Ventricular rate control during atrial fibrillation by cardiac parasympathetic nerve stimulation: a transvenous approach

Nonpharmacologic rate control of postoperative atrial fibrillation in the canine sterile pericarditis model

INTRODUCTION

Postoperative atrial fibrillation (POAF) is common following open heart surgery, and is associated with significant morbidity. Medications used for ventricular rate control of POAF may not be effective in controlling rapid ventricular rates during the postoperative period because of increased sympathetic tone. The purpose of this study was to develop nonpharmacologic rate control of POAF by atrioventricular node (AVN) fat pad stimulation using clinically available temporary pacing wires in the canine sterile pericarditis model.

METHODS

We studied 10 sterile pericarditis dogs in the closed-chest state on postoperative days 1-3. The AVN fat pad stimulation (amplitude 2-15 mA; frequency 20 Hz; pulse width 0.03-0.2 ms) was performed during sustained POAF (>5 min). We measured ventricular rate and inefficient ventricular contractions during sustained POAF and compared it with and without AVN fat pad stimulation. Also, the parameters of AVN fat pad stimulation to achieve a rate control of POAF were measured over the postoperative days.

RESULTS

Eleven episodes of sustained POAF were induced in 5/10 sterile pericarditis dogs in the closed-chest state on postoperative days 1-2. During POAF, the AVN fat pad stimulation decreased the ventricular rate from 178 ± 52 bpm to 100 ± 8 bpm in nine episodes. Nonpharmacologic rate control therapy successfully controlled the ventricular rate and eliminated inefficient ventricular contractions during POAF for the duration of the AVN fat pad stimulation. The AVN fat pad stimulation output remained relatively stable over the postoperative days.

CONCLUSION

During sustained POAF, nonpharmacologic rate control by AVN fat pad stimulation effectively and safely controlled rapid ventricular rates throughout the postoperative period.

Renal Denervation to Treat Heart Failure

Heart failure (HF) is a global pandemic with a poor prognosis after hospitalization. Despite HF syndrome complexities, evidence of significant sympathetic overactivity in the manifestation and progression of HF is universally accepted. Confirmation of this dogma is observed in guideline-directed use of neurohormonal pharmacotherapies as a standard of care in HF. Despite reductions in morbidity and mortality, a growing patient population is resistant to these medications, while off-target side effects lead to dismal patient adherence to lifelong drug regimens. Novel therapeutic strategies, devoid of these limitations, are necessary to attenuate the progression of HF pathophysiology while continuing to reduce morbidity and mortality. Renal denervation is an endovascular procedure, whereby the ablation of renal nerves results in reduced renal afferent and efferent sympathetic nerve activity in the kidney and globally. In this review, we discuss the current state of preclinical and clinical research related to renal sympathetic denervation to treat HF.

The role of the autonomic nervous system in cardiac arrhythmias: The neuro-cardiac axis, more foe than friend?

The autonomic nervous system (ANS) with its two limbs, the sympathetic (SNS) and parasympathetic nervous system (PSNS), plays a critical role in the modulation of cardiac arrhythmogenesis. It can be both pro- and/or anti-arrhythmic at both the atrial and ventricular level of the myocardium. Intricate mechanisms, different for specific cardiac arrhythmias, are involved in this modulatory process. More data are available for the arrhythmogenic effects of the SNS, which, when overactive, can trigger atrial and/or ventricular "adrenergic" arrhythmias in susceptible individuals (e.g. in patients with paroxysmal atrial fibrillation-PAF, ventricular pre-excitation, specific channelopathies, ischemic heart disease or cardiomyopathies), while it can also negate the protective anti-arrhythmic drug effects. However, there is also evidence that PSNS overactivity may be responsible for triggering "vagotonic" arrhythmias (e.g. PAF, Brugada syndrome, idiopathic ventricular fibrillation). Thus, a fine balance is necessary to attain in these two limbs of the ANS in order to maintain eurhythmia, which is a difficult task to accomplish. Over the years, in addition to classical drug therapies, where beta-blockers prevail, several ANS-modulating interventions have been developed aiming at prevention and management of arrhythmias. Among them, techniques of cardiac sympathetic denervation, renal denervation, vagal stimulation, ganglionated plexi ablation and the newer experimental method of optogenetics have been employed. However, in many arrhythmogenic diseases, ANS modulation is still an investigative tool. Initial data are encouraging; however, further studies are needed to explore the efficacy of such interventions. These issues are herein reviewed and old and recent literature data are discussed, tabulated and pictorially illustrated.

Role of the autonomic nervous system in atrial fibrillation: pathophysiology and therapy

Autonomic nervous system activation can induce significant and heterogeneous changes of atrial electrophysiology and induce atrial tachyarrhythmias, including atrial tachycardia and atrial fibrillation (AF). The importance of the autonomic nervous system in atrial arrhythmogenesis is also supported by circadian variation in the incidence of symptomatic AF in humans. Methods that reduce autonomic innervation or outflow have been shown to reduce the incidence of spontaneous or induced atrial arrhythmias, suggesting that neuromodulation may be helpful in controlling AF. In this review, we focus on the relationship between the autonomic nervous system and the pathophysiology of AF and the potential benefit and limitations of neuromodulation in the management of this arrhythmia. We conclude that autonomic nerve activity plays an important role in the initiation and maintenance of AF, and modulating autonomic nerve function may contribute to AF control. Potential therapeutic applications include ganglionated plexus ablation, renal sympathetic denervation, cervical vagal nerve stimulation, baroreflex stimulation, cutaneous stimulation, novel drug approaches, and biological therapies. Although the role of the autonomic nervous system has long been recognized, new science and new technologies promise exciting prospects for the future.

Computational modeling of an endovascular approach to deep brain stimulation

OBJECTIVE

Deep brain stimulation (DBS) therapy currently relies on a transcranial neurosurgical technique to implant one or more electrode leads into the brain parenchyma. In this study, we used computational modeling to investigate the feasibility of using an endovascular approach to target DBS therapy.

APPROACH

Image-based anatomical reconstructions of the human brain and vasculature were used to identify 17 established and hypothesized anatomical targets of DBS, of which five were found adjacent to a vein or artery with intraluminal diameter ≥1 mm. Two of these targets, the fornix and subgenual cingulate white matter (SgCwm) tracts, were further investigated using a computational modeling framework that combined segmented volumes of the vascularized brain, finite element models of the tissue voltage during DBS, and multi-compartment axon models to predict the direct electrophysiological effects of endovascular DBS.

MAIN RESULTS

The models showed that: (1) a ring-electrode conforming to the vessel wall was more efficient at neural activation than a guidewire design, (2) increasing the length of a ring-electrode had minimal effect on neural activation thresholds, (3) large variability in neural activation occurred with suboptimal placement of a ring-electrode along the targeted vessel, and (4) activation thresholds for the fornix and SgCwm tracts were comparable for endovascular and stereotactic DBS, though endovascular DBS was able to produce significantly larger contralateral activation for a unilateral implantation.

SIGNIFICANCE

Together, these results suggest that endovascular DBS can serve as a complementary approach to stereotactic DBS in select cases.

Therapeutic effects of selective atrioventricular node vagal stimulation in atrial fibrillation and heart failure

INTRODUCTION

Atrial fibrillation (AF) and heart failure (HF) frequently coexist. We have previously demonstrated that selective atrioventricular node (AVN) vagal stimulation (AVN-VS) can be used to control ventricular rate during AF. Due to withdrawal of vagal activity in HF, the therapeutic effects of AVN-VS may be compromised in the combined condition of AF and HF. Accordingly, this study was designed to evaluate the therapeutic effects of AVN-VS to control ventricular rate in AF and HF.

METHODS AND RESULTS

A combined model of AF and HF was created by implanting a dual chamber pacemaker in 24 dogs. A newly designed bipolar electrode was inserted into the ganglionic AVN fat pad and connected to a nerve stimulator for delivering AVN-VS. In all dogs, HF was induced by high rate ventricular pacing at 220 bpm for 4 weeks. AF was then induced and maintained by rapid atrial pacing at 600 bpm after discontinuation of ventricular pacing. These HF + AF dogs were randomized into control (n = 9) and AVN-VS (n = 15) groups. In the latter group, vagal stimulation (310 μs, 20 Hz, 3-7 mA) was delivered continuously for 6 months. Compared with the control, AVN-VS had a consistent effect on ventricular rate slowing (by >50 bpm, all P < 0.001) during the entire 6-month observation period that was associated with left ventricular functional improvement. Moreover, AVN-VS was well tolerated by the treated animals.

CONCLUSIONS

AVN-VS achieved consistent rate slowing, which was associated with improved ventricular function in a canine AF and HF model. Thus, AVN-VS may be a novel, effective therapeutic option in the combined condition of AF and HF.

Management of patients with drug-induced atrioventricular block

OBJECTIVE

To identify the frequency of atrioventricular (AV) conduction improvement after discontinuation of the culprit drug in patients with AV block.

BACKGROUND

AV blockers are considered as reversible causes of AV block that do not require pacemaker (PM) implantation. However, controversial reports declared that a major part of these drug-induced AV blocks are persistent or recurrent.

METHODS

Of 668 consecutive patients with symptomatic type II second- or third-degree AV block, 2:1 AV block, atrial fibrillation, and bradyarrhythmia, 108 patients (62 patients enrolled prospectively) using AV blockers without myocardial infarction, electrolyte abnormalities, digitalis toxicity, and vasovagal syncope were enrolled into the present study. The level of AV block (AV-nodal or infranodal) was defined according to electrocardiographic characteristics.

RESULTS

The most frequent culprit medications were β-blockers followed by digoxin. Drug discontinuation was followed by resolution of AV block in 72% of cases, whereas spontaneous resolution of AV block occurred in only 6.6% of patients who had AV block in the absence of medications. However, 27% of patients with improved AV conduction experienced a recurrence of AV block despite discontinuation of the culprit drug. Twenty-one of 24 carvedilol-induced AV blocks resolved after discontinuation of the drug and never recurred, whereas 24 of 36 metoprolol-induced AV blocks persisted or recurred. A digoxin-induced AV block usually improved (28 of 39) after withdrawal of the drug. Roughly half of the patients with drug-induced AV block underwent permanent PM implantation.

CONCLUSION

Drug-induced AV block is a serious disease that requires a permanent PM for almost half of the patients.

Chronic Electrical Neuronal Stimulation Increases Cardiac Parasympathetic Tone by Eliciting Neurotrophic Effects

Rationale:

Recently, we provided a technique of chronic high-frequency electric stimulation (HFES) of the right inferior ganglionated plexus for ventricular rate control during atrial fibrillation in dogs and humans. In these experiments, we observed a decrease of the intrinsic ventricular rate during the first 4 to 5 months when HFES was intermittently shut off.

Objective:

We thus hypothesized that HFES might elicit trophic effects on cardiac neurons, which in turn increase baseline parasympathetic tone of the atrioventricular node.

Methods and Results:

In mongrel dogs atrial fibrillation was induced by rapid atrial pacing. Endocardial HFES of the right inferior ganglionated plexus, which contains abundant fibers to the atrioventricular node, was performed for 2 years. Sham-operated nonstimulated dogs served as control. In chronic neurostimulated dogs, we found an increased neuronal cell size accompanied by an increase of choline acetyltransferase and unchanged tyrosine hydroxylase protein expression as compared with unstimulated dogs. Moreover, β-nerve growth factor (NGF) and neurotrophin (NT)-3 were upregulated in chronically neurostimulated dogs. In vitro, HFES of cultured neurons of interatrial ganglionated plexus from adult rats increased neuronal growth accompanied by upregulation of NGF, NT-3, glial-derived neurotrophic factor (GDNF), ciliary neurotrophic factor (CNTF) and brain-derived neurotrophic factor (BDNF) expression. NGF was identified as the main growth-inducing factor, whereas NT-3 did not affect HFES-induced growth. However, NT-3 could be identified as an important acetylcholine-upregulating factor.

Conclusions:

HFES of cardiac neurons in vivo and in vitro causes neuronal cellular hypertrophy, which is mediated by NGF and boosters cellular function by NT-3–mediated acetylcholine upregulation. This knowledge may contribute to develop HFES techniques to augment cardiac parasympathetic tone.

Augmentation of left ventricular contractility by cardiac sympathetic neural stimulation

BACKGROUND

Electric stimulation of mediastinal sympathetic cardiac nerves increases cardiac contractility but is not selective for the left ventricle because it elicits sinus tachycardia and enhanced atrioventricular conduction. The aim of this study was to identify sympathetic neural structures inside the heart that selectively control left ventricular inotropy and can be accessed by transvenous catheter stimulation.

METHODS AND RESULTS

In 20 sheep, high-frequency stimulation (200 Hz) during the myocardial refractory period with electrode catheters inside the coronary sinus evoked a systolic left ventricular pressure increase from 97+/-20 to 138+/-32 mm Hg (P<0.001) without changes in sinus rate or PR time. Likewise, the rate of systolic pressure development (1143+/-334 versus 1725+/-632 mm Hg/s; P=0.004) and rate of diastolic relaxation (531+/-128 versus 888+/-331 mm Hg/s; P=0.001) increased. The slope of the end-systolic pressure-volume relationship increased (2.3+/-0.8 versus 3.1+/-0.6 mm Hg/mL; P=0.04), as did cardiac output (3.5+/-0.8 versus 4.4+/-0.8 L/min; P<0.001). Systemic vascular resistance and right ventricular pressure remained unchanged. There was a sigmoid dose-response curve. Ultrasound analysis revealed an increase in circumferential and radial strain in all left ventricular segments that was significant for the posterior, lateral, and anterior segments. Pressure effects were maintained for at least 4 hours of continued high-frequency stimulation and abolished by beta1-receptor blockade. Histology showed distinct adrenergic nerve bundles at the high-frequency stimulation site.

CONCLUSIONS

Cardiac nerve fibers that innervate the left ventricle are amenable to transvenous electric catheter stimulation. This may permit direct interference with and modulation of the sympathetic tone of the left ventricle.

Chronic augmentation of the parasympathetic tone to the atrioventricular node: a nonthoracotomy neurostimulation technique for ventricular rate control during atrial fibrillation

INTRODUCTION

The right inferior ganglionated plexus (RIGP) selectively innervates the atrioventricular node. Temporary electrical stimulation of this plexus reduces the ventricular rate during atrial fibrillation (AF). We sought to assess the feasibility of chronic parasympathetic stimulation for ventricular rate control during AF with a nonthoracotomy intracardiac neurostimulation approach.

METHODS AND RESULTS

In 9 mongrel dogs, the small endocardial area inside the right atrium, which overlies the RIGP, was identified by 20 Hz stimulation over a guiding catheter with integrated electrodes. Once identified, an active-fixation lead was implanted. The lead was connected to a subcutaneous neurostimulator. An additional dual-chamber pacemaker was implanted for AF induction by rapid atrial pacing and ventricular rate monitoring. Continuous neurostimulation was delivered for 1-2 years to decrease the ventricular rate during AF to a range of 100-140 bpm. Implantation of a neurostimulation lead was achieved within 37 +/- 12 min. The latency of the negative dromotropic response after on/offset or modulation of neurostimulation was <1 s. Continuous neurostimulation was effective and well tolerated during a 1-2 year follow-up with a stimulation voltage <5 V. The neurostimulation effect displayed a chronaxie-rheobase behavior (chronaxie time of 0.07 +/- 0.02 ms for a 50% decrease of the ventricular rate during AF).

CONCLUSION

Chronic parasympathetic stimulation can be achieved via a cardiac neurostimulator. The approach is safe, effective, and well tolerated in the long term. The atrioventricular nodal selectivity and the opportunity to adjust the negative dromotropic effect within seconds may represent an advantage over pharmacological rate control.

Can atrial vagal denervation influence ventricular function in a failing heart?

Atrial fibrillation (AF) and congestive heart failure (CHF) often coexist (AF-CHF), and each adversely affects the other with respect to management and prognosis. Therapy with antiarrhythmic drugs to maintain sinus rhythm was disappointing. Ablation is more successful than antiarrhythmic drug therapy for the prevention of AF with few complications, although in patients with AF-CHF it is noted. Ablating autonomic nerves and ganglia on the large vessels and the heart can result in AF suppression with little damage to healthy myocardium. Our study in patients with AF-CHF found that cardiac function aggravation was more frequent in patients with AF recurrence than that of those who successfully maintain sinus rhythm. The autonomic nervous system is a fine network spreading throughout the myocytes; hence the elimination of atrial vagal with radiofrequency catheter ablation can influence the innervation in sinus and AV nodes even in the ventricular region. Thus we propose that atrial vagal denervation may result in paratherapeutic sympathovagal imbalance in the ventricular region, which has a negative effect in a failing heart, although it is neutralized by the benefit accrued from sinus rhythm after successful ablation.

Lead Design and Initial Applications of a New Lead for Long-Term Endovascular Vagal Stimulation

BACKGROUND

Vagal nerve stimulation (VNS) has negative chronotropic and dromotropic effects. We developed and tested an endovascular spiral vagal stimulation lead (ESVL) designed to follow the projection of the cardiac branches of the vagus nerve around the superior vena cava (SVC) to optimize VNS.

METHODS

ESVL contained six 5-mm coil electrodes, spaced 5-mm apart with a spiral guidewire to provide shape. The tightness and diameter of the guidewire were changed before each placement to simulate different lead designs. Various 2-, 3-, and 4-electrode combinations were used and several lead positions were tested each time. Each VNS protocol included 2-12 V, 15-second pulse trains at 20 Hz, with 2 ms pulse duration. A basket catheter (BC) was used as control and to approximate the initial VNS location. The VNS protocol was performed at the optimal location, using first the BC and then several ESVL configurations.

RESULTS

VNS caused a voltage-dependent decrease in heart rate (HR). Using the optimal ESVL configuration at 7 V, HR decreased by 30.4% (37.2 bpm) in dog no. 1 and 12.4% (16.6 bpm) in dog no. 2, versus 15.5% (16.6 bpm) and 16.7% (19.5 bpm) with the BC.

CONCLUSIONS

A new endovascular spiral lead that takes advantage of the anatomy of the cardiac branches of the vagus nerve in the SVC was developed. VNS using ESVL produced significant HR slowing at voltages slightly below the highest pulse generator output of 7.5 V, which may be suitable for long-term implantation.

Effects of Ca2+ channel antagonists on nerve stimulation-induced and ischemia-induced myocardial interstitial acetylcholine release in cats

Although an axoplasmic Ca2+ increase is associated with an exocytotic acetylcholine (ACh) release from the parasympathetic postganglionic nerve endings, the role of voltage-dependent Ca2+ channels in ACh release in the mammalian cardiac parasympathetic nerve is not clearly understood. Using a cardiac microdialysis technique, we examined the effects of Ca2+ channel antagonists on vagal nerve stimulation- and ischemia-induced myocardial interstitial ACh releases in anesthetized cats. The vagal stimulation-induced ACh release [22.4 nM (SD 10.6), n = 7] was significantly attenuated by local administration of an N-type Ca2+ channel antagonist ω-conotoxin GVIA [11.7 nM (SD 5.8), n = 7, P = 0.0054], or a P/Q-type Ca2+ channel antagonist ω-conotoxin MVIIC [3.8 nM (SD 2.3), n = 6, P = 0.0002] but not by local administration of an L-type Ca2+ channel antagonist verapamil [23.5 nM (SD 6.0), n = 5, P = 0.758]. The ischemia-induced myocardial interstitial ACh release [15.0 nM (SD 8.3), n = 8] was not attenuated by local administration of the L-, N-, or P/Q-type Ca2+ channel antagonists, by inhibition of Na+/Ca2+ exchange, or by blockade of inositol 1,4,5-trisphosphate [Ins( 1 , 4 , 5 )P3] receptor but was significantly suppressed by local administration of gadolinium [2.8 nM (SD 2.6), n = 6, P = 0.0283]. In conclusion, stimulation-induced ACh release from the cardiac postganglionic nerves depends on the N- and P/Q-type Ca2+ channels (with a dominance of P/Q-type) but probably not on the L-type Ca2+ channels in cats. In contrast, ischemia-induced ACh release depends on nonselective cation channels or cation-selective stretch activated channels but not on L-, N-, or P/Q type Ca2+ channels, Na+/Ca2+ exchange, or Ins( 1 , 4 , 5 )P3 receptor-mediated pathway.

Electrical and hemodynamic function produced by stimulation of atropine sensitive right ventricular nerves in humans

In mammalian ventricles including humans, it is recognized that parasympathetic ganglia innervate the heart. Little is known about the location and function of right ventricular parasympathetic nerves in humans. We hypothesized that in humans: (1) there are parasympathetic ganglia that supply the right ventricle that can be stimulated via an endocardial catheter and (2) stimulation of these fibers will alter the electrical and hemodynamic function of the right ventricle. Parasympathetic nerve stimulation was performed via an endocardial catheter placed along several sites of the right ventricle, superior vena cava, and right internal jugular area in humans. The spatial extent of parasympathetic innervation was mapped in 1-cm zones across the right ventricle. Cardiac output, heart rate, and atrioventricular conduction were monitored to provide independent assessment of parasympathetic innervation. In all 22 patients, ventricular refractoriness shortened from 12 +/- 3 to 3 +/- 1 ms during parasympathetic nerve stimulation, and the greatest shortening of refractoriness was observed at the base of the right ventricle (p = 0.01). No significant shortening in ventricular refractoriness occurred in areas beyond 2 cm from the right ventricular base. These results were compared by using T table test. The parasympathetic nerve stimulation protocol decreased cardiac output, reaffirming the principle effect of parasympathetic ganglia. Atropine was administered in seven patients. All effects from nerve stimulation were abolished after atropine administration. These results were also compared by using T table test. These data provide the first demonstration of the electrical and hemodynamic function by stimulation of atropine sensitive nerves of the human right ventricle. Greater understanding of parasympathetic innervation may lead to novel therapies for arrhythmias.

Determinants and Effects of Electrical Stimulation of the Inferior Interatrial Parasympathetic Plexus During Atrial Fibrillation

INTRODUCTION

Catheter stimulation of the inferior interatrial ganglionated parasympathetic plexus decreases the ventricular rate during atrial fibrillation (AF) in humans. However, the relatively high stimulation voltages might prevent implementation of neurostimulation in chronic implantable devices. From myocardial electrostimulation it is known that the required impulse energy and charge is lowest at the chronaxie time. In order to lower energy requirements for cardiac neurostimulation, the present study evaluates the impulse-strength versus impulse-duration relationship for a neurostimulation lead that was implanted into the inferior interatrial ganglionated plexus.

METHODS AND RESULTS

In nine dogs, permanent epicardial bipolar screw-in electrodes were fixed in the inferior interatrial ganglionated plexus. AF was maintained via rapid atrial pacing. During AF, neural stimulation was performed at various frequencies (1-100 Hz), impulse durations (0.05-2 msec), and voltages (0.02-11.5 V). There was a linear correlation between R-R interval lengthening and stimulus voltage (R = 0.99; P < 0.001) and a bell-shaped relationship between stimulation frequency and negative dromotropic effect with maximum rate slowing at 30-50 Hz. The rheobase for a 50% R-R interval prolongation during AF was 1.81 V and 2.72 V for high-grade AVB yielding a chronaxie time of 0.14 msec and 0.18 msec, respectively. The impulse energy (charge) at the chronaxie time was 4-6 microJ (6-8 microC).

CONCLUSIONS

Cardiac neurostimulation follows a chronaxie/rheobase behavior. Energy, charge, and voltage values needed to achieve significant negative dromotropic effects are within the limits of conventional cardiac pacemaker outputs, which may allow implementation of neurostimulation capabilities in current pacemaker technology.

Chronic atrioventricular nodal vagal stimulation: first evidence for long-term ventricular rate control in canine atrial fibrillation model

BACKGROUND

We have previously demonstrated that selective atrioventricular nodal (AVN) vagal stimulation (AVN-VS) can be used to control ventricular rate during atrial fibrillation (AF) in acute experiments. However, it is not known whether this approach could provide a long-term treatment in conscious animals. Thus, this study reports the first observations on the long-term efficacy and safety of this novel approach to control ventricular rate during AF in chronically instrumented dogs.

METHODS AND RESULTS

In 18 dogs, custom-made bipolar patch electrodes were sutured to the epicardial AVN fat pad for delivery of selective AVN-VS by a subcutaneously implanted nerve stimulator (pulse width 100 micros or 1 ms, frequency 20 or 160 Hz, amplitude 6 to 10 V). Fast-rate right atrial pacing (600 bpm) was used to induce and maintain AF. ECG, blood pressure, and body temperature were monitored telemetrically. One week after the induction of AF, AVN-VS was delivered and maintained for at least 5 weeks. It was found that AVN-VS had a consistent effect on ventricular rate slowing (on average 45+/-13 bpm) over the entire period of observation. Echocardiography showed improvement of cardiac indices with ventricular rate slowing. AVN-VS was well tolerated by the animals, causing no signs of distress or discomfort.

CONCLUSIONS

Beneficial long-term ventricular rate slowing during AF can be achieved by implantation of a nerve stimulator attached to the epicardial AVN fat pad. This novel concept is an attractive alternative to other methods of rate control and may be applicable in a selected group of patients.

Acute ventricular rate control in atrial fibrillation and atrial flutter

Atrioventricular node blocking agents including beta-adrenergic blockers, non-dihydropyridine calcium channel blockers and digoxin are usually effective in controlling ventricular rate in atrial fibrillation and flutter. Intravenous beta-blockers and non-dihydropyridine calcium channel blockers are equally effective in rapidly controlling the ventricular rate. The addition of digoxin to the regimen causes a favorable outcome but digoxin as a single agent is generally less effective in slowing the ventricular rate in acute setting. Clonidine, magnesium, and amiodarone have also been used for acute ventricular rate control in atrial fibrillation. Limited data suggest that combination regimens provide better ventricular rate control than any agent alone. The agent of first choice is usually individualized depending upon the clinical situation. Beta-blockers are preferable in patients with myocardial ischemia, myocardial infarction and hyperthyroidism and in post-operative state, but should be avoided in patients with bronchial asthma and chronic obstructive pulmonary disease where non-dihydropyridine calcium channel blockers are preferred. Beta-blockers are preferred drugs used for acute ventricular rate control in atrial fibrillation during pregnancy. In atrial fibrillation with Wolff-Parkinson-White syndrome, beta-blockers, calcium channel blockers and digoxin should be avoided, as these drugs are selective atrioventricular node blockers without slowing conduction through the accessory pathway, which can lead to increased transmission of impulses preferentially through the accessory pathway and precipitate ventricular fibrillation. The drug of choice for atrial fibrillation in pre-excitation syndrome is procainamide but propafenone, flecainide and disopyramide have also been used. When clinical condition is unstable or patient is hemodynamically compromised, immediate electrical cardioversion is the treatment of choice, as the best measure to control ventricular rate is by conversion to sinus rhythm. Factors precipitating rapid ventricular rate should be treated as well.

Achieving regular slow rhythm during atrial fibrillation without atrioventricular nodal ablation: Selective vagal stimulation plus ventricular pacing

OBJECTIVES

The aim of this study was to achieve regular slow ventricular rhythm during atrial fibrillation (AF) without destroying the AV node (AVN).

BACKGROUND

Recent experimental and clinical studies have demonstrated that selective AVN vagal stimulation (AVN-VS) can be used to slow ventricular rate during AF; however, an irregular rhythm remains. Alternatively, ventricular on-demand (VVI) pacing achieves rate regularization but at rates faster than the already fast intrinsic rate during AF. We hypothesized that AVN-VS combined with VVI pacing would achieve slow, regular rhythm during AF without requiring AVN ablation.

METHODS

AF was induced in eight dogs. AVN-VS was applied to the epicardial fat pad that projects vagal nerve fibers to the AVN. A computer-controlled algorithm adjusted AVN-VS intensity to achieve three levels of mean ventricular RR interval: 75%, 100%, or 125% of the spontaneous sinus cycle length. At each of the three levels, concomitant VVI pacing was delivered at a constant cycle length equal to the corresponding target. Hemodynamic measurements were performed during the study to elucidate the advantages of the proposed method.

RESULTS

AF resulted in rapid, irregular ventricular rates (RR = 287 +/- 36 ms, or 56% of sinus cycle length). AVN-VS achieved average ventricular rate slowing to the three target levels in all dogs (RR increased to 381 +/- 41, 508 +/- 54, and 632 +/- 68 ms, respectively). At each of the three target rate levels, AVN-VS combined with VVI pacing fully eliminated rate irregularities. The regular slow ventricular rhythms during AF were associated with significant hemodynamic improvement.

CONCLUSIONS

A novel approach combining AVN-VS with VVI pacing results in a regular, slow ventricular rhythm during AF that does not necessitate AVN ablation. Rate regularization achieved by this approach was associated with pronounced hemodynamic benefits during AF.

Parasympathetic cardiac nerve stimulation with implanted coronary sinus lead

A patient with drug-refractory paroxysmal atrial fibrillation associated with rapid ventricular rate underwent biatrial pacemaker implantation. During elective replacement of the pacemaker, a significant voltage- and frequency-dependent decrease in ventricular rate was achieved by high-frequency electrical stimulation (17 Hz) of parasympathetic cardiac nerves innervating the AV node with the implanted bipolar coronary sinus electrode. The negative dromotropic effect of parasympathetic stimulation was eliminated by intravenous administration of 1-mg atropine.

Endovascular stimulation of autonomic neural elements in the superior vena cava using a flexible loop catheter

It has been previously shown that parasympathetic nerve stimulation (PNS) can be achieved via basket electrode catheters (BEC) positioned in the superior vena cava (SVC). Since questions have been raised regarding formation of thrombi between and/or on the splines of the BECs, we investigated the use of a flexible loop "Lasso" catheter (LC) to achieve autonomic nerve stimulation in the SVC without clot formation. In 5 dogs, anesthetized with Na-pentobarbital, standard ECG leads II and aVR, blood pressure and right atrial electrograms were continuously monitored. The LC is a 7-French catheter at the end of which is a circular ring, 25 mm in diameter, equipped with ten 1-mm electrodes. The circular loop is made of a flexible, shape retaining, covered metal, which can be straightened in order to be inserted transvenously. The catheter was inserted through a sheath in the external jugular vein and positioned in the SVC. Stimulation was performed sequentially across each of the five bipolar pairs of electrodes, and consisted of square wave stimuli, each 0.1 msec duration, frequency 20 Hz at voltages from 1-40 V. The average voltage required to produce a 50% decrease in heart rate was 15 +/- 7 V, compared to 22 +/- 12 V with the standard BEC and 10 +/- 5 V with a modified BEC. We did not observe any thrombus formation at the end of a four-hour period during which the catheter was stabilized in the SVC. PNS can be achieved safely and effectively by the LC in the SVC in dogs.

Ventricular rate control by selective vagal stimulation is superior to rhythm regularization by atrioventricular nodal ablation and pacing during atrial fibrillation

BACKGROUND

Selective atrioventricular nodal (AVN) vagal stimulation (AVN-VS) has emerged as a novel strategy for ventricular rate (VR) control in atrial fibrillation (AF). Although AVN-VS preserves the physiological ventricular activation sequence, the resulting rate is slow but irregular. In contrast, AVN ablation with pacemaker implantation produces retrograde activation (starting at the apex), with regular ventricular rhythm. We tested the hypothesis that, at comparable levels of VR slowing, AVN-VS provides hemodynamic benefits similar to those of ablation with pacemaker implantation.

METHODS AND RESULTS

AVN-VS was delivered to the epicardial fat pad that projects parasympathetic nerve fibers to the AVN in 12 dogs during AF. A computer-controlled algorithm adjusted AVN-VS beat by beat to achieve a mean ventricular RR interval of 75%, 100%, 125%, or 150% of spontaneous sinus cycle length. The AVN was then ablated, and the right ventricular (RV) apex was paced either irregularly (i-RVP) using the RR intervals collected during AVN-VS or regularly (r-RVP) at the corresponding mean RR. The results indicated that all 3 strategies improved hemodynamics compared with AF. However, AVN-VS resulted in significantly better responses than either r-RVP or i-RVP. i-RVP resulted in worse hemodynamic responses than r-RVP. The differences among these modes became less significant when mean VR was slowed to 150% of sinus cycle length.

CONCLUSIONS

AVN-VS can produce graded slowing of the VR during AF without destroying the AVN. It was hemodynamically superior to AVN ablation with either r-RVP or i-RVP, indicating that the benefits of preserving the physiological antegrade ventricular activation sequence outweigh the detrimental effect of irregularity.

Optimal ventricular rate slowing during atrial fibrillation by feedback AV nodal-selective vagal stimulation

Although the beneficial effects of ventricular rate (VR) slowing during atrial fibrillation (AF) are axiomatic, the precise relationship between VR and hemodynamics has not been determined. We hypothesized that selective atrioventricular node (AVN) vagal stimulation (AVN-VS) by varying the nerve stimulation intensity could achieve precise graded slowing and permit evaluation of an optimal VR during AF. The aims of the present study were the following: 1) to develop a method for computerized vagally controlled VR slowing during AF, 2) to determine the hemodynamic changes at each level of VR slowing, and 3) to establish the optimal anterograde VR during AF. AVN-VS was delivered to the epicardial fat pad that projects parasympathetic nerve fibers to the AVN in 14 dogs. Four target average VR levels, corresponding to 75%, 100%, 125%, and 150% of the sinus cycle length (SCL), were achieved by computer feedback algorithm. VR slowing resulted in improved hemodynamics and polynomial fit analysis found an optimum for the cardiac output at VR slowing of 87% SCL. We conclude that this novel method can be used to maintain slow anterograde conduction with best hemodynamics during AF.

Catheter stimulation of cardiac parasympathetic nerves in humans: a novel approach to the cardiac autonomic nervous system

BACKGROUND

Cardiac parasympathetic nerves run alongside the superior vena cava (SVC) and accumulate particularly epicardially adjacent to the orifice of the coronary sinus (CS). In animals, these nerves can be electrically stimulated inside the SVC or CS, which results in negative chronotropic/dromotropic effects and negative inotropic effects in the atria but not the ventricles. Parasympathetic nerve stimulation (PS) with 20 Hz in the CS, however, also excites the atria, thereby inducing atrial fibrillation. The present study overcomes this limitation by applying high-frequency nerve stimuli within the atrial refractory period. Using this technique, we investigated for the first time whether neurophysiological effects similar to those in animals can be obtained in humans.

METHODS AND RESULTS

In 25 patients, parasympathetic nerves were stimulated via a multipolar electrode catheter placed in the SVC (stimulation with 20 Hz; n=14) or CS (pulsed 200-Hz stimuli; n=11). A significant sinus rate decrease and prolongation of the antegrade Wenckebach period was achieved during PS in the SVC. During PS in the CS, a graded-response prolongation of the antegrade Wenckebach interval was observed with increasing PS voltage until third-degree AV block occurred in 8 of 11 patients. The negative chronotropic/dromotropic effects started and terminated immediately after the onset and termination of PS, respectively. Atropine abolished these effects (n=11).

CONCLUSIONS

Human parasympathetic efferent nerve stimulation induces reversible negative chronotropic and dromotropic effects. PS may serve as an adjunctive tool for the diagnosis/treatment of supraventricular tachycardias and may be beneficial for ventricular rate slowing during tachycardic atrial fibrillation in patients with congestive heart failure.

Selective AV nodal vagal stimulation improves hemodynamics during acute atrial fibrillation in dogs

Although the atrioventricular node (AVN) plays a vital role in blocking many of the atrial impulses from reaching the ventricles during atrial fibrillation (AF), a rapid irregular ventricular rate nevertheless persists. The goals of the present study were to explore the feasibility of novel epicardial selective vagal nerve stimulation for slowing of the ventricular rate during AF and to characterize the hemodynamic benefits in vivo. Electrophysiological-echocardiographic experiments were performed on 11 anesthetized open-chest dogs. Hemodynamic measurements were performed during three distinct periods: 1) sinus rate, 2) AF, and 3) AF with vagal nerve stimulation. AF was associated with significant deterioration of all measured parameters (P < 0.025). The vagal nerve stimulation produced slowing of the ventricular rate, significant reversal of the pressure and contractile indexes (P < 0.025), and a sharp reduction in one-half of the abortive ventricular contractions. The present study provides comprehensive evidence that slowing of the ventricular rate during AF by selective ganglionic stimulation of the vagal nerves that innervate the AVN successfully improved the hemodynamic responses.

Endocardial stimulation of efferent parasympathetic nerves to the atrioventricular node in humans: optimal stimulation sites and the effects of digoxin

The purposes of this study were to identify optimal sites of stimulation of efferent parasympathetic nerve fibers to the human atrioventricular node via an endocardial catheter and to investigate the interaction between digoxin and vagal activation at the end organ.

METHODS

The ventricular rate was measured during atrial fibrillation, prior to and during parasympathetic nerve stimulation, in 8 patients taking digoxin and in 10 controls. High frequency electrical stimuli were delivered via an hexapolar or quadripolar electrode catheter, placed at the posteroseptal right atrium near the atrioventricular node (n=18 patients) or in the coronary sinus (n=12 of 18 patients). In 4 patients, stimulation was repeated after intravenous administration of 1 to 2 mg of atropine.

RESULTS

Nerve stimulation prolonged the R-R interval in all patients. Stimulation close to the posteroseptal right atrium led to maximal atrioventricular nodal slowing. The mean R-R intervals at baseline and during parasympathetic nerve stimulation (60 mA) from the posteroseptal right atrium and the proximal coronary sinus were 581+/-79 ms, 2440+/-466, and 900+/-228 ms respectively (p=0.0001). The response to nerve stimulation was greater in patients taking digoxin than in patients not taking the drug (p=0.02). Junctional rhythm occurred during nerve stimulation in 8/8 patients taking digoxin and 0/10 not taking the drug (p=0.0001). The response to stimulation was eliminated after atropine (p=0.01).

CONCLUSIONS

Parasympathetic nerves to the atrioventricular node were stimulated from the proximal coronary sinus as well as the posteroseptal right atrium. Stimulation at the posteroseptal right atrium resulted in the greatest response, and digoxin enhanced this response. The augmented response suggests that an interaction may exist between parasympathetic stimulation and digoxin at the end organ.

Catheter ablation of cardiac autonomic nerves for prevention of vagal atrial fibrillation

BACKGROUND

Vagal stimulation shortens the atrial effective refractory period (AERP) and maintains atrial fibrillation (AF). This study investigated whether the parasympathetic pathways that innervate the atria can be identified and ablated by use of transvenous catheter stimulation and radiofrequency current catheter ablation (RFCA) techniques.

METHODS AND RESULTS

In 11 dogs, AERPs were determined at 7 atrial sites during bilateral cervical vagal nerve stimulation (VNS) and electrical stimulation of the third fat pad (20 Hz) in the right pulmonary artery (RPA). VNS shortened the AERP at all sites (from 123+/-4 to 39+/-4 ms, P<0.001) and increased the covariance of AERP (COV-AERP) (from 9+/-3% to 27+/-13%, P<0.001). RPA stimulation shortened the AERP at all sites from 123+/-4 to 66+/-13 ms (P<0.001) and increased the COV-AERP from 9+/-3% to 30+/-12% (P<0.001). In 7 dogs, transvascular RFCA of the parasympathetic pathways along the RPA was performed, and in 3 dogs, additional RFCA of parasympathetic fibers along the inferior (n=2) or superior (n=1) vena cava was performed. RFCA blunted the AERP shortening at all sites during VNS (114+/-4 ms after RFCA), abolished the increase of COV-AERP during VNS (12+/-7% after RFCA), and led to an increase of the baseline AERP (123+/-4 ms before versus 127+/-3 ms after RFCA, P=0.002). Before RFCA, AF could be induced and maintained as long as VNS was continued, whereas after RFCA, AF was no longer inducible during VNS.

CONCLUSIONS

-Transvascular atrial parasympathetic nerve system modification by RFCA abolishes vagally mediated AF. This antifibrillatory procedure may provide a foundation for investigating the usefulness of neural ablation in chronic animal models of AF and eventually in patients with AF and high vagal tone.