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

Behavior of ectopic surface: effects of beta-adrenergic stimulation and uncoupling

By using both experimental and theoretical means, we have addressed the progression of ectopic activity from individual cardiac cells to a multicellular two-dimensional network. Experimental conditions that favor ectopic activity have been created by local perfusion of a small area of cardiomyocyte network (I-zone) with an isoproterenol-heptanol containing solution. The application of this solution initially slowed down and then fully blocked wave propagation inside the I-zone. After a brief lag period, ectopically active cells appeared in the I-zone, followed by evolution of the ectopic clusters into slowly propagating waves. The changing pattern of colliding and expanding ectopic waves confined to the I-zone persisted for as long as the isoproterenol-heptanol environment was present. On restoration of the control environment, the ectopic waves from the I-zone broke out into the surrounding network causing arrhythmias. The observed sequence of events was also modeled by FitzHugh-Nagumo equations and included a cell's arrangement of two adjacent square regions of 20 x 20 cells. The control zone consisted of well-connected, excitable cells, and the I-zone was made of weakly coupled cells (heptanol effect), which became spontaneously active as time evolved (isoproterenol effect). The dynamic events in the system have been studied numerically with the use of a finite difference method. Together, our experimental and computational data have revealed that the combination of low coupling, increased excitability, and spatial heterogeneity can lead to the development of ectopic waves confined to the injured network. This transient condition appears to serve as an essential step for the ectopic activity to "mature" before escaping into the surrounding control network.

Initiation and propagation of ectopic waves: insights from an in vitro model of ischemia-reperfusion injury

The objective of the present study was to directly visualize ectopic activity associated with ischemia-reperfusion and its progression to arrhythmia. To accomplish this goal, we employed a two-dimensional network of neonatal rat cardiomyocytes and a recently developed model of localized ischemia-reperfusion. Washout of the ischemia-like solution resulted in tachyarrhythmic episodes lasting 15-200 s. These episodes were preceded by the appearance of multiple ectopic sources and propagation of ectopic activity along the border of the former ischemic zone. The ectopic sources exhibited a slow rise in diastolic calcium, which disappeared upon return to the original pacing pattern. Border zone propagation of ectopic activity was followed by its escape into the surrounding control network, generating arrhythmias. Together, these observations suggest that upon reperfusion, a distinct layer, which consists of ectopically active, poorly coupled cells, is formed transiently over an injured area. Despite being neighbored by a conductive and excitable tissue, this transient functional layer is capable of sustaining autonomous waves and serving as a special conductive medium through which ectopic activity can propagate before spreading into the surrounding healthy tissue.

Basic mechanisms of reentrant arrhythmias

The mechanisms responsible for active cardiac arrhythmias are generally divided into two major categories: (1) enhanced or abnormal impulse formation and (2) reentry. Reentry can be subdivided into three subcategories: (1) circus movement, (2) reflection, and (3) Phase 2 reentry. Reentry occurs when a propagating impulse fails to die out after normal activation of the heart and persists to re-excite the heart after expiration of the refractory period. Evidence implicating reentry as a mechanism of cardiac arrhythmias stems back to the turn of the century. Amplification of intrinsic electrical heterogeneities provides the substrate responsible for developing Phase 2 and circus movement reentry, which underlie ventricular tachycardia in the long QT and Brugada syndromes.

Pharmacotherapy following myocardial infarction--a review of current treatment practices

Modern treatment of acute myocardial infarction (AMI), including thrombolysis and early interventional strategies, has reduced mortality rates but increased the number of patients requiring medical treatment. Post infarction treatment with aspirin, statins, beta-blockers and angiotensin-converting enzyme (ACE) inhibitors has improved morbidity and mortality and is cost-effective. The major successes of secondary prevention have been seen in the prevention of reinfarction, recurrent ischaemia and the development of heart failure. However, in spite of recent advances and increasing knowledge, the mortality rates remain high, partly due to the under use of the established and documented medical strategies. Implementation of the current treatment strategies into general practice remains a challenge. The field is undergoing rapid change due to the increasing use of early invasive strategies. The primary objective should remain the prevention of underlying aetiology: coronary artery disease (CAD). Secondary prevention following myocardial infarction (MI) will remain a major challenge in clinical practice within the foreseeable future.

Non-Q wave myocardial infarction. Assessment and management of a unique and diverse subset

Acute cardiac events involving coronary symptoms, elevated enzyme levels, and electrocardiographic changes without the development of Q waves often result in higher rates of reinfarction and unstable angina than do more severe myocardial infarctions. The incidence of these non-Q wave events is on the rise, possibly because of earlier detection and treatment of heart disease. Familiarity with the characteristics and management of the condition, therefore, is more important than ever.

Intermittent parasystole with exit block

A 53-year-old patient had a complex ventricular arrhythmia, which most likely was an intermittent pure (nonmodulated) parasystole, due to protection limited to the first part of the cycle coexisting with exit block. However, other interpretations of the observations were possible. Foremost among these was modulated parasystole with partial protection or with an attenuated or nondetectable early delaying phase, with exit block. Interestingly, the parasystole appeared to have fulfilled the dynamic rules regarding the number of sinus beats interposed between manifest parasystolic beats, as has been reported for pure or weakly modulated parasystole. This case corroborates unusual manifestations of an arrhythmia, which because of its newly found complexities and various possible interpretations seems to be discussed with decreasing frequency in most textbooks on general cardiology.

Transient entrainment: the evolution of a medical concept from description to prescription

Entrainment is a phenomenon that has come to have considerable utility in cardiac electro-physiology diagnosis and treatment; specifically, to identify a zone of slow conduction in a reentrant circuit, a zone hypothetically vulnerable to intervention from the application of RF energy. The observation of entrainment has gone through an evolutionary sequence in the literature, from the initial simple observations of the phenomenon to the present stage of relatively fixed criteria of identification. This article follows the evolution of the specific features of the criteria of entrainment to their current crystallization into features that are suggested to prescribe sites for attempted ablation. This examination of the evolutionary course of the development of the conception of entrainment is of interest not only to cardiac electrophysiology, but also to philosophers of science, by illustrating how scientists emphasize and develop certain observations with the ultimate aim of applying the observations for successful intervention in pathological entities.

Triggered activity as a mechanism for inherited ventricular arrhythmias in German shepherd Dogs

OBJECTIVES

This study sought to determine whether early afterdepolarization-induced triggered activity is responsible for the initiation of ventricular arrhythmias in dogs with an inherited predisposition to sudden death.

BACKGROUND

We have identified a colony of German shepherd dogs that display inherited ventricular ectopic activity and sudden cardiac death. The arrhythmias in these animals are pause dependent but are not associated with a prolonged QT interval, suggesting that they might be initiated by early afterdepolarization-induced triggered activity in Purkinje fibers.

METHODS

Cardiac Purkinje fibers obtained from dogs that either did or did not exhibit ventricular tachyarrhythmias at the time of study were superfused in vitro with normal Tyrode solution (extracellular potassium ion concentration 4 mmol/liter) and were studied using standard microelectrode techniques.

RESULTS

Early afterdepolarizations and triggered activity occurred spontaneously in Purkinje fibers obtained from affected dogs (n = 7) but not in fibers obtained from unaffected dogs (n = 13). Exit conduction block of triggered responses occurred to varying degrees within the Purkinje fiber but not at the Purkinje-muscle junction. Overdrive pacing suppressed triggered activity. The reemergence of triggered activity after cessation of pacing was both time and rate dependent. Triggered activity in fibers obtained from affected dogs was potentiated by phenylephrine and epinephrine and was suppressed by isoproterenol. Triggered activity was not induced by phenylephrine or epinephrine in fibers obtained from unaffected dogs.

CONCLUSIONS

These results support the hypothesis that early afterdepolarization-induced triggered activity in Purkinje fibers is responsible for the initiation of ventricular arrhythmias in this canine model of inherited sudden death.

Effect of intracardiac repair on biosynthesis of thromboxane A2 and prostacyclin in children with a left to right shunt

OBJECTIVE

To investigate the effect of intracardiac repair on the abnormal biosynthesis of prostacyclin (PGI2) and thromboxane A2 (TXA2) in children with congenital heart disease and increased pulmonary blood flow.

DESIGN

A prospective study with immunoaffinity chromatography and gas chromatography-mass spectrometry to measure the urinary excretion products of PGI2 (2,3-dinor-6-oxo-prostaglandin (PG) F1 alpha (2,3-dinor-6-oxo-PGF1 alpha)) and TXA2 (2,3-dinor-TXB2) before operation, in the first 12-24 h after operation, and at discharge from hospital.

SETTING

A supraregional referral centre for patients with congenital heart disease.

PATIENTS

15 patients aged 2 to 60 months (median 7 months) with a left to right shunt who underwent intracardiac repair.

RESULTS

The preoperative 2,3-dinor-TXB2 excretion rate was greater than that found previously in a control group of 16 healthy children with a median (range) age of 24 (6-36) months (1159(201) v 592(122) ng/g creatinine in controls, P = 0.006). The excretion rate rose after operation to 9600(3832) ng/g creatinine (P = 0.01) and decreased before discharge to 1071(191) ng/g creatinine (NS), but remained greater than that of the control group (P = 0.014). Before operation 2,3-dinor-6-oxo-PGF1 alpha excretion rates were similar to those of the healthy children (482(68) v 589(95) ng/g creatinine in controls) but increased after operation to 19,668(11,162) ng/creatinine (P = 0.002) and fell at discharge to 1621(245) ng/g creatinine although this was higher than both preoperative and control rates (P = 0.005 and P = 0.0002 respectively). The preoperative ratio of 2,3-dinor-TXB2 to 2,3-dinor-6-oxo-PGF1 alpha excretion was greater than that of the control group (3.2(0.8) v 1.3(0.22) in controls, (P = 0.005)), decreased significantly after operation to 0.9(0.13) (P = 0.016), and changed little, to 0.7(0.12), before discharge. The last two ratios were similar to those in normal children and significantly lower than those before operation (P = 0.004).

CONCLUSION

In children with a left to right shunt the ratio of the excretion rates of the metabolites of TXA2 and PGI2 was abnormal before operation, which favoured vasoconstriction and platelet aggregation, but had decreased at discharge from hospital. The increase in excretion of PGI2 metabolites over TXA2 metabolite after intracardiac repair augurs well for pulmonary vascular recovery.

Reflected reentry in depolarized foci with variable conduction impairment in 1 day old infarcted canine cardiac tissue

A recent study in 1 day old infarcted canine cardiac tissue demonstrated that variations in entrance and exit characteristics of depolarized spontaneous foci could profoundly affect their expression, resulting in behaviors such as modulated parasystole and entrainment, second and third degree exit block "autoentrainment" and annihilation (abrupt termination) of spontaneous activity. Foci with exit and entrance delay should also allow the occurrence of reflected reentry. To test this, the left circumflex coronary artery was ligated and, after 1 day, simultaneous microelectrode impalements were made in infarcted and uninfarcted portions of isolated ventricular preparations. Preparations were stimulated from the uninfarcted portions. Reflected reentry was demonstrated in 5 of 11 preparations. It occurred when exit conduction delay resulted in reexcitation of the focus or, alternatively, when entrance conduction delay resulted in reexcitation of extrafocal tissue. Reflection occurred in which the action potentials were reexcited during phase 2 or 3, resulting in prolongation of action potential duration (type I), and in which reexcitation occurred after full or nearly full repolarization, resulting in a closely coupled extrasystole (type II). Electrotonic modulation and reflection could coexist, the type of behavior depending on the phase relation between focal and extra-focal action potentials. An example is illustrated in which type I reflection occurred only when preceding driven activity induced overdrive suppression of exit conduction from the focus. The results suggest that, in infarcted tissue, entrance and exit conduction delays to depolarized foci may form the basis for the occurrence of reflected reentry.

The paradox of sudden slowing of idioventricular rhythm during exercise in complete heart block: modulation of automatic focus, concealed reentry, or Wedensky inhibition?

As a rule, exercise enhances the rate of discharge of idioventricular rhythms in complete atrioventricular (AV) block. The opposite effect, a sudden drop in the rate of discharge with subsequent resetting of the firing rate, was observed in an 18-year-old boy with chronic complete AV block. Two mechanisms for this paradoxical response to exercise are proposed: (1) delay in discharge of the automatic pacemaker focus by successful penetration of electrotonic potentials from nonconducted P waves across the area of block in the AV conduction system and into the pacemaker focus; and (2) discharge of the pacemaker focus by concealed reentrant extrasystoles. Wedensky inhibition is ruled out because of the resetting of the firing rate.

Concealed ventricular bigeminy with exceptions due to time-dependent modulation of an ectopic rhythm

This presentation reflects a case of atypical concealed bigeminy, where some interectopic intervals contain even numbers of sinus beats. Exceptions to the rule of concealed bigeminy only occur during slowing of the sinus node. The pattern is explained on the basis of modulated parasystole, by drawing a phase-response curve which explains all the interectopic intervals on the basis of the modulating effect exerted by the sinus impulses upon a parasystolic focus.

Effects of clofilium on ischemic subendocardial Purkinje fibers 1 day postinfarction

One day after ligation of the canine anterior descending coronary artery, clofilium, a long-acting class III antiarrhythmic agent, was studied for its effects on normal and ischemic Purkinje fibers. In normal Tyrode's solution (4 mM potassium, 2.7 mM calcium) clofilium (10(-7) to 10(-5) M) increased action potential duration. Although only 2 of 10 normal Purkinje fibers developed early after depolarizations and early afterdepolarization-initiated triggered activity, 10 of 11 ischemic Purkinje fibers developed these features. Consequently, action potentials in ischemic fibers were prolonged to durations greater than 10 s. The triggered activity in the ischemic Purkinje fibers produced repetitive activity in adjacent normal ventricular muscle. In vivo, 3 days after ligation, the administration of 3 to 10 mg/kg clofilium induced grouped beating. Action potentials recorded subsequently from these same hearts in vitro showed early afterdepolarizations, triggered activity and a similar grouping of responses. Therefore, clofilium differentially produced early afterdepolarizations in ischemic Purkinje fibers. This is a mechanism by which clofilium could be arrhythmogenic in an ischemic heart.

Phase resetting in a model of cardiac Purkinje fiber

The phase-resetting response of a model of spontaneously active cardiac Purkinje fiber is investigated. The effect on the interbeat interval of injecting a 20-ms duration depolarizing current pulse is studied as a function of the phase in the cycle at which the pulse is delivered. At low current amplitudes, a triphasic response is recorded as the pulse is advanced through the cycle. At intermediate current amplitudes, the response becomes quinquephasic, due to the presence of supernormal excitability. At high current amplitudes, a triphasic response is seen once more. At low stimulus amplitudes, type 1 phase resetting occurs; at medium amplitudes, a type could not be ascribed to the phase resetting because of the presence of effectively all-or-none depolarization; at high amplitudes, type 0 phase resetting occurs. The modeling results closely correspond with published experimental data; in particular type 1 and type 0 phase resetting are seen. Implications for the induction of ventricular arrhythmias are considered.

Distribution patterns of ventricular premature complexes at different heart rates

The relation between distribution patterns of ventricular premature complexes (VPCs) and heart rate in Holter recordings abundant in VPCs was analyzed using computer-assisted determination of the number of interectopic sinus beats at different heart rates. Within the complete 24-hour heart rate spectrum, zones were demonstrated that were characterized by manifest or concealed bigeminy or manifest or concealed trigeminy. Bigeminy zones were found in 26 and trigeminy zones in 21 of 42 patients. Bigeminy zones were found at a significantly lower heart rate, on the average, then trigeminy zones. In 10 patients both bigeminy and trigeminy zones were observed. Bigeminy and trigeminy zones probably correspond to the distribution patterns of VPCs predicted from modulation of a pacemaker and reflected reentry, both of which can be induced by electrotonically mediated impulses across a zone of impaired conduction in isolated bundles of Purkinje fibers. The bigeminy and trigeminy zones will correspond at least partly to the entrainment zones found during electrotonic modulation of parasystolic foci. The bigeminy zones will correspond to 2:1 entrainment and the trigeminy zones mainly to 3:1 entrainment.

A study of modulated ventricular parasystole by programmed stimulation

To analyze the phase-dependent sensitivity of the parasystolic pacemaker to nonparasystolic beats, 11 patients with modulated ventricular parasystole were studied using the ventricular extrastimulus method. The intrinsic parasystolic cycle lengths ranged from 1,100 to 1,800 ms. Premature stimuli altered the duration of the parasystolic cycle lengths by amounts that depended on timing of the test impulses within the parasystolic cycles. Premature impulses delivered during the first part of the parasystolic cycles prolonged the parasystolic cycle lengths to 107 to 151% of the intrinsic parasystolic cycle lengths and impulses applied during the second part abbreviated them to 70 to 81% of the intrinsic parasystolic cycle lengths. In 10 patients the accelerating effects were of greater magnitude than the decelerating effects. Transition from the accelerating to slowing phases was progressive or unstable in 9 patients and abrupt in 2. Changes induced by individual stimuli were short-lived and the parasystolic pacemakers returned immediately to their original rates. In 1 patient the biphasic sensitivity of parasystole to premature stimuli was shown to sustain for 21 days.

Modulated parasystole originating in the sinoatrial node

A computer model of "modulated sinus parasystole" was devised in which two sinus pacemakers interacted electrotonically, entraining each other's periodicity according to their beat-to-beat phasic relationships. Depending on the preestablished rules, the model gave rise to various rhythm patterns that were similar to those recorded in patients with sinoatrial arrhythmias. The validity of the model in predicting clinically observed rhythm disturbances was tested in a case of sinoatrial extrasystolic activity. The sinoatrial origin of parasystolic discharges giving rise to various patterns of group beating in this case was diagnosed according to the following electrocardiographic criteria: premature P waves having contour identical to P waves of basic beats, variable coupling intervals, and absence of compensatory pauses (i.e., returning cycles having duration similar to that of the basic P-P interval). For the analysis, it was assumed that two distinct but closely apposed sinoatrial pacemaker centers were competing for activation of the heart. The model accurately simulated the arrhythmias in the electrocardiographic trace. The best fit was found when the two pacemakers interacted on the basis of "resetting" in one direction and electronic modulation in the other. In fact, under appropriate conditions, the model matched precisely all frequency-dependent patterns of extrasystolic activity observed in the trace. We conclude that the modulated parasystole hypothesis can readily explain the mechanism of sinus extrasystolic discharges whose returning cycle equals the basic P-P interval. Moreover, the model predicts that, when the rules for mutual entrainment between "dominant" and parasystolic sinus pacemaker are appropriate, the returning cycle can be shorter than the basic cycle.

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.

Abrupt withdrawal of beta-blockade therapy in patients with myocardial infarction: effects on infarct size, left ventricular function, and hospital course

The effects of abrupt withdrawal or continuation of beta-blockade therapy during acute myocardial infarction were evaluated in 326 patients participating in the Multicenter Investigation of the Limitation of Infarct Size (MILIS). Thirty-nine patients previously receiving a beta-blocker and randomly selected for withdrawal of beta-blockers and placebo treatment during infarction (group 1) were compared with 272 patients previously untreated with beta-blockers who were also randomly assigned to placebo therapy (group 2). There were no significant differences between the two groups in MB creatine kinase isoenzyme (15.8 +/- 10.9 vs 18.2 +/- 14.4 g-eq/m2, respectively) estimates of infarct size, radionuclide-determined left ventricular ejection fractions within 18 hr of infarction (0.44 +/- 0.15 vs 0.47 +/- 0.16) or 10 days later (0.42 +/- 0.14 vs 0.47 +/- 0.16), creatine kinase-determined incidence of infarct extension (13% vs 6%), congestive heart failure (43% vs 37%), nonfatal ventricular fibrillation (5% vs 7%), or in-hospital mortality (13% vs 9%). Patients in group 1 had more recurrent ischemic chest pain (p = .002) within the first 24 hr after infarction, but not thereafter. However, this did not appear to be related to a rebound increase in systolic blood pressure, heart rate, or double product. In a separate analysis, 20 propranolol-eligible group 1 patients randomly selected for withdrawal of beta-blockade (group 3) were compared with 15 patients randomly selected for continuation of prior beta-blockade therapy (group 4). This comparison yielded similar results. These data indicate that the beta-blockade withdrawal phenomenon is not a major clinical problem in patients with acute myocardial infarction. beta-Blockade therapy can be discontinued abruptly during acute myocardial infarction if clinically indicated.

Effect of propranolol after acute myocardial infarction in patients with congestive heart failure

The incidence of congestive heart failure was studied in the Beta Blocker Heart Attack Trial in which postmyocardial infarction patients between the ages of 30 and 69 years, with no contraindication to propranolol, were randomly assigned to receive placebo (n = 1921) or propranolol 180 or 240 mg daily (n = 1916) 5 to 21 days after admission to the hospital for the event. Survivors of acute myocardial infarction with compensated or mild congestive heart failure, including those on digitalis and diuretics, were included in the study. A history of congestive heart failure before randomization characterized 710 (18.5%) patients: 345 (18.0%) in the propranolol group and 365 (19.0%) in the placebo group. The incidence of definite congestive heart failure after randomization and during the study was 6.7% in both groups. In patients with a history of congestive heart failure before randomization, 51 of 345 (14.8%) in the propranolol group and 46 of 365 (12.6%) in the placebo group developed congestive heart failure during an average 25 month follow-up. In the patients with no history of congestive heart failure, 5% in the propranolol group developed congestive heart failure and 5.3% in the placebo group developed congestive heart failure. Baseline characteristics predictive of the occurrence of congestive heart failure by multivariate analyses included an increased cardiothoracic ratio, diabetes, increased heart rate, low baseline weight, prior myocardial infarction, age, and more than 10 ventricular premature beats per hour.(ABSTRACT TRUNCATED AT 250 WORDS)

Rate-dependent patterns of modulated ventricular parasystole

The effects of ventricular pacing on the arrangement of ventricular parasystolic beats were studied in 14 patients. By analyzing the effects of various pacing rates and modalities, it was found that both rate and pattern of manifest parasystolic beats were intimately related to the rate and coupling interval of the paced rhythm. Our findings indicate that fixed coupling of ectopic beats is not incompatible with parasystole; modulated parasystole may manifest as fixed rate classic parasystole; a parasystolic pacemaker can be entrained by a wide range of driving rates both above and below the intrinsic rate of the parasystole; and with different driving rates, these rate-dependent patterns can be observed in the same patient. Our observations suggest that spontaneous or drug-induced changes in the heart rate can lead to major alterations in the frequency and patterns of ventricular parasystole.

Concealment of manifest, and exposure of concealed, ventricular parasystole produced by isoproterenol

Few studies have dealt with the effects of isoproterenol on ventricular parasystole. Intravenous isoproterenol (2 to 4 micrograms/min) was administered to 11 nonmedicated patients with ventricular parasystole. At the onset of the drip infusion, 8 patients had continuous parasystole, 2 had intermittent parasystole, and 1 patient (in whom intermittent parasystole was documented 2 to 5 days earlier) showed no manifest parasystolic activity. In all patients, whose control parasystolic cycle length varied between 960 and 2,530 ms, isoproterenol caused a decrease of the parasystolic cycle lengths ranging from 12 to 36%. Therefore, isoproterenol produced a consistent increase of the parasystolic rate. In 4 patients, parasystolic activity ceased to be manifest when the concomitantly enhanced (by isoproterenol) sinus cycle lengths became shorter than 430 ms. This phenomenon reflected a tachycardia-dependent parasystolic concealment, presumably as a result of interference in the parasystolic-ventricular junction. In every case, the arrhythmia reappeared at its initial rate upon stopping the drip infusion. In no patient did parasystolic ventricular tachycardia develop. In the patient without manifest parasystolic beats, isoproterenol unmasked the intermittent parasystole that previously had been intrinsically manifest. The latter effect reflected a true exposure, or unmasking of a latent, rate-independent concealed, parasystolic focus.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible mechanisms of ventricular arrhythmias elicited by ischemia followed by reperfusion. Studies on isolated canine ventricular tissues

The purpose of this study was to develop an isolated tissue model in which arrhythmic activity could be generated in response to conditions encountered in ischemia followed by reperfusion, and in which intracellular recordings could be used to identify and study arrhythmogenic mechanisms. Isolated canine Purkinje fiber-papillary muscle preparations were superfused with modified Tyrode's solutions. Tissues were exposed to conditions observed in ischemia (hypoxia, acidosis, elevated lactate, zero substrate for 40 minutes). Superfusion with Tyrode's solution of "normal" composition was then reinstituted. Transmembrane recordings from Purkinje and muscle tissues were made, using standard microelectrode techniques. Ischemic conditions caused loss of membrane potential, shortened action potentials, depressed excitability, and progressive bidirectional conduction block between muscle and Purkinje tissues. Spontaneous activity, probably reentrant in origin, was observed. Return to nonischemic conditions resulted in a multiphasic sequence of responses in Purkinje fibers: prompt hyperpolarization, progressive depolarization to unresponsiveness, and final repolarization to control. The depolarization phase was accompanied by oscillatory afterpotentials which initiated extrasystoles. Final repolarization included a phase of automaticity at low membrane potentials, during which Purkinje tissue functioned as a parasystolic focus. Elevation of potassium concentration to 10 mM during the ischemic period did not alter the sequence of electrophysiological events during ischemic conditions or upon reperfusion. This study demonstrates that ischemia followed by reperfusion elicits an orderly sequence of electrophysiological events which may constitute important mechanisms of arrhythmia in vivo.

Mutual entrainment and electrical coupling as mechanisms for synchronous firing of rabbit sino-atrial pace-maker cells

The mechanisms of synchronous firing of cardiac pace-makers were studied using thin (0.3-0.5 mm) rabbit sino-atrial (s.a.) node strips placed in a three-compartment tissue bath. Superfusion of the central segment (1 mm in length) with ion-free sucrose solution permitted the electrical insulation of the external segments and the development of two independent pace-maker 'centres': one fast (F); one slow (S). An external shunt pathway was used to modulate the degree of coupling between F and S. Superfusion of the central segment with Tyrode solution containing heptanol (3.5 mM) instead of sucrose induced progressive decrease in the amplitude of responses in this segment and led to progressive loss of F:S synchronization. Eventually the two pace-makers became totally independent from each other. These changes were reversible upon wash-out of heptanol. When a pace-maker centre was within the range of influence of local circuit (i.e. electronic) currents from the pace-maker in the opposite side of the sucrose (or heptanol) compartment, its period was prolonged or abbreviated, depending on phase and frequency relations. Dynamic F:S interactions at various degrees of electrical coupling resulted in mutual entrainment with both pace-makers beating at simple harmonic (i.e. 1:1, 2:1, 1:2, etc.) or more complex (3:2, 5:4, etc.) ratios that depended on the degree of coupling and the intrinsic periods of the individual pace-maker centres. The patterns of synchronization could be predicted by the phasic sensitivity of each pace-maker to brief electrotonic inputs. The results suggest that when two individual pace-maker cells are connected through low resistance junctions, the period resulting from their mutual entrainment should be a function of their respective intrinsic frequencies, their phase relations and the degree of electrical coupling. The data further suggest that the heart beat is initiated by a 'democratic' type of synchronous firing of cells in the s.a. node, with each pace-maker cell contributing to an aggregate signal and involving mutual entrainment between cells.

Annihilation, entrainment and modulation of ventricular parasystolic rhythms

Annihilation and one-to-one entrainment of modulated parasystolic rhythms in humans has not been previously discussed. In 9 nonmedicated patients, it was possible to measure the intrinsic, parasystolic ectopic cycle length given by the intervals between 2 consecutive parasystolic beats without any interposed nonparasystolic beat. The corresponding values varied between 960 and 2,350 ms (corresponding to rates between 62 and 26 beats/min). In addition, modulation could be determined, because nonparasystolic beats falling during the initial 59% of the cycle prolonged the parasystolic cycle length (by 12 to 37.5%), whereas those that fell later in the cycle shortened it (by 9 to 25%). Plotting this prolongation or shortening as a function of the temporal position of the nonparasystolic beats in the cycle yielded biphasic response curves, of which 7 were symmetric and 2 asymmetric. In 2 patients, episodes of concealed one-to-one entrainment were initiated by late nonparasystolic (sinus) beats and, later on, terminated by early ventricular extrasystoles. In 2 other patients (and in 2 separate occasions) nonparasystolic beats, falling in part of the cycle located in between those of maximal delay and acceleration, produced pacemaker annihilation (cessation of automatic activity for the remaining monitoring time). Parasystolic annihilation and concealed entrainment may be one of the causes that can explain the large, spontaneous, day-to-day variability in the incidence of ectopic ventricular beats reported in Holter recordings. Nevertheless, future prospective studies performing interventions that can change the sinus and ectopic rates are required to corroborate our finding.

Effect of propranolol on myocardial-infarct size in a randomized blinded multicenter trial

A multicenter randomized single-blind study was performed to evaluate the effects of propranolol administered during the evolution of myocardial infarction. Five centers enrolled a total of 269 patients, with 134 receiving propranolol and 135 placebo. Propranolol or placebo was given intravenously upon randomization (0.1 mg per kilogram of body weight) and then orally for nine days to keep the heart rate between 45 and 60 beats per minute. Less than 2 per cent of patients were treated within 4 hours after the onset of symptoms, but 50 per cent received therapy within 8 hours of onset of chest pain, and the remainder between 8 and 18 hours. The heart rates in the propranolol-treated group were significantly lower than those in the placebo group (P less than 0.001). Base-line characteristics, including the mean heart rate (79.6 vs. 81.3) and the left ventricular ejection fraction (49.0 vs. 49.5), were similar in the two groups. The primary end point evaluated--infarct size as estimated from plasma MB creatine kinase activity--was virtually identical in the two groups, averaging 13.3 and 13.6 gram-equivalents of MB creatine kinase per square meter of body-surface area. Peak plasma levels of the enzyme were also similar in the two groups. No significant difference was observed between the propranolol and placebo groups in the change in left ventricular ejection fraction, extent of area involved in pyrophosphate uptake, R-wave loss on electrocardiograms, or mortality (after three years). These results do not support the use of propranolol administered four or more hours after the onset of symptoms to limit infarct size.

Multiform ventricular ectopic rhythm by combined parasystolic and reentrant activities

Multiform ventricular ectopic rhythm (MVER), i.e., at least two QRS configurations of ventricular ectopic beats (VEBs), was assessed by 24-hour ambulatory ECG recording in four patients with ventricular parasystole (VP). In two of these four patients, VEBs with fixed coupling to the preceding impulses coexisted with VP beats of different configuration. In case no. 1, the VEBs had an identical coupling interval to sinus beats and VP beats, suggesting a mechanism of reentry elicited from both dominant pacemakers. In case no. 2, an intermittent form of VP due to type II second-degree entrance block was present. In this patient, the VEBs were coupled to sinus beats and to sinus-VP fusion beats and appeared to be dependent on the sinus beats reaching the VP focus. A mechanism of reentry determined by the penetration of sinus beats into the VP area, with prematurity-dependent aberrancy of VEBs, was suggested for the coupled VEBs in this patient. These observations suggest that the coexistence of an automatic ventricular ectopic focus and of a reentrant activity determined by, or elicited from, an area of automaticity may constitute the underlying mechanism of MVER in some patients.