Effects of sympathetic stimulation on refractory periods of ischemic canine ventricular myocardium

Autonomic modulation and cardiac arrhythmias: old insights and novel strategies

The autonomic nervous system (ANS) plays a critical role in both health and states of cardiovascular disease. There has been a long-recognized role of the ANS in the pathogenesis of both atrial and ventricular arrhythmias (VAs). This historical understanding has been expanded in the context of evolving insights into the anatomy and physiology of the ANS, including dysfunction of the ANS in cardiovascular disease such as heart failure and myocardial infarction. An expanding armamentarium of therapeutic strategies-both invasive and non-invasive-have brought the potential of ANS modulation to contemporary clinical practice. Here, we summarize the integrative neuro-cardiac anatomy underlying the ANS, review the physiological rationale for autonomic modulation in atrial and VAs, highlight strategies for autonomic modulation, and finally frame future challenges and opportunities for ANS therapeutics.

Neuromodulation for the Treatment of Heart Rhythm Disorders

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Derangement of autonomic nervous signaling is an important contributor to cardiac arrhythmogenesis.

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Modulation of autonomic nervous signaling holds significant promise for the prevention and treatment of cardiac arrhythmias.

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Further clinical investigation is necessary to establish the efficacy and safety of autonomic modulatory therapies in reducing cardiac arrhythmias.

There is an increasing recognition of the importance of interactions between the heart and the autonomic nervous system in the pathophysiology of arrhythmias. These interactions play a role in both the initiation and maintenance of arrhythmias and are important in both atrial and ventricular arrhythmia. Given the importance of the autonomic nervous system in the pathophysiology of arrhythmias, there has been notable effort in the field to improve existing therapies and pioneer additional interventions directed at cardiac-autonomic targets. The interventions are targeted to multiple and different anatomic targets across the neurocardiac axis. The purpose of this review is to provide an overview of the rationale for neuromodulation in the treatment of arrhythmias and to review the specific treatments under evaluation and development for the treatment of both atrial fibrillation and ventricular arrhythmias.

Stellate ganglion blockade for the treatment of refractory ventricular arrhythmias: A systematic review and meta-analysis

INTRODUCTION

Treatment refractory ventricular arrhythmias (VAs) are often driven and exacerbated by heightened sympathetic tone. We aim to conduct a systematic review and meta-analysis of published studies of a temporary percutaneous stellate ganglion block (SGB) on VA burden and defibrillation episodes in patients with treatment refractory VAs.

METHODS

Relevant studies from January 1960 through May 2017 were identified in PubMed and Google Scholar. We performed a patient-level analysis using Student's t-test to compare outcomes before and after SGB.

RESULTS

We identified 22 unique case series with a total of 35 patients. Patients were 57 ± 17 years old and 69% were males with a high burden of VA. A unilateral (left)-sided SGB was used in 85.7% (30 of 35) of cases and the remaining were bilateral SGB. The use of a unilateral or bilateral SGB resulted in a significant reduction of VA episodes (24-hours pre: mean 16.5 [CI 9.7-23.1] events vs. post: mean 1.4 [CI 0.85-2.01] events; P = 0.0002) and need for defibrillation (24-hours pre: mean 14.2 [CI 6.8-21.6] vs. post: mean 0.6 [CI 0.3-0.9]; P = 0.0026). Furthermore, SGB was significantly associated with a reduction of VA burden regardless of etiology of cardiomyopathy, type of ventricular rhythm, and degree of contractile dysfunction. SGB was followed by surgical sympathectomy in 21% of cases.

CONCLUSIONS

Early experience suggests that SGB is associated with an acute reduction in the VA burden and offers potential promise for a broader use in high-risk populations. Randomized controlled studies are needed to confirm the safety and efficacy of this therapy.

Ventricular repolarization: An overview of (patho)physiology, sympathetic effects and genetic aspects

Most textbook knowledge on ventricular repolarization is based on animal data rather than on data from the in vivo human heart. Yet, these data have been extrapolated to the human heart, often without an appropriate caveat. Here, we review multiple aspects of repolarization, from basic membrane currents to cellular aspects including extrinsic factors such as the effects of the sympathetic nervous system. We critically discuss some mechanistic aspects of the genesis of the T-wave of the ECG in the human heart. Obviously, the T-wave results from the summation of repolarization all over the heart. The T-wave in a local electrogram ideally reflects local repolarization. The repolarization moment is composed of the moment of local activation plus local action potential duration (APD) at 90% repolarization (APD90). The duration of the latter largely depends on the balance between L-type Ca2+ current and the delayed rectifier currents. Generally speaking, there is an inverse relationship between local activation time and local APD90, leading to less dispersion in repolarization moments than in activation moments or in APD90. In transmural direction, the time needed for activation from endocardium toward epicardium has been considered to be overcompensated by shorter APD90 at the epicardium, leading to the earliest repolarization at the subepicardium. In addition, mid-myocardial cells would display the latest repolarization moments. The sparse human data available, however, do not show any transmural dispersion in repolarization moment. Also, the effect of adrenergic stimulation on APD90 has been studied mainly in animals. Again, sparse human data suggest that the effect of adrenergic stimulation is different in the human heart compared to many other mammalian hearts. Finally, aspects of the long QT syndrome are discussed, because this intrinsic genetic disease results from repolarization disorders with extrinsic aspects.

Effects of premature beats on repolarization of postextrasystolic beats

BACKGROUND

A short-long-short sequence of cycle lengths predisposes to reentrant tachyarrhythmias. There is limited information about the effects of premature ventricular contractions (PVCs) on repolarization of postextrasystolic depolarizations (PEDs). Such information would contribute to understanding the mechanism for facilitating reentry with short-long-short cycle lengths.

METHODS AND RESULTS

We introduced PVCs, over a range of coupling intervals and during a range of basic drive cycle lengths (BCLs), and determined PED repolarization. Our results from whole-animal experiments, isolated cell studies, and computer simulations are reported. In the whole-animal experiments, PED refractory periods (RPs) were longer than RPBCL. The greatest difference between RPPED and RPBCL (delta RPmax) occurred after short coupling interval PVCs and was 4.3 +/- 0.8, 4.2 +/- 0.8, and 2.1 +/- 0.5 ms (mean +/- SEM) during drives with short, intermediate, and long BCLs, respectively. The diastolic interval preceding the PED (DIPED) was inversely related to the coupling interval between the basic drive beat and the PVC and directly related to RPPED. PED action potential durations (APDs) of isolated canine myocytes were 9.8 +/- 4.9 ms (mean +/- SEM) longer than APD BCL (n = 19). The DiFrancesco-Noble membrane equations were used in simulations of action potential propagation in a one-dimensional cable, with stimulation protocols duplicating those in the animal experiments. PVCs prolonged APDPED, and APDPED was prolonged more during short than during long BCL drives. There was a direct relation between DIPED and APDPED. Analysis of the membrane currents over the time course of the PVCs and PEDs suggested that the ionic basis for PED repolarization prolongation was the interaction of Ito and Ik. Hyperpolarizing constant-current injections introduced immediately after the spike of isolated myocyte action potentials caused APD prolongation. This observation is consistent with the Ito and Ik interaction causing PED repolarization prolongation.

CONCLUSIONS

PED repolarization prolongation could provide sites for unidirectional block to propagation of PVCs after PEDs and could facilitate initiation of reentrant tachyarrhythmias after short-long-short sequences of cycle lengths.

Alternation in refractoriness and in conduction delay in the ischemic myocardium associated with the alternation in the ST-T complex during acute coronary occlusion in anesthetized dogs

The effective refractory period (ERP) during the period of the alternation of the ST-T complex (STTA) in the ischemic myocardium was determined using a synchronized system of electrical stimulators and a bipolar epicardial electrode during acute coronary occlusion in anesthetized dogs. Epicardial unipolar electrograms (UP), epicardial bipolar electrograms (BP) and monophasic action potentials (MAP) were also recorded from ischemic areas. ERP in ischemic areas was prolonged, as ischemia progressed, and during the period of STTA ERP also alternately changed. A lower or negative deflection of the ST-T complex was accompanied by a longer ERP, and a higher or monophasic ST-T complex was accompanied by a shorter ERP. A good correlation was observed between the degrees of alternation in ERP and of STTA. MAP also showed an alternation in its duration and its amplitude. The alternation in duration preceded the alternation in amplitude. A lower or negative deflection of the ST-T complex corresponded to a MAP with a longer duration and a larger amplitude. During the period of STTA, a long conduction delay followed the negative deflection of the ST-T complex. A marked conduction delay appeared only during a period of STTA. Diltiazem and nifedipine inhibited STTA and an alternation in ERP. In conclusion, STTA is associated with an alternation in ERP and in conduction delay, and calcium antagonists attenuate both STTA and an alternation in ERP.

Right ventricular monophasic action potentials in healthy young men

The right ventricular repolarization phase was studied in 48 healthy men between 20 and 40 years of age. The assessment of the repolarization time included the measurement of ventricular effective refractory periods and monophasic action potentials during constant ventricular stimulation. Computer-based analysis of the monophasic action potential allowed the duration at 90% and 50% repolarization, the amplitude, the maximal upstroke velocity and the total rise time of the depolarization to be determined. These results may serve as reference values in further studies on ventricular repolarization using the same monophasic action potential recording technique.

Effects of sympathetic stimulation on ventricular refractory periods in cats with acute coronary artery ligation

Ventricular refractory periods shorten in the ischemic area following acute coronary artery ligation. Subsequent bilateral sympathetic nerve stimulation reduces disparity in refractory periods across normal, border (peri-ischemic) and ischemic areas.

Effects of the ventricular premature beat on the alternation of the repolarization phase in ischemic myocardium during acute coronary occlusion in dogs

Effects of ventricular premature beats (VPB) on the alternans of the ST segment (ST alternans, STA) in the epicardial ECG and of the monophasic action potential (MAP) were examined during acute coronary occlusion in dogs. When STA was recorded simultaneously from four different points it was discordant in most cases. The discordant STA was accompanied by discordant alternation of the repolarization phase of MAP. A VPB transformed the discordant alternans into a concordant one and potentiated the degree of alternans. Transient prolongation or shortening of the cycle length showed effects similar to those of the VPB. The concordant alternans which was transformed from a discordant one by a VPB was not remarkably potentiated by a VPB, or by prolongation or shortening of the cycle length. It is possible that the transformation of the discordant alternans into a concordant one may contribute to the potentiation of STA by a VPB, and that the effects of VPB are due to the effects of the compensatory pause as well as the short cycle length associated with the VPB.