André G. Kléber

Pathophysiology and Prevention of Atrial Fibrillation

Atrial fibrillation (AF) is a ubiquitous yet diverse cardiac arrhythmia whose incidence increases with age; with most forms of cardiac and some pulmonary diseases; and with a number of metabolic, toxic, endocrine, or genetic abnormalities.1 2 Classification of clinical AF subtypes can be achieved on the basis of the ease by which episodes of the arrhythmia terminate as follows3 : “Paroxysmal” AF refers to episodes that generally stop spontaneously after no more than a few days. “Persistent” AF occurs less frequently than paroxysmal AF and, rather than self-terminating, requires cardioversion to restore sinus rhythm. “Permanent” AF cannot be converted to sinus rhythm. These terms apply strictly to chronic AF, because a single episode of the arrhythmia cannot be fully categorized. Although there are some mixed patterns, they generally derive from physician impatience for early cardioversion or from pragmatic clinical considerations (eg, to avoid thrombus formation or hemodynamic decompensation). Patients initially presenting with paroxysmal AF often progress to longer, non–self-terminating bouts. An exception may be paroxysmal AF during intense vagotonia. Moreover, AF initially responsive to pharmacological or electrical cardioversion tends to become resistant and cannot then be converted to sinus rhythm. To some extent, the failure of the physician to suggest or the patient to accept further cardioversion attempts may lead to diagnosis of “permanent” AF. Thus, the “point of no return” may be determined by true pathophysiological abnormalities or may merely be an artifact of clinical pragmatism. Effective prevention is essential in managing this arrhythmia whose occurrence is widespread, progression is relentless, and morbidity and mortality are significant. To focus on means for prevention necessitates considering both clinical risk factors and pathophysiology. AF derives from a complex continuum predisposing factors, summarized in Table 1⇓. In the West, about 5% of the population >65 years of age …

Resting membrane potential, extracellular potassium activity, and intracellular sodium activity during acute global ischemia in isolated perfused guinea pig hearts.

Transmembrane potentials, extracellular potassium activity, and intracellular sodium activity were determined during acute global ischemia in Langendorff perfused guinea pig ventricles by microelectrode techniques. Resting membrane potential decreased with a sigmoidal time course from −82 mV to −49.5 ± 2.7 mV (SD, n = 6) and extracellular potassium activity increased from 4 to 5 mM to 14.7 ± 1.3 ITIM (n = 8) during 15 minutes of ischemia. The estimated potassium equilibrium potential was 7 mV more negative than resting membrane potential prior to occlusion, but approached resting membrane potential during ischemia. An increase in extracellular potassium accumulation occurred when heart rate was increased abruptly from 60 to 170 beats/min. After rapid stimulation, a transient decrease of extracellular potassium activity occurred which was abolished in the presence of 10−6 M strophanthidin. If the preparations were paced before and after aortic occlusion at a constant rate, potassium accumulation was independent of heart rate within a range of 50–170 beats/min. Intracellular sodium activity was 8.8 ± 2.8 mM (n = 8) prior to occlusion and decreased slightly to values between 4.7 and 7.6 mM after 10–15 minutes of ischemia. The results suggest that relative potassium permeability largely predominates over relative sodium permeability during the decrease of resting membrane potential after interruption of aortic flow. Furthermore, active sodium-potassium exchange compensates for the rate-dependent fraction of potassium efflux and maintains a low intracellular sodium activity. For reasons of electroneutrality, the potassium efflux underlying extracellular potassium accumulation must be balanced by an equivalent charge movement which is not carried by sodium. The most probable hypothesis regarding the charge carriers is that net potassium efflux occurs secondary to efflux of phosphate and lactate generated during ischemia.

Electrical uncoupling and increase of extracellular resistance after induction of ischemia in isolated, arterially perfused rabbit papillary muscle.

Extracellular and intracellular longitudinal resistances (ro and ri), transmembrane potentials, and conduction velocity were determined in arterially blood-perfused rabbit papillary muscles. Cable analysis was made possible by placing the muscle in a H2O-saturated gaseous environment, which acted as an electrical insulator. Ischemia was produced by exchanging the O2 in the atmosphere by N2 (94% N2-6% CO2) in addition to arresting coronary flow. The first 10-15 minutes of ischemia were characterized by an increase in ro, while ri remained constant. The early part of the increase in ro coincided with the drop in perfusion pressure and was probably due to the diminution of intravascular volume. Rapid electrical uncoupling, reflected by an increase in ri (threefold within 5 minutes), occurred thereafter. The dissociation between the early increase in ro and the delayed increase in ri produced an initial increase in the ratio ro:ri, which subsequently decreased. The decrease in conduction velocity was less than observed in intact hearts with ischemia. This difference is explained by the relatively small changes in resting membrane potential and action potential amplitude in the preparation used. Our results suggest that in the early, reversible phase of ischemia, the increase in ro contributes to a small but significant extent to the slowing of conduction. After 15-20 minutes, the rapid cellular uncoupling, which was most likely coincident with breakdown of cellular homeostasis, may contribute to the occurrence of arrhythmias during this phase of ischemia. Moreover, the early change in the ratio ro:ri will influence the amplitude of the extracellular electrograms following coronary occlusion (TQ-segment and ST-segment shifts).

Genetic and molecular basis of cardiac arrhythmias: impact on clinical management parts I and II

Genetic approaches have succeeded in defining the molecular basis of an increasing array of heart diseases, such as hypertrophic cardiomyopathy and the long-QT syndromes, associated with serious arrhythmias. Importantly, the way in which this new knowledge can be applied to managing patients and to the development of syndrome-specific antiarrhythmic strategies is evolving rapidly because of these recent advances. In addition, the extent to which new knowledge represents a purely research tool versus the extent to which it can be applied clinically is also evolving. The present article represents a consensus report of a meeting of the European Working Group on Arrhythmias. The current state of the art of the molecular and genetic basis of inherited arrhythmias is first reviewed, followed by practical advice on the role of genetic testing in these and other syndromes and the way in which new findings have influenced current understanding of the molecular and biophysical basis of arrhythmogenesis.

Slow Conduction in Cardiac Tissue, I: Effects of a Reduction of Excitability Versus a Reduction of Electrical Coupling on Microconduction

It was the aim of this study to characterize the spread of activation at the cellular level in cardiac tissue during conduction slowing, a key element of reentrant arrhythmias; therefore, activation patterns were assessed at high spatiotemporal resolution in narrow (70 to 80 microm) and wide (230 to 270 microm) linear strands of cultured neonatal rat ventricular myocytes, using multiple site optical recording of transmembrane voltage. Slow conduction was induced by graded elevation of [K+]o, by applying tetrodotoxin, or by exposing the preparations to the gap junctional uncouplers palmitoleic acid or 1-octanol. The main findings of the study are 4-fold: (1) gap junctional uncoupling reduced conduction velocity (range, 37 to 47 cm/s under control conditions) to a substantially larger extent before block (</=1 cm/s; ultra-slow conduction) than did a reduction of excitability (range, approximately 10 to 15 cm/s); (2) activation wavefronts during uncoupling meandered within the boundaries of the preparations, resulting in a pronounced additional slowing of conduction in wide cell strands; (3) at the cellular level, propagation during uncoupling-induced ultra-slow conduction was sustained by sequentially activated tissue patches, each of which consisted of a few cells being activated simultaneously; and (4) depending on the uncoupler used, maximal action potential upstroke velocities during ultra-slow conduction were either slightly (palmitoleic acid) or highly (1-octanol) depressed. Thus, depolarizing inward currents, the spatial pattern and degree of gap junctional coupling, and geometrical factors all contribute in a concerted manner to conduction slowing, which, at its extreme (0.25 cm/s measured over 1 mm), can reach values low enough to permit, theoretically, reentrant excitation to occur in minuscule areas of cardiac tissue (<<1 mm2).

Role of wavefront curvature in propagation of cardiac impulse

It is traditionally assumed that impulse propagation in cardiac muscle is determined by the combination of two factors: (1) the active properties of cardiac cell membranes and (2) the passive electrical characteristics of the network formed by cardiac cells. However, advances made recently in the theory of generic excitable media suggest that an additional factor-the geometry of excitation wavefronts -may play an important role. In particular, impulse propagation strongly depends on the wavefront curvature on a small spatial scale. In the heart, excitation wavefronts have pronounced curvatures in several situations including waves initiated by small electrodes, waves emerging from narrow tissue structures, and waves propagating around the sharp edges of anatomical obstacles or around a zone of functional conduction block during spiral wave rotation. In this short review we consider the theoretical background relating impulse propagation to wavefront curvature and we estimate the role of wavefront curvature in electrical stimulation, formation of conduction block, and the dynamic behavior of spiral waves.

Extracellular potassium accumulation in acute myocardial ischemia.

In acute reversible myocardial ischemia extracellular potassium activity rises 4 to 5 fold within 10 to 15 min after interruption of coronary perfusion. The underlying net cellular K+ loss is likely to be caused by an increased K+ efflux in the presence of maintained Na+/K+ pumping (i.e. active K+ influx). A hypothesis is discussed which relates increased K+ efflux to the formation of weak acids in the cells during anaerobic metabolism. However, the exact mechanism remains to be determined by further experimental investigations.

Cardiac tissue geometry as a determinant of unidirectional conduction block: assessment of microscopic excitation spread by optical mapping in patterned cell cultures and in a computer model.

OBJECTIVE Unidirectional conduction block (UCB) and reentry may occur as a consequence of an abrupt tissue expansion and a related change in the electrical load. The aim of this study was to evaluate critical dimensions of the tissue necessary for establishing UCB in heart cell culture. METHODS Neonatal rat heart cell cultures with cell strands of variable width emerging into a large cell area were grown using a technique of patterned cell growth. Action potential upstrokes were measured using a voltage sensitive dye (RH-237) and a linear array of 10 photodiodes with a 15 microns resolution. A mathematical model was used to relate action potential wave shapes to underlying ionic currents. RESULTS UCB (block of a single impulse in anterograde direction - from a strand to a large area - and conduction in the retrograde direction) occurred in narrow cell strands with a width of 15(SD 4) microns (1-2 cells in width, n = 7) and there was no conduction block in strands with a width of 31(8) microns (n = 9, P < 0.001) or larger. The analysis of action potential waveshapes indicated that conduction block was either due to geometrical expansion alone (n = 5) or to additional local depression of conduction (n = 2). In wide strands, action potential upstrokes during anterograde conduction were characterised by multiple rising phases. Mathematical modelling showed that two rising phases were caused by electronic current flow, whereas local ionic current did not coincide with the rising portions of the upstrokes. CONCLUSIONS (1) High resolution optical mapping shows multiphasic action potential upstrokes at the region of abrupt expansion. At the site of the maximum decrement in conduction, these peaks were largely determined by the electrotonus and not by the local ionic current. (2) Unidirectional conduction block occurred in strands with a width of 15(4) microns (1-2 cells).

Impulse Propagation in Synthetic Strands of Neonatal Cardiac Myocytes With Genetically Reduced Levels of Connexin43

Abstract —Connexin43 (Cx43) is a major determinant of the electrical properties of the myocardium. Closure of gap junctions causes rapid slowing of propagation velocity (&thgr;), but the precise effect of a reduction in Cx43 levels due to genetic manipulation has only partially been clarified. In this study, morphological and electrical properties of synthetic strands of cultured neonatal ventricular myocytes from Cx43+/+ (wild type, WT) and Cx+/− (heterozygote, HZ) mice were compared. Quantitative immunofluorescence analysis of Cx43 demonstrated a 43% reduction of Cx43 expression in the HZ versus WT mice. Cell dimensions, connectivity, and alignment were independent of genotype. Measurement of electrical properties by microelectrodes and optical mapping showed no differences in action potential amplitude or minimum diastolic potential between WT and HZ. However, maximal upstroke velocity of the transmembrane action potential, dV/dtmax, was increased and action potential duration was reduced in HZ versus WT. &thgr; was similar in the two genotypes. Computer simulation of propagation and dV/dtmax showed a relatively small dependence of &thgr; on gap junction coupling, thus explaining the lack of observed differences in &thgr; between WT and HZ. Importantly, the simulations suggested that the difference in dV/dtmax is due to an upregulation of INa in HZ versus WT. Thus, heterozygote‐null mutation of Cx43 produces a complex electrical phenotype in synthetic strands that is characterized by both changes in ion channel function and cell‐to‐cell coupling. The lack of changes in &thgr; in this tissue is explained by the dominating role of myoplasmic resistance and the compensatory increase of dV/dtmax. (Circ Res. 2003;92:1209–1216.)

Autocrine Regulation of Myocyte Cx43 Expression by VEGF

Cardiac myocytes can rapidly adjust their expression of gap junction channel proteins in response to changes in load. Previously, we showed that after only 1 hour of linear pulsatile stretch (110% of resting cell length; 3 Hz), expression of connexin43 (Cx43) by cultured neonatal rat ventricular myocytes is increased by ≈2-fold and impulse propagation is significantly more rapid. In the present study, we tested the hypothesis that vascular endothelial growth factor (VEGF), acting downstream of transforming growth factor-&bgr; (TGF-&bgr;), mediates stretch-induced upregulation of Cx43 expression by cardiac myocytes. Incubation of nonstretched cells with exogenous VEGF (100 ng/mL) or TGF-&bgr; (10 ng/mL) for 1 hour increased Cx43 expression by ≈1.8-fold, comparable to that observed in cells subjected to pulsatile stretch for 1 hour. Stretch-induced upregulation of Cx43 expression was blocked by either anti-VEGF antibody or anti-TGF-&bgr; antibody. Stretch-induced enhancement of conduction was also blocked by anti-VEGF antibody. ELISA assay showed that VEGF was secreted into the culture medium during stretch. Furthermore, stretch-conditioned medium stimulated Cx43 expression in nonstretched cells. This effect was also blocked by anti-VEGF antibody. Upregulation of Cx43 expression stimulated by exogenous TGF-&bgr; was blocked by anti-VEGF antibody, but VEGF-stimulation of Cx43 expression was not blocked by anti-TGF-&bgr; antibody. Thus, stretch-induced upregulation of Cx43 expression is mediated, at least in part, by VEGF, which acts downstream of TGF-&bgr;. Because the cultures contained only ≈5% nonmyocytic cells, these results indicate that myocyte-derived VEGF, secreted in response to stretch, acts in an autocrine fashion to enhance intercellular coupling.