摘要:

Enantioselective Electrophysiologic Actions of Tocainide.The electrophysiologic effects of the R (−) and S(+) enantiomers of tocainide, a Class IB antiarrhythmic drug, were studied in the guinea pig papillary muscle, using the standard microelectrode technique. The results can be summarized as follows: (1) R(−) tocainide HCI, 100 mg/L (437μM), was more potent in reducing the maximal rate of depolarization (τmax) than S(+) tocainide HCl. At a stimulation frequency of 1 Hz, in the presence of 5.4 mM external K+, τmaxwas reduced an average of 44% in the presence of R(−) tocainide while only 26% in the presence of S(+) tocainide. The inhibition was mainly due to tonic block. The amount of tonic block, as well as phasic block, were different for both enantiomers. (2) The recovery of drug‐occupied channels during the diastolic interval was slower in the presence of R(−) tocainide (τ= 491 msec) than in the presence of S(+) tocainide (τ= 241 msec). Decreasing the stimulation interval caused proportionally more reduction of Vmaxin the presence of R(−) tocainide than in the presence of S(+) tocainide. (3) Voltage dependence of block of τmaxwas also more pronounced with R(−) tocainide. Inactivation curves were shifted toward more negative potentials of 14 mV in the presence of R(−) tocainide, while S(+) tocainide provoked

ISSN:1045-3873    

DOI:10.1111/j.1540-8167.1991.tb01327.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Propagation Delays and Extracellular Potentials.There is increasing evidence that the complex microscopic structure of the myocardium (distribution of cell‐to‐cell connections) plays an important role in determining propagation of excitation in cardiac muscle. To study this phenomenon, extracellular potentials must be measured with microscopic spatial resolution in cardiac tissue. In this article, the relationships between propagation delays across cardiac gap junctions and extracellular potential waveforms are analyzed using a one‐dimensional model of a propagating action potential, for different degrees of cellular coupling. The central question addressed by this study is whether propagation discontinuities at the cellular level can be detected by microscopic resolution extracellular potential measurements, and whether the discrete junctional propagation delay can be measured by extracellular electrodes. Results demonstrate that the discontinuous nature of propagation at the cellular level is reflected as nonuniformities in the local (microscopic) conduction velocity measured by extracellular electrodes with microscopic resolution (interelectrode distance of 50μm); the degree of nonuniformities in the microscopic velocity is amplified as a result of decreased cellular coupling. These observations are in agreement with recent experimental findings. Propagation time between the middle regions of adjacent cells can be estimated accurately from extracellular potentials at these sites. In contrast, two extracellular electrodes positioned at opposite ends of the gap junction (electrode spacing of 5μm) cannot detect or measure the propagation delay across the junction. This limitation results from the fact that extracellular potentials do not reflect only the localized underlying electrical sources on such a microscopic scale. As cells become less coupled, irregularities appear in the unipolar extracellular potential waveforms. The simulations relate these irregularities to depolarization of individual cells in the fiber. Under the condition of reduced cellular coupling, the depolarization events of neighboring cells are sufficiently separated in time to appear as separate events in the extracellular wa

ISSN:1045-3873    

DOI:10.1111/j.1540-8167.1991.tb01328.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Signal‐Averaged ECG.Signal averaging of the surface QRS complex has been used in recent years to detect electrical signals of small amplitude not otherwise evident on the surface ECG. The technique enables detection of low‐amplitude signals in the terminal portion of the QRS complex and the ST segment. These low‐amplitude signals termed late potentials (LPs) have been correlated with the presence of fragmented electrical activity recorded from endocardial and epicardial sites due to inhomogeneous propagation of conduction in scarred myocardium. Recent studies have supported the value of the signal‐averaged ECG for risk stratification postmyocardial infarction, in the management of patients with syncope and patients with sustained ventricular tachycardia (VT), and survivors of cardiac

ISSN:1045-3873    

DOI:10.1111/j.1540-8167.1991.tb01329.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Intrinsic Cardiac Neurons.Physiological evidence indicates that afferent neurons, local circuit neurons, as well as efferent sympathetic and efferent parasympathetic neurons, are located in the mammalian intrinsic cardiac nervous system. Complex interneuronal interactions can occur between these neurons, as well as between such neurons and other intrathoracic and central nervous system neurons. A variety of neurochemicals have been proposed to be involved in such interneuronal interactions. Thus the electrophysiologic properties and synaptology of intrinsic cardiac neurons may be more varied than has been appreciated accounting, at least in part, for the variety of neuronal responses that in situ intrinsic cardiac neurons are capable of displaying. The various interactions that occur between intrinsic cardiac neurons and other intrathoracic neurons, as well as between neurons in all intrathoracic ganglia and the central nervous system, will have to be characterized in order to clarify the role of the autonomic nervous system regulating the heart throughout each cardiac cycle.

ISSN:1045-3873    

DOI:10.1111/j.1540-8167.1991.tb01330.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Chaos in Cardiology.The field of nonlinear dynamics, including chaos theory, has received considerable attention of late. There is a growing body of research relating chaos to basic and clinical cardiology. We present some of the concepts underlying nonlinear dynamics at an introductory level, and review the research relating chaos to cardiology in three areas: cellular‐level cardiac electrophysiology; ventricular fibrillation; and sinus rhyth

ISSN:1045-3873    

DOI:10.1111/j.1540-8167.1991.tb01331.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Sympathetic Imbalance.The demonstration of QT interval prolongation after left stellate ganglion stimulation led to the successful trial of left cervicothoracic sympathectomy for the control of arrhythmias in long QT syndromes. Based on those findings, sympathetic imbalance with excessive left‐sided effects became widely regarded as the mechanism of QT prolongation and arrhythmias in the syndromes. Prolongation of the interval was attributed to regional reduction of repolarization durations with exposure of previously cancelled electrocardiographic effects and arrhythmias to nonuniform recovery of excitability permitting reentrant excitation. Subsequent findings have not, however, supported this hypothesis. It has been shown that QT prolongation following left stellate ganglion stimulation is transient and unlikely to account for the sustained prolongation in the syndromes. It has also been shown that QT prolongation is not unique for left stellate stimulation and also occurs transiently after right stellate stimulation or bolus injections of catecholamines. It remains possible that sympathetic imbalance in the form of deficient right‐sided sympathetic effects on the heart that experimentally results in prolonged repolarization and sustained QT interval prolongation is a mechanism in long QT syndromes. ECG mapping suggests prolongation of repolarization in the region normally innervated by the right stellate ganglion and a plausible mechanism of torsade de pointes based on regionally disparate recovery has been propo

ISSN:1045-3873    

DOI:10.1111/j.1540-8167.1991.tb01332.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

ISSN:1045-3873    

DOI:10.1111/j.1540-8167.1991.tb01333.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Digitalis and Ventricular Tachycardia.Digoxin was used to treat a patient with an adenosine‐sensitive ventricular arrhythmia. The patient had an exercise‐induced ventricular tachycardia that was evaluated electrophysiologically and displayed characteristics of a triggered arrhythmia. The tachycardia was terminated reproducibly with 12 mg of intravenous adenosine. After treatment with digoxin (serum level = 1.7 ng/mL), the arrhythmia could no longer be initiated with programmed electrical stimulation or exercise treadmill testing. The patient has since remained symptom free for 10 months. The autonomic effects of digitalis are proposed to mediate drug efficacy in this form of ventricular tachycar

ISSN:1045-3873    

DOI:10.1111/j.1540-8167.1991.tb01334.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

ISSN:1045-3873    

DOI:10.1111/j.1540-8167.1991.tb01336.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY