occur at all or provide insufficient time to intervene pharmacologically. Many of the new class I antiarrhythmic agents effectively reduce the frequency of premature ventricular depolarizations, but lack specific antifibrillatory activity. However, the recent introduction of bretylium into clinical cardiology opens a new approach to preventing life-threatening ventricular dysrhythmias. Along with other members of class III, bretylium exerts different cardiac electrophysiologic effects than do the other 3 classes of drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Mapping of ventricular tachycardia induced by programmed stimulation in canine preparations of myocardial infarction

To investigate the mechanism of uniform ventricular tachycardia induced by programmed stimulation, we recorded His bundle electrograms and unipolar electrograms from 64 subepicardial, subendocardial, and intramural sites in dogs. Isochronal maps were generated off-line by computer. Two groups of dogs were studied 3 days after occlusion of their left anterior descending coronary arteries; one group underwent reperfusion after 2 to 2.5 hr of occlusion and the other methylprednisolone treatment before permanent occlusion. In the former, subepicardial sequences presented either a pattern suggesting circus movement or a radial pattern in which excitation at intramural sites could precede earliest subepicardial excitation. In the latter preparations, subepicardial excitation patterns consistently suggested circus movement in the subepicardial muscle layer surviving over necrotic tissue. Assuming complete circus movement, the "missed" time interval, measured as the interval left unaccounted for by actual recording of local excitation between ventricular tachycardia cycles, ranged from 3% to 64% of the cycle length of ventricular tachycardia. While surviving subepicardial and intramural layers appeared to be involved in the mechanism of ventricular tachycardia, a late second breakthrough on the right ventricle, in conjunction with fixed-coupled H deflections on the His bundle electrograms, suggested the involvement of the conducting system in propagation of the impulse.

New directions in the development of antiarrhythmic drugs

To identify new antiarrhythmic drugs and their mechanisms of actions, it has been necessary to use appropriate models to describe arrhythmias. Recently introduced concepts in cellular electrophysiology of arrhythmias provide the newest models in which drug development have been attempted. Arrhythmias have been attributed to abnormalities of conduction, abnormalities of impulse formation, or a combination of both. Recent descriptions of abnormally triggered rhythms have been ascribed to a phenomenon called after depolarizations. An ionic basis for the formation of these arrhythmias has been recently described. Further, specificity of the drug effects at a cellular level to modify these arrhythmias are being observed by a variety of new techniques. Microelectrode methods have provided direct access to the intracellular milieu to determine the biophysical changes brought about by antiarrhythmic agents. These observations have led to a greater understanding of the underlying abnormalities at the cellular level. Testing of antiarrhythmic drugs in the intact heart has led to the development of clinical use of invasive electrophysiologic techniques. Intracardiac catheters are being used to describe abnormalities of impulse initiation as well as conduction. Efficacy of antiarrhythmic drugs in the intact heart may be determined with greater accuracy using these techniques. Abnormal rhythms can be initiated and terminated using intracardiac catheter methods with antiarrhythmic agents as therapeutic adjuncts.

Electrophysiologic basis for arrhythmias in ischemic heart disease

Substantial gains have been made toward clarifying the mechanisms of arrhythmia in ischemia in animal models. After coronary occlusion in the dog, ischemic myocardial cells have reduced resting potential and slowed and diminished upstrokes of action potentials due to depression of fast channels. As a result, conduction is slow and irregular, especially at shorter cycle lengths, because refractoriness is altered by a delay in recovery of the fast channels beyond the completion of repolarization. These abnormalities occur during the acute phase of arrhythmia in the first half hour after occlusion and persist in surviving the subepicardial layers of myocardial cells for days to weeks. Reentry has been mapped in these surviving layers. Reentrant circuits form around regions of functional block formed by interfaces between responding and refractory myocardium. Standard antiarrhythmic agents generally are fast-channel blockers that further depress conduction and prolong refractoriness in ischemic tissue, causing block in slow conducting segments of the reentry circuits. However, antiarrhythmic agents may cause or accentuate reentrant arrhythmias by virtue of the same depressant actions. The greater likelihood of antiarrhythmic agents suppressing rather than producing reentrant arrhythmias may be due to enhanced depressant effects of antiarrhythmic agents on very slowly conducting tissues that are involved in reentry circuits. After the acute phase, arrhythmias occurring 1 to 4 days after coronary occlusion are probably largely automatic, although the potential for reentry remains if the cycle length is shortened. Abnormally enhanced automaticity and triggered activity are demonstrable in the surviving Purkinje network in regions of infarction, but the role of these phenomena in vivo has not been clarified.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of nifedipine on triggered activity in 1-day-old myocardial infarction in dogs

Triggered activity arising from a delayed afterdepolarization occurs in canine subendocardial Purkinje fibers 1 day after myocardial infarction (MI). Standard microelectrode techniques were used to study small preparations (20 to 48 mm2) in vitro. Nifedipine, 1 mg/liter, reversibly suppressed triggered activity by reducing maximum diastolic potential, action potential amplitude and the rate of depolarization of the delayed afterdepolarization. Complete quiescence or exit block resulted. The effects of nifedipine were antagonized by elevating extracellular calcium ion concentration. These results suggest that spontaneous ectopic rhythms 1 day after MI that are the result of triggered activity are dependent on transmembrane calcium ion movement, which nifedipine can directly antagonize.

Pathophysiologic mechanisms of cardiac arrhythmias

Arrhythmias result from abnormalities of impulse initiation or impulse conduction or a combination of both. Abnormal impulse initiation results from either automaticity or triggered activity. Automaticity can further be subdivided into (1) automaticity caused by the normal automatic mechanism (a normal property of cardiac cells in the sinus node, in some parts of the atria, in the atrioventricular junctional region, and in the His-Purkinje system) and (2) automaticity caused by an abnormal mechanism (resulting from a decrease in membrane potential of cardiac fibers, which normally have a high level of membrane potential). Triggered activity is caused by afterdepolarizations, which are second depolarizations that occur either during repolarization (referred to as early afterdepolarizations) or after repolarization is complete or nearly complete (referred to as delayed afterdepolarizations). Abnormal impulse conduction results in reentrant excitation. Usually a combination of slowed conduction and unidirectional conduction block provides the conditions necessary for reentry to occur. Slow conduction and block may result from a decrease in the resting potential and velocity of depolarization of the action potential or may be a consequence of the anisotropic structure of cardiac muscle, in which case resting potential and action potential upstroke velocity may be normal.

Recurrent sustained ventricular tachycardia: structure and ultrastructure of subendocardial regions in which tachycardia originates

Surgical resection of the endocardium and subendocardium often abolishes chronic recurrent sustained ventricular tachycardia in patients with healed myocardial infarcts or ventricular aneurysms, presumably by interrupting the reentrant pathway. To define the morphologic characteristics of cells in the reentrant pathway, we studied the histology and ultrastructure of the endocardial resections of 23 patients who underwent this procedure. Bundles of apparently viable myocardial fibers embedded in dense fibrous tissue were identified throughout the endocardial resections from all patients. These bundles of cells were separated from one another by fibrous tissue but extended uninterrupted to the margins of the surgical resection. In 14 patients Purkinje fibers were identified beneath the thickened endocardium whereas the remaining bundles were composed of ventricular muscle. The Purkinje fibers appeared to have normal ultrastructure and ventricular cells with both normal and abnormal ultrastructures were present. The abnormal muscle cells were characterized by loss of contractile elements, aggregates of dilated sarcoplasmic reticulum, and osmiophilic dense bodies. The sarcolemma was intact and the nuclear chromatin was evenly dispersed suggesting that these cells were still viable. The abnormal structure and arrangement of the surviving cardiac fibers in the endocardium may cause the abnormal electrophysiologic function that results in ventricular tachycardia.

Sustained ventricular tachyarrhythmias during the early postinfarction period: electrophysiologic findings and prognosis for survival

Forty patients with sustained tachycardia occurring 3 to 65 days after myocardial infarction underwent programmed ventricular stimulation within 3 months of the infarction. Patients were characterized clinically by a complicated initial 48 hours of hospitalization for their acute infarction (85% of study group). The development of bundle branch block in association with infarction occurred with an unusually high frequency (32%). Ventricular tachycardia similar in configuration to spontaneous arrhythmia was induced with programmed ventricular stimulation in 33 (83%) of the 40 patients. In 15 (45%) of these 33 patients, additional morphologically distinct ventricular tachycardia not seen clinically was initiated. The induction of ventricular tachycardia was not significantly related to the time after myocardial infarction at which spontaneous ventricular tachycardia was initially observed. Only 20 of the 40 patients are alive after a mean follow-up period of 20 +/- 15 months. Twelve of the 20 deaths were sudden cardiac deaths. Sixteen of the 33 patients with inducible ventricular tachycardia died; 8 of the 16 deaths were sudden. By comparison, four of the seven patients with no inducible ventricular tachycardia died (probability [p] = not significant), all suddenly. The mode of therapy did not influence subsequent survival. It appears that in patients with sustained ventricular tachycardia occurring more than 48 hours after a recent myocardial infarction, ventricular tachycardia similar to that clinically observed can usually be induced by programmed stimulation. In addition, multiple morphologically distinct ventricular tachycardias, some of which have not been previously observed, are frequently induced. Finally, the prognosis for survival is poor, regardless of inducibility or mode of therapy, and may in part be related to a changing arrhythmia substrate.

The efficacy of Ajmaline in ventricular arrhythmias after failure of lidocaine therapy in the acute phase of myocardial infarction

Forty-three patients in the acute phase of myocardial infarction who were resistant to conventional doses of lidocaine received Ajmaline intravenously (50 mg bolus followed by constant infusion rate of 1-1.5 mg/min). Dangerous ventricular arrhythmias were abolished in 72% of this group of patients (group A). In the remaining patients (28%), Ajmaline was found to be ineffective (group B). There was no reduction of systolic or diastolic blood pressure and there was an insignificant increase in heart rate. Atrio-ventricular or intraventricular conduction defects appeared in 46% of the patients described. There was a statistically significant increase in occurrence of heart blocks in group B patients and among these complete left bundle branch block (CLBBB) was the most prevalent. Atrio-ventricular or intraventricular conduction defects were transient, appearing between 8-36 h (mean 23 h), and were not accompanied by reduction of ventricular rate. Conduction defects disappeared within several hours (up to 24 hours) after Ajmaline was discontinued. It is concluded that Ajmaline administered by this regimen is an effective alternative agent for patients with ventricular arrhythmia not controlled by lidocaine in the acute phase of myocardial infarction.

Termination of ventricular tachycardia by single extrastimulation during the ventricular effective refractory period

Termination of ventricular tachycardia by low-energy shocks delivered during the ventricular refractory period has been reported. We describe a case of reproducible termination of multiple episodes of sustained ventricular tachycardia by a low-current extrastimulus delivered during the effective refractory period of the right ventricle, from the distal bipole of a quadripolar electrode catheter.

Electrophysiologic observations during the spontaneous initiation of ischemia-induced ventricular fibrillation

Electrophysiologic features of spontaneous, ischemia-induced ventricular fibrillation were studied in 17 dogs using multiple endocardial bipoles positioned in normal and ischemic zones and at the border of ischemic myocardium. All dogs showed ventricular tachyarrhythmias prior to the initiation of ventricular fibrillation. The heart rate prior to the fatal arrhythmia in the ventricular fibrillation dogs was significantly faster than that of nonventricular fibrillation dogs. There was no difference in the coupling intervals of the initial premature complex between episodic and sustained ventricular arrhythmia in most dogs. However, shorter coupling intervals initiated sustained arrhythmia in some dogs. Sites of initiation of arrhythmia were mostly in the ischemic zone. Furthermore, diastolic electrical activity was consistently observed in the ischemic zone during fatal arrhythmia in dogs showing diastolic activity. Cycle length during the fatal arrhythmia prior to ventricular fibrillation gradually shortened, whereas cycle length of episodic ventricular tachycardia remained approximately 200 msec followed by lengthening prior to restoration of sinus rhythm. The disparity of local activation (time differences between the earliest and latest onset of the activation in the five recording sites) increased during the fatal arrhythmia. Examples of progressive intraventricular block (Wenckebach-like) between the border and the center of ischemic myocardium leading to ventricular fibrillation are resynchronization of this disparity leading to the termination of ventricular tachycardia are shown. The recording of continuous electrical activity using bipolar electrodes with an interelectrode distance of 1 mm suggests a smaller reentrant pathway during fatal arrhythmia. Our observations confirm the importance of endocardial recordings within ischemic myocardium, and adds new insight into the events leading to both episodic and sustained ventricular tachycardia.

Contribution of variable entrance and exit block in protected foci to arrhythmogenesis in isolated ventricular tissues

Automatic foci with membrane potentials in the range characterized by depolarization-induced automaticity exhibit entrance block. The present study demonstrates a role of variable entrance and exit block in arrhythmogenesis. We studied canine interventricular septa with the right bundle branch exposed, isolated false tendons and isolated feline papillary muscle using standard microelectrode techniques. Foci of automaticity were produced either by focal application of electric current or by exposure of the preparations to Tyrode's solution containing 1.5-2.0 mM KCl. Foci induced by mild depolarization exhibited entrance block with exit conduction and were subject to electrotonic modulation. With greater depolarization, varying degrees of exit block developed. Various rhythms, including Wenckebach periodicity, resulted. Delayed emergence of electrotonically accelerated activity led to closely coupled extrasystoles resembling reentrant activity. Exit conduction in some preparations was facilitated by enhanced normal pacemaker activity (membrane potentials -- 70 mV or greater) in tissue peripheral to the focus. Also, when there were two sites of automaticity separated by an area of depressed conduction, intermodulation between the two automatic regions generated complex arrhythmias. Shifts in maximum diastolic potential also changed conduction and led to changes in arrhythmic patterns. In some experiments, focal automaticity was terminated by single stimuli. We conclude that complex and variable behavior of automatic foci may result in activity with characteristics previously attributed to other arrhythmic mechanisms.

Effects of premature depolarization on refractoriness of ischemic canine myocardium

In 25 pentobarbital anesthetized dogs we measured refractory periods (RPs) of regularly driven complexes and premature ventricular depolarizations (PVDs) with a range of coupling intervals or of regularly driven complexes and the complex following the PVD, i.e. the postextrasystolic depolarization (PED). Measurements were made during control periods and during occlusion of a branch of the left anterior descending coronary artery. The difference in control and occlusion RPs was less following some PVDs with short coupling intervals than following other PVDs with longer coupling intervals. Variations in the coupling interval of PVDs had less effect on RPs of the PVDs in ischemic than in nonischemic tissue. RPs of PEDs were prolonged with respect to RPs of regularly driven complexes in both ischemic and nonischemic tissue, but the prolongation in ischemic tissue was significantly greater than that in nonischemic tissue, 8 +/- 4 msec and 2 +/- 2 msec respectively, p less than .001. The difference in effect of PVDs on RPs of ischemic and nonischemic tissue results in greater disparity of refractoriness between ischemic and nonischemic tissue following some long coupling interval PVDs than following some PVDs with shorter coupling intervals. In addition the greater prolongation of RPs of PEDs in ischemic than in nonischemic tissue can result in increased disparity in RPs than the disparity between ischemic and nonischemic tissue present during regular drive.

Ventricular activation patterns of spontaneous and induced ventricular rhythms in canine one-day-old myocardial infarction. Evidence for focal and reentrant mechanisms

We studied isochronal maps of ventricular activation during spontaneous multiform ventricular rhythms (rates 120-190/min) and pacing-induced ventricular tachyarrhythmias (rate 230-450/min) in dogs 1 day after myocardial infarction. Recordings were obtained from the entire epicardial surface and from selected endocardial and intramural sites utilizing a computerized multiplexing technique. Spontaneous ventricular rhythms had a focal origin from the surviving subendocardial Purkinje network underlying the infarction and showed frequent shift of the pacemaker site. On the other hand, fast ventricular tachyarrhythmias were consistently induced in the same dogs by bursts of rapid ventricular pacing or programmed premature stimulation and had a tendency to degenerate into ventricular fibrillation. Pacing-induced rhythms were due to reentrant activation that developed mainly in the surviving, electrophysiologically abnormal, epicardial layer overlying the infarction. The last stimulated heat that initiated reentry resulted in a continuous arc of functional conduction block and two slowly circulating activation fronts around both ends of the arc of block. The activation fronts rejoined on the distal side of the arc of block before breaking through the arc to reactivate an area proximal to the block. This resulted in splitting of the initial single arc of block into two separate arcs. Reentrant activation subsequently continued as two synchronous circuits which conducted in clockwise and counterclockwise directions, respectively. Reentry spontaneously terminated when the leading edge of both reentrant circuits encountered refractory tissue and failed to advance. The presence of two synchronous circuits was the hallmark of a stable reentrant activation. The development of three or more asynchronous circuits resulted in an activation pattern that was "prefibrillatory." This pattern was seen to develop during pleomorphic ventricular rhythms and ventricular tachycardias of the torsades de pointes type that degenerated into ventricular fibrillation. Ventricular fibrillation was maintained by continuously changing multiple asynchronous circuits. The transition from a stable reentrant activation pattern to that of ventricular fibrillation was probably related to nonhomogeneous shortening of refractoriness in different parts of the myocardium.

Method for evaluating the effects of antiarrhythmic drugs on ventricular tachycardias with different electrophysiologic characteristics and different mechanisms in the infarcted canine heart

In a canine model of myocardial infarction caused by coronary occlusion and reperfusion, ventricular tachycardia occurs spontaneously at 24 hours and has many of the characteristics of an accelerated idioventricular rhythm. It cannot be induced by premature or rapid stimulation of the ventricles; overdrive pacing during this tachycardia usually causes some transient overdrive suppression but occasionally there is overdrive acceleration. We suggest that this arrhythmia is caused mainly by enhanced automaticity. Ventricular tachycardia also can be induced by premature or rapid ventricular pacing 3 to 5 days after infarction, but it does not occur spontaneously. It can be stopped by overdrive pacing, suggesting that it is caused by reentry. Electrocardiographic features are identical to chronic, recurrent sustained ventricular tachycardia in human patients. Because the arrhythmias occurring at different times probably result from different mechanisms this canine model is useful for comparing the actions of drugs on different kinds of arrhythmias. Lidocaine and procainamide generally abolished tachycardia 24 hours after infarction, but only procainamide abolished tachycardia 3 to 5 days after infarction. Isoproterenol accelerated tachycardia at both times. Verapamil did not abolish tachycardia 3 to 5 days after infarction.

Early electrophysiologic and anatomic alterations in cat ventricular muscle after coronary artery ligation

The effect of coronary artery ligation on electrophysiologic properties of cat ventricular muscle cells was studied. Depression of resting potential, action potential rate of rise and amplitude was observed in infarcted cells, 30 min to 5 days after ligation. Action potential duration was markedly shortened in acute stages (30-120 min) but gradually lengthened to above control by 48 h. Anatomic sequelae included oedema, loss of fibre striation and cellular necrosis.

Coronary artery ligation in anesthetized rats as a method for the production of experimental dysrhythmias and for the determination of infarct size

Coronary artery ligation in anesthetized rats was assessed as a method for the production of experimental dysrhythmias and for the determination of infarct size. Following occlusion of the left main coronary artery, very marked ventricular dysrhythmias occurred in two distinct phases, an early and a late phase, at 0-30 minutes and at 1 1/2--4 hours after ligation respectively. Infarct size was measured at 4 hours after ligation using nitroblue tetrazolium to stain for the depletion of two cytoplasmic enzymes, lactate dehydrogenase and NADPH diaphorase. In this model, lignocaine (10 mg/kg plus an infusion of 5 mg/kg/hr) and ORG 6001 (10 mg/kg plus an infusion of 2.5 mg/kg/hr) had marked antidysrhythmic activity but neither drug influenced infarct size. The model, because of its simplicity, reproducibility, and low cost, should be useful for the screening of potential antidysrhythmic agents and may also be used to determine infarct size following coronary artery ligation.

Electrophysiological effects of encainide on isolated cardiac muscle and Purkinje fibers and on the Langendorff-perfused guinea-pig heart

The effect of encainide (0.1, 0.3 and 1.0 mg/l) on transmembrane electrical activity was studied in left auricle and papillary muscles of the guinea pig and in Purkinje fibers of cow and sheep hearts. Conduction times and refractoriness of the AV node and the His-Purkinje system were measured in the Langendorff-perfused guinea-pig heart by His-bundle recording. Encainide had no effect on the maximum diastolic potential. Action potential duration (100% repolarization) remained constant or slightly prolonged in guinea-pig auricle and ventricle preparations but was shortened in cow and sheep Purkinje fibers. Action potential amplitude and Vmax decreased in a dose-dependent manner, the effect being more pronounced at higher frequencies. The frequency effect was not related to a change in the recovery of Vmax following the repolarization. The drug reduced Vmax at all membrane potential values, the relative effect being slightly more pronounced at low membrane potentials. In three different tests for pacemaker activity encainide had no effect on the rate of diastolic depolarization at high levels of membrane potential, but clearly reduced the spontaneous oscillations at the plateau level and the transient depolarizations characteristic for triggered pacemaker activity induced by ouabain. Ca-mediated action potentials were not affected even at concentrations of encainide up to 8 mg/l. In the Langendorff-perfused guinea-pig heart, conduction through the His-Purkinje system was slowly more than conduction through the AV node. All effects were quickly reversible by drug-free perfusion. In comparison with lidocaine, the effect of encainide on electrical parameters showed the following differences: the decrease in Vmax in the presence of encainide was more frequency-dependent, and less potential-dependent, the effect of encainide on pacemaker activity was more selective.

Relationship between changes in left ventricular bipolar electrograms and regional myocardial blood flow during acute coronary artery occlusion in the dog

The purpose of this study was to determine whether a quantitative relationship existed between a reduction in regional myocardial blood flow, measured by radiolabeled microspheres, and the degree and type of changes in myocardial activation recorded in bipolar left ventricular subepicardial and subendocardial electrograms, in open-chest dogs following acute coronary artery occlusion. We found that the degree of regional myocardial ischemia was related quantitatively to the reduction in amplitude recorded with bipolar electrograms in the subepicardium and subendocardium, and to the increase in duration of subepicardial electrograms. Other characteristics measured in electrograms did not relate to the degree of ischemia. Despite a comparable reduction in regional myocardial blood flow, subepicardial conduction delay exceeded that recorded in the subendocardium, which often exhibited accelerated conduction.

Management of intractable ventricular tachyarrhythmias after myocardial infarction

Twenty-five patients with recent or old myocardial infarction were studied because they had life-threatening ventricular arrhythmias that required repeated cardioversions and were intractable to medical management. All patients had had a large anterior infarction a mean of 4.6 weeks before the emergence of the arrhythmias and all had severe left ventricular dysfunction. Cardiac catheterization or autopsy revealed a left ventricular aneurysm in 18 of 18 patients and obstruction of the left anterior descending coronary artery in 20 of 20 patients. Of 16 patients treated surgically with aneurysm resection or coronary bypass grafting, or both, 10 (62 percent) were alive after 3 to 39 (mean 26) months of follow-up. The perioperative mortality rate was 31 percent and only one patient died during the postoperative follow-up period 4 months after discharge from the hospital. By contrast, all nine medically treated patients died either in the hospital (four patients) or suddenly within 2 months of discharge (five patients). Ventricular fibrillation was documented as the cause of death in five of these patients. Surgical intervention was found to improve significantly the survival of these patients (P less than 0.02). The perioperative mortality rate was lower when at least 4 weeks had elapsed from acute infarction to surgery (10 versus 67 percent) and when the procedure included coronary bypass grafting (13 versus 50 percent), although these differences were not statistically significant (P greater than 0.05).

Effects of some components of ischemia on electrical activity and reentry in the canine ventricular conducting system

We used intracellular microelectrodes to study the electrophysiological effects of combinations of components of ischemia and their relation to the occurrence of ventricular arrhythmias in the specialized conducting system of isolated canine right ventricles. The middle area of the free wall was exposed to various test solutions in the center compartment of a three-chambered bath; the base and apex of the preparation were superfused with normal Tyrode's solution in the outer control compartments. Hypoxia (Po2 40 mm Hg), lactic acidosis (pH 6.5), and orciprenaline (10(-6) M), either alone or combined, failed to affect the action potential amplitude or the conduction velocity of the subendocardial fibers, and no arrhythmias occurred. The action potential duration and the effective refractory period were markedly prolonged by lactic acidosis. Exposure of the test regions to 15 mM K+ plus orciprenaline resulted in marked decreases in action potential amplitude and conduction velocity. Abnormalities of impulse transmission through the depressed area included high degrees of rate-dependent block, one-way block, warming-up phenomenon, and the Wenckebach phenomenon. Such conditions regularly provoked the appearance of single, sustained, or concealed reentrant depolarizations. The combined effects of hypoxia, 15 mM K+, and orciprenaline resulted in further depression of the already depressed action potential in the depolarized fibers. Our results indicate that regional increases of extracellular K+ may be the predominant factor of the components of ischemia we studied which facilitates the initiation of reentrant arrhythmias.