The relationship among cardiac pacemakers. Overdrive suppression

Emergent Synchronous Volumetric Oscillation in Hierarchically Structured Self-Oscillating Gel Clusters

Emergent properties accompanying synchronization among oscillators are vital characteristics in biological systems. Belousov-Zhabotinsky (BZ) oscillators are an artificial model to study the emergence and synchronization in life. This research represents a self-oscillating gel system with clusterable properties to experimentally examine synchronous and emergent properties at a fundamental hierarchical level. Incorporating acrylic acid (AAc) moieties within the gel network facilitates cluster formation through hydrogen bonding in an acidic BZ substrate solution. Upon clustering, both homogeneous and heterogeneous gel assemblies─ranging from double to quadruple clusters─exhibit increased and synchronized periods and amplitudes during the BZ reaction. Notably, in heterogeneous clusters, gel units with initially short periods and small volumetric amplitudes display a significant increase, aligning with the lonfger periods and larger amplitudes of other elements within the cluster, an emergent property. This research can pave the way for a better understanding of synchronous and emergent properties in biological oscillators such as cardiomyocytes.

A bioelectrical phase transition patterns the first vertebrate heartbeats

A regular heartbeat is essential to vertebrate life. In the mature heart, this function is driven by an anatomically localized pacemaker. By contrast, pacemaking capability is broadly distributed in the early embryonic heart1-3, raising the question of how tissue-scale activity is first established and then maintained during embryonic development. The initial transition of the heart from silent to beating has never been characterized at the timescale of individual electrical events, and the structure in space and time of the early heartbeats remains poorly understood. Using all-optical electrophysiology, we captured the very first heartbeat of a zebrafish and analysed the development of cardiac excitability and conduction around this singular event. The first few beats appeared suddenly, had irregular interbeat intervals, propagated coherently across the primordial heart and emanated from loci that varied between animals and over time. The bioelectrical dynamics were well described by a noisy saddle-node on invariant circle bifurcation with action potential upstroke driven by CaV1.2. Our work shows how gradual and largely asynchronous development of single-cell bioelectrical properties produces a stereotyped and robust tissue-scale transition from quiescence to coordinated beating.

Sinus node dysfunction and atrial fibrillation-Relationships, clinical phenotypes, new mechanisms, and treatment approaches

Although the anatomical basis of the pathogenesis of sinus node dysfunction (SND) and atrial fibrillation (AF) is located primarily in the left and right atria, increasing evidence suggests a strong correlation between SND and AF, in terms of both clinical presentation and formation mechanisms. However, the exact mechanisms underlying this association are unclear. The relationship between SND and AF may not be causal, but is likely to involve common factors and mechanisms, including ion channel remodeling, gap junction abnormalities, structural remodeling, genetic mutations, neuromodulation abnormalities, the effects of adenosine on cardiomyocytes, oxidative stress, and viral infections. Ion channel remodeling manifests primarily as alterations in the "funny" current (I_f) and Ca²⁺ clock associated with cardiomyocyte autoregulation, and gap junction abnormalities are manifested primarily as decreased expression of connexins (Cxs) mediating electrical impulse propagation in cardiomyocytes. Structural remodeling refers primarily to fibrosis and cardiac amyloidosis (CA). Some genetic mutations can also cause arrhythmias, such as SCN5A, HCN4, EMD, and PITX2. The intrinsic cardiac autonomic nervous system (ICANS), a regulator of the heart's physiological functions, triggers arrhythmias.In addition, we discuss arrhythmias caused by viral infections, notably Coronavirus Disease 2019 (COVID-19). Similarly to upstream treatments for atrial cardiomyopathy such as alleviating CA, ganglionated plexus (GP) ablation acts on the common mechanisms between SND and AF, thus achieving a dual therapeutic effect.

Emergent synchronous beating behavior in spontaneous beating cardiomyocyte clusters

We investigated the dominant rule determining synchronization of beating intervals of cardiomyocytes after the clustering of mouse primary and human embryonic-stem-cell (hES)-derived cardiomyocytes. Cardiomyocyte clusters were formed in concave agarose cultivation chambers and their beating intervals were compared with those of dispersed isolated single cells. Distribution analysis revealed that the clusters' synchronized interbeat intervals (IBIs) were longer than the majority of those of isolated single cells, which is against the conventional faster firing regulation or "overdrive suppression." IBI distribution of the isolated individual cardiomyocytes acquired from the beating clusters also confirmed that the clusters' IBI was longer than those of the majority of constituent cardiomyocytes. In the complementary experiment in which cell clusters were connected together and then separated again, two cardiomyocyte clusters having different IBIs were attached and synchronized to the longer IBIs than those of the two clusters' original IBIs, and recovered to shorter IBIs after their separation. This is not only against overdrive suppression but also mathematical synchronization models, such as the Kuramoto model, in which synchronized beating becomes intermediate between the two clusters' IBIs. These results suggest that emergent slower synchronous beating occurred in homogeneous cardiomyocyte clusters as a community effect of spontaneously beating cells.

Geometric Understanding of Local Fluctuation Distribution of Conduction Time in Lined-Up Cardiomyocyte Network in Agarose-Microfabrication Multi-Electrode Measurement Assay

We examined characteristics of the propagation of conduction in width-controlled cardiomyocyte cell networks for understanding the contribution of the geometrical arrangement of cardiomyocytes for their local fluctuation distribution. We tracked a series of extracellular field potentials of linearly lined-up human embryonic stem (ES) cell-derived cardiomyocytes and mouse primary cardiomyocytes with 100 kHz sampling intervals of multi-electrodes signal acquisitions and an agarose microfabrication technology to localize the cardiomyocyte geometries in the lined-up cell networks with 100–300 μm wide agarose microstructures. Conduction time between two neighbor microelectrodes (300 μm) showed Gaussian distribution. However, the distributions maintained their form regardless of its propagation distances up to 1.5 mm, meaning propagation diffusion did not occur. In contrast, when Quinidine was applied, the propagation time distributions were increased as the faster firing regulation simulation predicted. The results indicate the “faster firing regulation” is not sufficient to explain the conservation of the propagation time distribution in cardiomyocyte networks but should be expanded with a kind of community effect of cell networks, such as the lower fluctuation regulation.

Dominant rule of community effect in synchronized beating behavior of cardiomyocyte networks

Exploiting the combination of latest microfabrication technologies and single cell measurement technologies, we can measure the interactions of single cells, and cell networks from "algebraic" and "geometric" perspectives under the full control of their environments and interactions. However, the experimental constructive single cell-based approach still remains the limitations regarding the quality and condition control of those cells. To overcome these limitations, mathematical modeling is one of the most powerful complementary approaches. In this review, we first explain our on-chip experimental methods for constructive approach, and we introduce the results of the "community effect" of beating cardiomyocyte networks as an example of this approach. On-chip analysis revealed that (1) synchronized interbeat intervals (IBIs) of cell networks were followed to the more stable beating cells even their IBIs were slower than the other cells, which is against the conventional faster firing regulation or "overdrive suppression," and (2) fluctuation of IBIs of cardiomyocyte networks decreased according to the increase of the number of connected cells regardless of their geometry. The mathematical simulation of this synchronous behavior of cardiomyocyte networks also fitted well with the experimental results after incorporating the fluctuation-dissipation theorem into the oscillating stochastic phase model, in which the concept of spatially arranged cardiomyocyte networks was involved. The constructive experiments and mathematical modeling indicated the dominant rule of synchronization behavior of beating cardiomyocyte networks is a kind of stability-oriented synchronization phenomenon as the "community effect" or a fluctuation-dissipation phenomenon. Finally, as a practical application of this approach, the predictive cardiotoxicity is introduced.

A case of ventricular standstill in a patient with acute gastrointestinal bleeding

Ventricular stand still is an electrophysiologic phenomenon characterized by the absence of ventricular activity or contraction. It is the result of the lack of impulse formation in ventricles (absence of idioventricular automaticity) or the failure of impulse transmission to ventricles (conduction disturbance) [1]. It is an uncommon condition that can affect a wide range of age groups with life threatening consequences. There are no set guidelines on the treatment of ventricular standstill – swift and sound clinical judgment is required. The condition should be treated as a cardiac arrest, with resolution of precipitating factors. Here we present a case of a 59-year-old man with multiple comorbidities, who presented with massive gastrointestinal hemorrhage and recurrent episodes of ventricular standstill during hospitalization, with his immediate treatment and stabilization.

Ivabradine and metoprolol in fixed dose combination: When, why and how to use it

Heart rate is an important factor in coronary artery disease and its manifestations, and as such has been considered as a possible target for therapy. Although in epidemiological, and in less degree, in clinical studies derived indications of a possible pathogenetic role of heart rate in major cardiac diseases, clinical trials did not provided any strong evidence. However, even as a simple risk marker, remains important in the treatment of coronary artery disease and heart failure. Beta-blockers are the drugs most frequently used for heart rate control. However, recent studies constantly find insufficient effectiveness of beta-blockers in heart rate control and go further to question their efficacy on outcomes, making clear the need for an additional therapy. Ivabradine, a pure heart rate inhibitor, added to classic beta-blocker treatment represent the new therapeutic option in stable coronary disease and heart failure.

Cardiac automaticity: basic concepts and clinical observations

PURPOSE

The purpose of this report was to review the basic mechanisms underlying cardiac automaticity. Second, we describe our clinical observations related to the anatomical and functional characteristics of sinus automaticity.

METHODS

We first reviewed the main discoveries regarding the mechanisms responsible for cardiac automaticity. We then analyzed our clinical experience regarding the location of sinus automaticity in two unique populations: those with inappropriate sinus tachycardia and those with a dominant pacemaker located outside the crista terminalis region.

RESULTS

We studied 26 patients with inappropriate sinus tachycardia (age 34 ± 8 years; 21 females). Non-contact endocardial mapping (Ensite 3000, Endocardial Solutions) was performed in 19 patients and high-density contact mapping (Carto-3, Biosense Webster with PentaRay catheter) in 7 patients. The site of earliest atrial activation shifted after each RF application within and outside the crista terminalis region, indicating a wide distribution of atrial pacemaker sites. We also analyzed 11 patients with dominant pacemakers located outside the crista terminalis (age 27 ± 7 years; five females). In all patients, the rhythm was the dominant pacemaker both at rest and during exercise and located in the right atrial appendage in 6 patients, in the left atrial appendage in 4 patients, and in the mitral annulus in 1 patient. Following ablation, earliest atrial activation shifted to the region of the crista terminalis at a slower rate.

CONCLUSIONS

Membrane and sub-membrane mechanisms interact to generate cardiac automaticity. The present observations in patients with inappropriate sinus tachycardia and dominant pacemakers are consistent with a wide distribution of pacemaker sites within and outside the boundaries of the crista terminalis.

Sinoatrial and Atrioventricular Blocks: Who First Described Them and How? [Retrospectroscope]

The relationship among cardiac pacemakers is characterized by the fact that one pacemaker is usually dominant and all the others are subsidiary. The sinoatrial node acts as the dominant pacemaker, and all other potential pacemaker tissues are discharged by a conducted impulse before their respective diastolic depolarizations attain threshold. These pacemakers are called subsidiary to emphasize the fact that, under normal circumstances, they are engaged in conducting impulses, but, under abnormal circumstances, they may become actual pacemakers.

Cardiac electrical defects in progeroid mice and Hutchinson-Gilford progeria syndrome patients with nuclear lamina alterations

Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disease caused by defective prelamin A processing, leading to nuclear lamina alterations, severe cardiovascular pathology, and premature death. Prelamin A alterations also occur in physiological aging. It remains unknown how defective prelamin A processing affects the cardiac rhythm. We show age-dependent cardiac repolarization abnormalities in HGPS patients that are also present in the Zmpste24^-/- mouse model of HGPS. Challenge of Zmpste24^-/- mice with the β-adrenergic agonist isoproterenol did not trigger ventricular arrhythmia but caused bradycardia-related premature ventricular complexes and slow-rate polymorphic ventricular rhythms during recovery. Patch-clamping in Zmpste24^-/- cardiomyocytes revealed prolonged calcium-transient duration and reduced sarcoplasmic reticulum calcium loading and release, consistent with the absence of isoproterenol-induced ventricular arrhythmia. Zmpste24^-/- progeroid mice also developed severe fibrosis-unrelated bradycardia and PQ interval and QRS complex prolongation. These conduction defects were accompanied by overt mislocalization of the gap junction protein connexin43 (Cx43). Remarkably, Cx43 mislocalization was also evident in autopsied left ventricle tissue from HGPS patients, suggesting intercellular connectivity alterations at late stages of the disease. The similarities between HGPS patients and progeroid mice reported here strongly suggest that defective cardiac repolarization and cardiomyocyte connectivity are important abnormalities in the HGPS pathogenesis that increase the risk of arrhythmia and premature death.

DIFFERENCES IN IONIC CURRENTS BETWEEN CANINE MYOCARDIAL AND PURKINJE CELLS

An electrophysiological analysis of canine single ventricular myocardial (VM) and Purkinje (P) cells was carried out by means of whole cell voltage clamp method. The following results in VM versus P cells were obtained. I_Na3 was present, had a threshold negative to the fast activating-inactivating I_Na1, its slow inactivation was cut off by I_Na1, and contributed to Na⁺ influx at I_Na1 threshold. I_Na1 was smaller and had a less negative threshold. There was no comparable slowly inactivating I_Na2, accounting for the shorter action potential. Slope conductance at resting potential was about double and decreased to a minimum value at the larger and less negative I_K1 peak. The negative slope region of I-V relation was smaller during fast ramps and larger during slow ramps than in P cells, occurred in the voltage range of I_K1 block by Mg²⁺, was not affected by a lower V_h and TTX and was eliminated by Ba²⁺, in contrast to P cells. I_Ca was larger, peaked at positive potentials and was eliminated by Ni²⁺. I_to was much smaller, began at more positive values, was abolished by less negative V_h and by 4-aminopyridine, included a sustained current that 4-aminopyridine decreased but did not eliminate. Steeper ramps increased I_K1 peak as well as the fall in outward current during repolarization, consistent with a time-dependent block and unblock of I_K1 by polyamines. During repolarization, the positive slope region was consistently present and was similar in amplitude to I_K1 peak, whereas it was small or altogether missing in P cells. The total outward current at positive potentials comprised a larger I_K1 component whereas it included a larger I_to and sustained current in P cells. These and other results provide a better understanding of the mechanisms underlying the action potential of VM and P cells under normal and some abnormal (arrhythmias) conditions.

Post-partum variation in the expression of paternal care is unrelated to urinary steroid metabolites in marmoset fathers

The organization and activation of maternal care are known to be highly regulated by hormones and there is growing evidence that expression of paternal care is also related to endocrine substrates. We examined the relationship between paternal behavior and steroid hormones in marmoset fathers (Callithrix geoffroyi) and evaluated whether hormone-paternal behavior relationships were altered by previous offspring-care experience in males. Based on previous findings, we predicted that testosterone, estradiol, and cortisol would decrease following the birth of offspring and would be lowest during the period of maximal infant carrying. Furthermore, we predicted that post-partum changes in carrying effort and hormone levels would be influenced by the level of offspring-care experience. Carrying effort and other paternal care behaviors underwent temporal changes over the post-partum period, but these patterns were not related to variation in hormone concentrations over the same period. There was a limited effect of offspring-care experience on hormone concentrations, but experience was found to play a role in the expression of paternal care, with experienced fathers engaging in significantly more infant allogrooming than inexperienced fathers. Furthermore, inexperienced fathers increased the frequency of food sharing in response to infant begging across the post-partum period, while experienced fathers displayed consistently low levels. We posit that a combination of experiential factors and an increased role for alloparents in offspring-care led to these changes. However, it appears that hormonal changes may not influence paternal responsiveness in white-faced marmoset fathers and that hormone-paternal behavior relationships are not critically dependent on a male's previous offspring-care experience.

I(f) channels as a therapeutic target in heart disease

In the normal heart, impulses are generated from the sinoatrial node. It is generally accepted that the pacemaker current, I(f), plays a major role in the spontaneous rhythmic activity. Recently, several electrophysiological and molecular data demonstrate that I(f) channels are present in embryonic and post-natal ventricular myocytes and undergo a downregulation during maturation. Interestingly, the I(f) current is re-expressed in some pathological conditions such as cardiac hypertrophy and heart failure. In these conditions, the overexpression of f-channels is a consequence of electrophysiological remodeling and may represent an arrhythmogenic mechanism in heart failure, a condition associated with high risk for sudden cardiac death. For its physiological and pathophysiological role and the availability of selective f-channel blockers, I(f) may be a suitable therapeutic target in heart failure.

Mathematical models of the electrical action potential of Purkinje fibre cells

Early development of ionic models for cardiac myocytes, from the pioneering modification of the Hodgkin-Huxley giant squid axon model by Noble to the iconic DiFrancesco-Noble model integrating voltage-gated ionic currents, ion pumps and exchangers, Ca(2+) sequestration and Ca(2+)-induced Ca(2+) release, provided a general description for a mammalian Purkinje fibre (PF) and the framework for modern cardiac models. In the past two decades, development has focused on tissue-specific models with an emphasis on the sino-atrial (SA) node, atria and ventricles, while the PFs have largely been neglected. However, achieving the ultimate goal of creating a virtual human heart will require detailed models of all distinctive regions of the cardiac conduction system, including the PFs, which play an important role in conducting cardiac excitation and ensuring the synchronized timing and sequencing of ventricular contraction. In this paper, we present details of our newly developed model for the human PF cell including validation against experimental data. Ionic mechanisms underlying the heterogeneity between the PF and ventricular action potentials in humans and other species are analysed. The newly developed PF cell model adds a new member to the family of human cardiac cell models developed previously for the SA node, atrial and ventricular cells, which can be incorporated into an anatomical model of the human heart with details of its electrophysiological heterogeneity and anatomical complexity.

Reentrant waves in a ring of embryonic chick ventricular cells imaged with a Ca2+ sensitive dye

According to the classic model initially formulated by Mines, reentrant cardiac arrhythmias may be associated with waves circulating in a ring geometry. This study was designed to study the dynamics of reentry in a ring geometry of cardiac tissue culture. Reentrant calcium waves in rings of cultured embryonic chick cardiac myocytes were imaged using a macroscope to monitor the fluorescence of intracellular Calcium Green-1 dye. The rings displayed a variety of stable rhythms including pacemaker activity and spontaneous reentry. Waves originating from a localized pacemaker could lead to reentry as a consequence of unidirectional block. In addition, more complex patterns were observed due to the interactions between reentrant and pacemaker rhythms. These rhythms included instances in which pacemakers accelerated the reentrant rhythm, and instances in which the excitation was blocked in the vicinity of pacemakers. During reentrant activity an appropriately timed electrical stimulus could induce resetting of activity or cause complete annihilation of the propagating waves. This experimental preparation reveals many spontaneously occuring complex rhythms. These complex rhythms are hypothesized to reflect interactions between spontaneous pacemakers, wave propagation, refractory period, and overdrive suppression. This preparation may serve as a useful model system to further investigate complex dynamics arising during reentrant rhythms in cardiac tissue.

Paroxysmal starting and stopping of circulating waves in excitable media

Levels of intracellular Ca2+ were monitored using fluorescence from Ca2+-sensitive dyes in chick embryonic heart cells cultured in an annular geometry. There was spontaneous starting and stopping of reentrant waves of activity. The results are modeled using modified FitzHugh-Nagumo equations representing pacemakers embedded in a conducting medium. These results provide a potential mechanism for spontaneous abnormal cardiac rhythms in which there are rapid heart beats (tachycardias) that repetitively start and stop.

Cellular electrophysiologic mechanisms of cardiac arrhythmias

Cardiac arrhythmias are caused by alterations in the electrophysiologic properties of the cardiac cells, which affect the characteristics of the transmembrane potentials. The electrophysiologic properties that cause arrhythmias are automaticity, triggered activity, and reentrant excitation. Each of these mechanisms is described in terms of the characteristics of the transmembrane potentials and how these influence the appearance of the arrhythmia on the electrocardiogram.

Bursting behavior during fixed-delay stimulation of spontaneously beating chick heart cell aggregates

Spontaneously beating embryonic chick atrial heart cell aggregates were stimulated with depolarizing current pulses delivered at a fixed delay after each action potential. This preparation is an experimental model of a reentrant tachycardia. During fixed-delay stimulation, bursting behavior was typically observed for a wide range of delays. Episodes of bursting at a rate faster (slower) than control were followed by overdrive suppression (underdrive acceleration). We use a simple nonlinear model, based on the interaction between excitability and overdrive suppression, to describe these dynamics. A modified version of the Shrier-Clay ionic model of electrical activity of the embryonic chick heart cell aggregates that includes a simplified Na+ pump term is also presented. We show that the complex patterns during fixed-delay stimulation arise as a result of delicate interactions between overdrive suppression and phase resetting, which can be described in terms of the underlying ionic mechanisms. This study may provide a basis for understanding incessant tachycardias in the intact heart, as well as an alternative mechanism for the emergence of bursting activity in other biologic tissue.

Pacemakers

Rapid advances in pacing technology will continue to affect the quality of life of many patients with cardiovascular disease. A truly "smart" device that seemed fanciful 30 years ago now seems to be a virtual certainty by early in the next century. The surgical contributions and expertise of individuals trained in cardiothoracic surgery in these bradypacing developments is highly desirable to minimize morbidity to the greatest possible degree, to optimize the outcome of the procedure for the individual patient, and to conserve health care costs as much as possible. To maintain this cardiothoracic presence in cardiac pacing, acquisition of knowledge and expertise in the basic electrophysiology and technology of cardiac pacing, to go along with surgical expertise, is necessary on the part of individuals with the interest and opportunity to do so.

Temporary Cardiac Pacing in the Intensive Care Unit

Indications for temporary cardiac pacing have increased substantially in the last several years. Although most temporary cardiac pacing is still carried out to treat symptomatic bradycardia due to atrioventricular conduction system disease or atrial bradycardia (i.e., sinus node dysfunction), temporary pacing is currently used to induce and to terminate some supraventricular tachyarrhythmias, prevent pause-dependent ventricular tachycardia (usually torsades de pointes), and vagally mediated atrial fibrillation, to allow the maintenance of hemodynamic competence in postoperative cardiac patients and to evaluate selected patients with hypertrophic and dilated cardiomyopathies who might benefit hemodynamically from cardiac pacing. The roles of transcutaneous and esophageal pacing have also expanded; transcutaneous pacing is now commonly used in patients at high risk for the development of atrioventricular block, such as those with acute myocardial infarction and bifascicular block. We review available types of temporary pacing leads and pulse generators, the methods by which temporary pacing is accomplished, complications of pacing system insertion, and current indications for this therapy. Guidelines for troubleshooting normal and abnormal pacemaker function in the intensive care unit setting are provided.

Electromechanical effects of acetylcholine on the atrial tissues of the cultured tilapia (Oreochromis nilotica × O. aureus)

The effects of acetylcholine (ACh) on the action potential and twitch force of atrial tissues isolated from 15 tilapia (Oreochromis nilotica × O. aureus) were studied by means of conventional microelectrode techniques. In isolated whole atrium or sinoatrial tissue, scattered pacemaker-like cells with spontaneous diastolic depolarization were found mainly near the sinoatrial junction but also occasionally throughout the atrial wall. However, most of the atrial cells recorded were myocardial fibers as judged by a stable diastolic potential and a markedly reduced action potential duration (APD) in response to low concentrations of ACh (0.1-1 μM). The shortening in APD in atrial myocardial fibers was correlated with a significant fall in twitch force in the atrial preparations. ACh at high concentrations (10-300 μM) decreased moderately the APD and the slope of diastolic depolarization of the pacemakers and prolonged the spontaneous cycle length but did not induce hyperpolarization. The negative chronotropic action of ACh was competitively inhibited by atropine, a muscarinic antagonist. The means (± SEM) negative logarithm of the dissociation constant (pKb or pA2 value) for atropine against the ACh action on muscarinic receptors were 9.10 (± 0.13) (n = 6), similar to those values obtained in mammalian atria. The present findings indicate that while the negative inotropic effects of ACh in tilapia atria are comparable to those observed in mammalian hearts, unique electrophysiological responses to ACh exist in different types of tilapia atrial cells.

Phase resetting and dynamics in isolated atrioventricular nodal cell clusters

In the heart, the AV node is the primary conduction pathway between the atria and ventricles and subserves an important function by virtue of its rate-dependent properties. Cell clusters isolated from the rabbit atrioventricular (AV) node beat with a stable rhythm (cycle length: 300-520 ms) and are characterized by slow action potential upstroke velocities (7 to 30 V/s). The goal of this study is to better characterize the phase resetting and the rhythms during periodic stimulation of this slow inward current system. Single or periodic depolarizing pulses (20 ms in duration) were injected into AV nodal cell clusters using glass microelectrodes. Phase resetting curves of both strong, weak as well as discontinuous types were obtained by applying single current pulses of different intensities and latencies following every ten action potentials. Graded responses were elicited in a wide range of stimulus phases and amplitudes. A single premature stimulus caused a transient prolongation of the cycle length. Sustained periodic stimulation, at rates faster than the intrinsic beat rate, resulted in various N:M (stimulus frequency: action potential frequency) entrainment rhythms as well as periodic or irregular changes in action potential morphology. The changes in action potential characteristics were evaluated by computing the area under the action potential trace and above a fixed threshold (-45 mV). We show that the variations in action potential morphology play a major role in the onset of complicated dynamics observed in this experimental preparation. In this context, the prediction of entrainment rhythms using techniques based on the iteration of phase resetting curves (PRCs) is inadequate since the PRC does not carry information directly related to the changes in action potential morphology. This study demonstrates the need to consider graded events which, though not propagated, have important implications in the understanding of dynamical diseases of the heart. (c) 1995 American Institute of Physics.

Role of Intracellular Sodium Activity in the Control of Contraction in Cardiac Purkinje Fibers

The role of intracellular sodium activity (a(i)(Na)) in the control of force was studied in sheep cardiac Purkinje fibers exposed to norepinephrine (NE) and high [Ca](o) in the absence and presence of overdrive or of a low concentration of strophanthidin. Both NE and high [Ca](o) decrease a(i)(Na) and increae force, while overdrive increases and low strophanthidin decreases both parameters. In the presence of NE, overdrive increases a(i)(Na) less than force and is followed by a more pronounced undershoot in a(i)(Na) and force. In contrast, in high [Ca](o) overdrive increases a(i)(Na) more than force and is followed by a less pronounced undershoot in a(i)(Na) and force than in NE. High [Ca](o) increases force to a peak, but then the decreasing a(i)(Na) reduces force. In all these conditions, a(i)(Na) determines force changes during recovery from overdrive. NE and high [Ca](o) decrease a(i)(Na) less and increase force more in low strophanthidin. Thus, changes in a(i)(Na) modulate the increase in force due to increased Ca influx and control force development when Ca influx is either unchanged (low strophanthidin) or has reached a steady state (high [Ca](o), recovery from overdrive). Copyright 1994 S. Karger AG, Basel

Stress-induced arrhythmic disease of the heart--Part I

This review deals with the following principal concepts: (1) Heart injuries in single severe stress episodes manifested primarily in disturbances of membrane lipid bilayer, sarcolemmal Na, K-pump, and sarcoplasmic Ca-pump with concurrent limited disturbances of the heart energy supply, namely, of the creatine kinase and glycolysis systems. These disturbances cause small focal myocardial lesions and reduce cardiac electrical stability: the fibrillation threshold falls and ectopic activity increases. In repeated stress, this damage, localized mainly in the richly innervated conduction system, accumulates to cause even more pronounced disturbances of electrical stability and severe arrhythmias. (2) Severe stress and beta-adrenergic effects on the heart regularly result in coronary vasodilation and increased coronary blood flow. However, the entire primary complex of stress-induced injuries and disturbances of the heart's electrical stability occurs despite the increased coronary blood flow. Thus, beta-adrenergic stress-induced injuries may indeed develop as primary stress damage to cardiomyocytes without any relation to ischemia. (3) The main factor determining high vulnerability or, on the contrary, resistance of the heart to stress is the state of stress-limiting systems, namely, the opioidergic, GABAergic, cholinergic, adenosinergic, and other systems. Activation of these systems by adaptation to repeated stress or other factors prevents serious injuries to the heart in severe stress. Conversely, genetically determined or acquired dysfunction of these systems predisposes to severe arrhythmias and sudden death. Thus, in stress-induced arrhythmic disease as well as in ischemic heart disease, the main pathogenetic links are outside the heart, but they differ from those observed in ischemia. (4) The clinical picture of stress-induced arrhythmic disease, that is, alterations in electrocardiogram, coronarogram, and patient responses to stress, physical loads, and tranquilizers differ, as do pathologic alterations in the heart. These differences are summarized at the end of this review.

Effect of stimulus intensity on atrial refractoriness and sinus node recovery

INTRODUCTION

Prior studies of sinus node function in man stated that the stimulus intensity of overdrive pacing has no effect on the response of the sinus node to overdrive suppression; however, data documenting these statements were lacking. Previous studies have also suggested that drive train stimulus intensity can alter ventricular refractoriness, but similar studies have not been performed on the human atrium. The purpose of this study was to evaluate the effects of drive train stimulus intensity on atrial effective refractory period and sinus node recovery time.

METHODS AND RESULTS

The effect of drive train stimulus intensity on atrial effective refractory period and sinus node recovery time was studied in 42 patients undergoing clinical electrophysiologic tests. The atrial effective refractory period was shorter at 10 mA (221 +/- 20 msec) and 5 mA (232 +/- 25 msec) than at a drive train stimulus intensity of 1.5 times late diastolic threshold (248 +/- 24 msec, P < 0.05 for pairwise comparison). The sinus node recovery time did not demonstrate a similar effect in the baseline state, following beta-adrenergic blockade, or following combined parasympathetic and beta-adrenergic blockade. However, following isolated parasympathetic blockade with atropine, the corrected sinus node recovery time shortened from 88 +/- 51 msec at 1.5 times late diastolic threshold to 48 +/- 55 msec at 10 mA (P < 0.05). Significant variability was present in sinus node recovery time measurements at baseline and following beta blockade; this variability decreased following parasympathetic blockade.

CONCLUSION

These data suggest that drive train stimulus intensity can affect the electrophysiologic properties of sinus node and atrial tissue. This effect appears to be mediated by local catecholamine and acetylcholine release and provides further evidence that the interaction between pacing stimuli and the cardiac autonomic system may need to be considered in evaluating electrophysiologic effects.

5-Hydroxytryptamine causes rate-dependent arrhythmias through 5-HT4 receptors in human atrium: facilitation by chronic beta-adrenoceptor blockade

We have investigated the ability of 5-hydroxytryptamine (5-HT) to elicit arrhythmic contractions in isolated human atrial strips as a function of pacing rate (0.1-2 Hz) using a method recently introduced by us (Kaumann and Sanders, this journal, 1993 b) and examined the nature of the 5-HT receptors involved. Right atrial appendage tissue was obtained from 14 patients undergoing cardiac surgery. None of the patients had advanced heart failure. 5-HT (0.6-20 mumol/l) induced arrhythmic contractions during pacing in 4/11 atrial strips from 3/4 patients who had not received beta blockers and in 21/27 atrial strips from 9/10 patients who had been chronically treated with beta blockers (primarily beta 1-selective). The incidence of arrhythmic contractions evoked by 5-HT did not reach statistical significance in the atrial tissue from the non-beta blocked patients but was highly significant in the atrial tissue from the chronically beta blocked patients. The arrhythmic contractions usually occurred more frequently at low than at high pacing rates and were observed at the physiological frequency of 1 Hz in 1/4 atrial strips from 1/4 of the non-beta blocked patients and 6/11 strips from 5/10 of the beta blocked patients. The 5-HT-evoked arrhythmic contractions were observed during blockade of beta 1-adrenoceptors, beta 2-adrenoceptors and 5-HT3 receptors, ruling out the participation of these receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal automaticity as mechanism of atrial tachycardia in the human heart--electrophysiologic and histologic correlation: a case report

INTRODUCTION

A 32-year-old woman was operated upon because of drug refractory atrial tachycardia.

METHODS AND RESULTS

Electrophysiologic study was performed prior to operation. During surgery, epicardial mapping of the electrical activity of the left atrium was performed. The left atrial appendage was resected and studied in a tissue bath. Thereafter, histologic examination was performed. Polarity of the P wave in the surface ECG suggested that the tachycardia originated high in the left atrium. Epicardial mapping disclosed earliest activation in the apex of the left atrial appendage. Intracellular recordings from surgical specimen made at the site of origin, which was marked during surgery, revealed cells with phase 4 depolarization at cycle lengths ranging from 360 to 540 msec. Exit block prevented spread of activation from the spontaneously firing cells to surrounding tissue. Histology showed that spontaneous activity arose in an area with abnormal cells--characterized by an amorphous, pale eosinophilic staining cytoplasm and absence of nuclei--surrounded by normal myocytes.

CONCLUSION

The observations indicate that the mechanism of the atrial tachycardia was based on abnormal automaticity in an area consisting of a conglomeration of normal and abnormal myocytes.

Modulations of membrane potential oscillations with drive, calcium overload, ryanodine, and caffeine

The objective of this study is to determine the conditions that generate overdrive excitation and overdrive suppression in canine cardiac Purkinje tissue superfused in vitro. Drive-induced (3 Hz) perturbations in the membrane potential of a calcium overloaded (induced by strophanthidin) Purkinje fiber (from a canine heart) were differently modulated by caffeine and ryanodine. Whereas the postdrive oscillations in the membrane potential Vos (single or multiple oscillations in the diastole of the action potential) and/or spontaneous rate (postdrive suppression or postdrive excitation [PDE]) depended on the concentration of strophanthidin (PDE occurred at 2.5 x 10(-7) M, and Vos were seen variably at several concentrations), caffeine (2-3 mM) in the presence of a lower concentration of strophanthidin (1.25 x 10(-7) M) induced PDE. At these lower concentrations, either drug administered alone only induced Vos. On the contrary, the characteristic effects of ryanodine (10(-8) M) in the presence of strophanthidin (2.5 x 10(-7) M) were either a consistent postdrive suppression immediately or the induction of a pronounced afterdepolarization ([AD] a depolarization following the repolarization of the action potential) whose amplitude decreased with time and suppression. At higher concentrations of ryanodine (10(-5) M-10(-6) M) in a calcium overloaded tissue (strophanthidin, 1.25 x 10(-7) M) overdrive induced a pronounced AD in most cases, with subsequent depolarization and cessation of activity in less than 20 minutes. Ryanodine alone caused suppression of postdrive diastolic potential at lower concentrations (10(-9) M-10(-8) M), a pronounced AD (amplitude diminished with later drives), and suppression at higher concentrations (10(-6) M-10(-5) M).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of overdrive pacing rate and duration on ventricular escape rhythms in patients with chronic complete atrioventricular block

The effect of overdrive (OD) pacing rate and duration on subsidiary pacemakers was evaluated in 54 patients with third-degree AV block. They had a permanent pacemaker implanted 61 +/- 56 months earlier because of complete AV block in 38 patients and, in 16 patients because of second-degree AV block, which in the interim advanced to complete AV block. The patients had a reliable infranodal escape rhythm, with a mean cycle length of 2,022 +/- 603 msec, upon discontinuation of the ventricular OD pacing, at a rate of 40 beats/min. The escape interval and escape rhythm cycle length was evaluated after OD pacing at 40, 50, 60, 70, 90, and 100 beats/min for 30 seconds, at each rate. In 100% of the patients the subsidiary pacemaker recovered after OD pacing at 40 and 50 beats/min and the number decreased to 59% at a rate of 100 beats/min. The escape interval prolonged gradually between OD pacing at 40 and 100 beats/min, by 56%. The effect of OD pacing duration at 50 and 70 beats/min was evaluated. At an OD pacing rate of 70 beats/min there was a significant effect of the pacing duration on the escape interval. There were significant differences in the escape interval duration and escape rhythm cycle length between males and females, patients with or without coronary artery disease, and patients with narrow or wide QRS escape. However, the increase in the OD pacing rate had a similar effect on the escape interval in the above mentioned groups. There was no effect on the paced QRS duration and sinus cycle length at each OD pacing rate.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of chronic atrial overdrive suppression pacing on the incidence of supraventricular tachyarrhythmias

Chronic overdrive suppression pacing has been suggested as an effective adjunctive method for reducing the incidence of cardiac tachyarrhythmias. Documentation of effectiveness during prolonged monitoring is lacking, however. To assess more accurately the long-term utility of this treatment modality for medically refractory supraventricular tachyarrhythmias (SVTs), 10 patients with atrially implanted Intermedics Intertach pacemakers were randomly assigned to either a low or a high bradycardia (back-up) pacing rate. SVT counts were performed during matching follow-up periods both at the initial rate and after rate crossover. The primary antitachycardia modality of this pacemaker (P mod) provides burst pacing to terminate tachycardia episodes, and P mod counters were utilized to quantitate SVT episodes. Tachycardia termination algorithms were programmed to "no restart" and were not changed during the study. The P mod use counter, therefore, reflected the number of discrete episodes of SVTs. Pacemaker implantation diagnoses include atrial flutter, concealed bypass tract, AV nodal reentry, intraatrial reentry, and Wolff-Parkinson-White associated tachycardia. Patient age was 59 +/- 18 yrs. The average pacemaker back-up low rate was 45.7 +/- 4 versus a back-up high rate of 85.1 +/- 2 beats/min. Follow-up was for 57.4 days +/- 33 days at the low rate and 57.3 days +/- 34 days at the high rate (r = 0.99). There was no difference in SVT incidence with a P mod usage of 98.4 +/- 106 at the low rate and 100.8 +/- 94 at the high rate (p = NS). In this blinded, randomized cross-over trial, chronic atrial overdrive suppression pacing did not reduce the overall incidence of SVT episodes during prolonged monitoring.

Age-related effects of ischemia, lidocaine and verapamil on overdrive-induced suppression of ventricular pacemakers in isolated rat heart

The effect of age on ventricular automaticity in the isolated perfused rat heart was determined under different conditions. When the ventricle is electrically stimulated at a faster rate, drive cessation is followed by a temporary suppression of ventricular automaticity (overdrive suppression). The effects of ischemia, lidocaine and verapamil on overdrive suppression were studied in isolated perfused adult and senescent rat hearts with complete atrio-ventricular block, by monitoring ventricular escape rate and escape rhythm recovery time after 1 minute of overdrive at a constant multiple (x3) of the spontaneous rate. The results demonstrated that: 1) lidocaine decreases ventricular automaticity especially in senescent hearts; 2) verapamil does not modify ventricular automaticity in basal conditions in either adult or senescent hearts; 3) myocardial ischemia causes a reduction in ventricular automaticity and more markedly in senescent hearts; and 4) lidocaine exaggerates the effect of ischemia, while verapamil seems to antagonize its depressant effect more in adult than in senescent hearts.

Electrophysiologic mechanisms of ventricular arrhythmias

In this work the electrophysiologic mechanisms of ventricular arrhythmias have been briefly summarized. Ventricular arrhythmias can be caused either by pacemaker activity or by reentrant excitation. Enhancement of normal automaticity can generate a parasystolic rhythm in normal fibers. Abnormal automaticity may arise from fibers in which maximum diastolic potential has been reduced by a variety of interventions. Triggered activity is caused by either an early (EAD) or delayed (DAD) afterdepolarization and requires a prior normal action potential for initiation. While there is growing evidence that EAD-induced triggered activity plays a significant role in the Long QTU syndrome and Torsade de Pointes, no clinical arrhythmias has definitely been ascribed to DADs, although DADs have been recorded in man after acute digoxin intoxication. Ventricular arrhythmias can be also caused by reentrant excitation, which can be subdivided into reflection or circus movement reentry (CMR). In the reflection model impulses in both directions are transmitted over the same pathway. In the CMR three models can be differentiated: the ring model, which requires a fixed anatomical obstacle; the figure-eight model and the leading circle model, where functional rather than fixed anatomical obstacles are involved.

Determinants of subsidiary ventricular pacemaker suppression in man

To investigate the relative contribution of the duration and rate of overdrive to subsidiary ventricular pacemaker suppression, in six patients with complete heart block after His-bundle ablation, ventricular overdrive stimulation studies were performed. The studies, which were spread over a mean follow-up period of 745 days, were carried out invasively with a temporary lead (one patient) as well as noninvasively with the implanted pacemakers and chest wall inhibition (five patients). The overdrive pacing rate was increased in steps of 10 beats/min, and the pacing duration was 15, 30, 60, 90, and 120 seconds at each level. A recovery period of 2 minutes was allowed after each overdrive stimulation. Incremental ventricular overdrive stimulation at increasing pacing durations consistently caused progressive suppression of ventricular impulse formation. Nonparametric variance analysis demonstrated a significant (P less than 0.0001) influence of both the pacing rate and duration on ventricular recovery time. Nonlinear regression showed an exponential increase in recovery time with incremental pacing rate and a biphasic increase in recovery time with incremental pacing duration. Beyond a pacing duration of 60 seconds ventricular impulse suppression was primarily dependent upon the pacing rate. A nonlinear regression model was applied to predict the number of beats required for return of the escape rhythm toward prepacing control values. The predicted maximum mean number of beats was 15.4 +/- 5.9 and independent of the rate and duration of pacing, although, the initial temporary instability of the escape rhythm was directly related to the degree of overdrive.

Mechanisms of lidocaine actions on normal and abnormal rhythms in canine cardiac tissues in vivo and in vitro

1. The actions of lidocaine on cardiac pacemaker rhythms were studied in anaesthetized dogs and in Purkinje fibres from hearts of the same animals. 2. In vivo, lidocaine (1 mg/kg, intravenously) slowed the sino-atrial (SA) node rhythm (-5.0%), and (during vagal stimulation) prolonged ventricular standstill by +25.1% and slowed the idioventricular rhythm (-16.7%). A higher dose (4 mg/kg) had more pronounced effects. 3. Propranolol also slowed sinus (-26.2%) and idioventricular (-27.2%) rhythms, and prolonged ventricular standstill (+36.8%). In the presence of propranolol, the effects of lidocaine on idioventricular rhythm were exaggerated. 4. In Purkinje fibres driven in vitro, lidocaine (10 mumol/L) decreased contractile force (-47.9%) and (during the interruption of drive) prolonged the suppression of (+53.2%) and slowed the escape rhythm (-67.0%). 5. In the presence of lidocaine the threshold potential was shifted to less negative values and diastolic depolarization slope was decreased (-23.6%). 6. Lidocaine slowed spontaneously active Purkinje fibres, abolished early afterdepolarizations in low [K]o and slow responses in high [K]o (by shifting the threshold to less negative values), and antagonized strophanthidin arrhythmias. 7. TTX reduced the hyperpolarization by lidocaine in low [K]o and vice versa. 8. We conclude that lidocaine enhances vagally-induced ventricular standstill by depressing the idioventricular rhythm far more than the sinus rhythm, an action enhanced by beta-blockade. Furthermore, lidocaine depresses normal and different types of abnormal automaticity through direct and indirect effects of the blockade of the fast sodium channel.

NASPE young investigator awardee-1990. Complex rhythms resulting from overdrive suppression in electrically stimulated heart cell aggregates

Spontaneously beating embryonic chick heart cell aggregates were stimulated with current pulses delivered either as periodic trains, or at a fixed delay after each action potential. Following stimulation at fixed rates faster than the intrinsic rate, there was a transient slowing of the spontaneous rhythm. This response, called overdrive suppression, can lead to a complex evolution of rhythms. During periodic stimulation there is a continuum of dropped beat patterns, and during fixed delay stimulation bursting activity appears. This study provides a conceptual basis for understanding analogous rhythms in the intact heart.

Strontium induces oscillatory potentials in sheep cardiac Purkinje fibers

The induction of strontium overload and its electromechanical manifestations, the factors influencing and the mechanism underlying Sr overload were studied in Purkinje fibers perfused in vitro. Strontium: (1) can induce an oscillatory potential (Vos) and repetitive spontaneous activity at low concentrations (1.35-2.7 mM); (2) at high concentrations (5.4-10.8 mM) less frequently causes a Vos but during recovery in Tyrode solution Vos appears as Sr overload recedes; (3) decreases the maximum diastolic potential by inducing a prolonged depolarization (Vex) which subsides slowly during an interruption of drive; (4) induces a larger Vex after procedures that increase Sr loading (fast driving rates, higher [Sr]o or longer action potentials); (5) does not induce Vos and Vex when the slow channel is blocked; (6) exaggerates Vex (but not Vos) in calcium overloaded fibers; (7) exchanges with Na since in low [Na]o the twitch amplitude increases; (8) is removed from the cell at the resting potential since after a period of quiescence the first resumed twitch decreases as a function of the preceding pause duration; (9) needs Na as charge carrier since the slope of diastolic depolarization decreases in low [Na]o. Thus, Sr causes overload even at low concentrations and induces an oscillatory potential and the prolonged depolarization Vex, whose mechanism appears to be an electrogenic Sr extrusion through Na-Sr exchange.

Relation between Na+-K+ pump, Na+ activity and force in strophanthidin inotropy in sheep cardiac Purkinje fibres

1. The effects of different concentrations of strophanthidin on intracellular sodium activity (aiNa), membrane potential and contractile force have been studied in cardiac sheep Purkinje fibres under conditions (overdrive) that stimulate Na+-K+ pump activity. 2. In fibres driven at 1 Hz, a 5 min overdrive at 2 Hz in the steady state increased force by +74.2%, aiNa by +10.9% and the maximum diastolic potential (Emax) by 3.32 +/- 0.52 mV. 3. During the recovery from overdrive (the fibres being driven again at 1 Hz), both contractile force and aiNa transiently undershot the control value by -10.5 and -3.7%, respectively. When the fibres were quiescent during the recovery from overdrive, no aiNa undershoot was present. 4. During overdrive, force and aiNa were closely correlated when plotted either on linear (correlation coefficient, R = 0.98) or logarithmic (R = 0.98) co-ordinates. 5. A low concentration of strophanthidin (0.01 microM) decreased force (-31.7%) and aiNa (-7.2%): overdrive increased force and Emax more and aiNa less than in the absence of strophanthidin. During the recovery, the undershoot in force (-12.9%) and aiNa (-5.4%) was larger and longer than in the absence of strophanthidin. 6. An intermediate concentration of strophanthidin (0.05 microM) increased force (+43.5%) and aiNa (+6.4%): overdrive increased force and aiNa as usual, but during the recovery the force remained above the value prior to overdrive and there was no aiNa undershoot. 7. A high concentration of strophanthidin (0.1 microM) increased force (+91.4%) and aiNa (+11.7%): overdrive further increased force and aiNa more than in control but there was no increase in Emax. During the recovery, both force and aiNa remained well above the values prior to overdrive. 8. Force and aiNa were closely correlated whether aiNa decreased in 0.01 microM-strophanthidin (R = 0.99 both on linear and logarithmic co-ordinates) or increased in 0.05-microM- (R = 1.00 on both co-ordinates) and in 0.1 microM- (R = 0.98 and 0.99, respectively) strophanthidin. The two parameters were well correlated also during overdrive in the three strophanthidin solutions. However, the slope of the relation was less steep in the low- than in the higher-strophanthidin solutions. 9. For a 1 mM change in aiNa, force decreased less in the low- than it increased in the intermediate-strophanthidin solution. Also, in low-strophanthidin solution, at the end of overdrive the aNao/aNai ratio was similar to that in Tyrode solution but force was well above control (+73.2%).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Effects of beta-adrenergic blockade on verapamil-responsive and verapamil-irresponsive sustained ventricular tachycardias

To assess effects of beta-adrenergic blockade on ventricular tachycardia (VT) of various mechanisms, electrophysiology studies were performed before and after intravenous infusion of propranolol (0.2 mg/kg) in 33 patients with chronic recurrent VT, who had previously been tested with intravenous verapamil (0.15 mg/kg followed by 0.005 mg/kg/min infusion). In the verapamil-irresponsive group, 10 patients (group IA) had VT that could be initiated by programmed ventricular extrastimulation and terminated by overdrive ventricular pacing, and 11 patients (group IB) had VT that could be provoked by isoproterenol infusion (3-8 micrograms/min) but not by programmed electrical stimulation, and that could not be converted to a sustained sinus rhythm by overdrive ventricular pacing. Notably, in the group IA patients, all 10 patients had structural heart disease (coronary arteriosclerosis or idiopathic cardiomyopathy); beta-adrenergic blockade accelerated the VT rate in one patient but exerted no effects on the VT rate in the remaining 9 patients, and VT remained inducible in all 10 patients. By contrast, in the group IB patients, 7 of the 11 patients had no apparent structural heart disease; beta-adrenergic blockade completely suppressed the VT inducibility during isoproterenol infusion in all 11 patients. There were 12 patients with verapamil-responsive VT (group II). 11 of the 12 patients had no apparent structural heart disease. In these patients, the initiation of VT was related to attaining a critical range of cycle lengths during sinus, atrial-paced or ventricular-paced rhythm; beta-adrenergic blockade could only slow the VT rate without suppressing its inducibility. Of note, 14 of the total 33 patients had exercise provocable VT: two in group IA, five in group IB, and seven in group II. Thus, mechanisms of VT vary among patients, and so do their pharmacologic responses. Although reentry, catecholamine-sensitive automaticity, and triggered activity related to delayed afterdepolarizations are merely speculative, results of this study indicate that beta-adrenergic blockade is only specifically effective in a subset group (group IB) of patients with VT suggestive of catecholamine-sensitive automaticity.

Mechanisms for arrhythmias

Two broad descriptors have been applied to the genesis of cardiac arrhythmias: abnormalities of impulse initiation and abnormalities of propagation. The first category include arrhythmias that are automatic, arising over a wide range of membrane potentials, and those that are triggered by early or delayed afterdepolarizations. Among conduction abnormalities, the mechanisms most widely referred to are reentry and reflection. Although it is relatively easy to distinguish among mechanisms at the cellular electrophysiologic level, distinction is far more difficult in the intact heart and in the clinic. The patterns of response to electrical stimulation of the heart are reviewed to demonstrate the confusion that can occur when one attempts to distinguish mechanisms. In addition, means to aid in the process of distinction are suggested.

Contribution of the Na+/K+-pump to the membrane potential

The inward movement of sodium ions and the outward movement of potassium ions are passive and the reverse movements against the electrochemical gradients require the activity of a metabolism-driven Na+/K+-pump. The activity of the Na+/K+-pump influences the membrane potential directly and indirectly. Thus, the maintenance of a normal electrical function requires that the Na+/K+-pump maintain normal ionic concentrations within the cell. The activity of the Na+/K+-pump also influences the membrane potential directly by generating an outward sodium current that is larger when the Na+/K+-pump activity is greater. The activity of the Na+/K+-pump is regulated by several factors including the intracellular sodium concentration and the neuromediators norepinephrine and acetylcholine. The inhibition of the Na+/K+-pump can lead indirectly to the development of inward currents that may cause repetitive activity. Therefore, the Na+/K+-pump modifies the membrane potential in different ways both under normal and abnormal conditions and influences in an essential way many cardiac functions, including automaticity, conduction and contraction. Key words. Active transport of ions; cardiac tissues; electroneutral and electrogenic Na+/K/-pump; control of Na+/K+-pump; normal and abnormal electrical events.

Arrhythmogenic effects of theophylline in human atrial tissue

We studied the effects of theophylline on the transmembrane action potential and the contractile force of human atrial fibers obtained from the hearts of 15 patients, undergoing corrective open-heart surgery. Atrial fibers were perfused with Tyrode solution and driven electrically at a constant rate of 60 beats per min. Theophylline (0.1-1 mM) steepened the diastolic depolarization, increased the amplitude of oscillatory potential during diastole and facilitated the development of spontaneous slow response action potentials. These arrhythmogenic effects of theophylline were suppressed after diltiazem (0.1-0.3 microM) pretreatment. The present findings provide the electrophysiologic evidence that abnormal atrial automaticity as a result of triggered activity may be the underlying cause for atrial ectopic activity and multifocal atrial tachycardia in patients taking theophylline.