Interactive atrial neural network: Determining the connections between ganglionated plexi

Autonomic mechanism for initiation of rapid firing from atria and pulmonary veins: evidence by ablation of ganglionated plexi

AIMS

Previous studies showed that autonomic activation by high-frequency electrical stimulation (HFS) during myocardial refractoriness evokes rapid firing from pulmonary vein (PV) and atria, both in vitro and in vivo. This study sought to investigate the autonomic mechanism underlying the rapid firings at various sites by systematic ablation of multiple ganglionated plexi (GP).

METHODS AND RESULTS

In 43 mongrel dogs, rapid firing-mediated atrial fibrillation (AF) was induced by local HFS (200 Hz, impulse duration 0.1 ms, train duration 40 ms) to the PVs and atria during myocardial refractoriness. The main GP in the atrial fat pads or the ganglia along the ligament of Marshall (LOM) were then ablated. Ablation of the anterior right GP and inferior right GP significantly increased the AF threshold by HFS at the right atrium and PVs. The AF threshold at left atrium and PVs was significantly increased by ablation of the superior left GP and inferior left GP, and was further increased by ablation of the LOM. Ablation of left- or right-sided GP on the atria had a significant effect on contralateral PVs and atrium. Administration of esmolol (1 mg/kg) or atropine (1 mg) significantly increased AF threshold at all sites.

CONCLUSION

HFS applied to local atrial and PV sites initiated rapid firing via activation of the interactive autonomic network in the heart. GP in either left side or right side contributes to the rapid firings and AF originating from ipsolateral and contralateral PVs and atrium. Autonomic denervation suppresses or eliminates those rapid firings.

Autonomic elements within the ligament of Marshall and inferior left ganglionated plexus mediate functions of the atrial neural network

OBJECTIVES

We sought to systematically investigate the role of the ligament of Marshall (LOM) and inferior left ganglionated plexi (ILGP) in modulating electrophysiological functions.

METHODS

The following structures were exposed in 36 dogs: (1) LOM, (2) superior left GP (SLGP) near the junction of left superior pulmonary vein (LSPV) and left atrium, (3) ILGP near the left inferior pulmonary vein-atrial junction, (4) anterior right GP (ARGP) near the sino-atrial node, and (5) inferior right GP (IRGP) at the junction of inferior vena cava and atria. High frequency stimulation (HFS; 0.6-8.0 V, 20 Hz, 0.1 msec in duration) was applied to the LOM, SLGP, ILGP, ARGP, IRGP, or vagosympathetic trunk. Ventricular rate (VR) during atrial fibrillation (AF) was compared before and after ablation of GP in different sequences.

RESULTS

ARGP + ILGP ablation but not ARGP ablation alone eliminated the VR slowing response induced by LOM stimulation, suggesting that all the autonomic innervation from the LOM to AV node passes the ILGP. LOM ablation attenuated the VR slowing response caused by SLGP or left vagosympathetic stimulation, suggesting that LOM modulates the autonomic innervation between the AV node and the left vagosympathetic trunk or SLGP. ARGP attenuated while ARGP + ILGP ablation eliminated the VR slowing response induced by left vagosympathetic stimulation, suggesting that both ARGP and ILGP modulate the AV nodal innervation of the extrinsic and intrinsic cardiac autonomic nervous system (ANS).

CONCLUSION

The LOM and ILGP function as the "integration centers" that modulate the autonomic interactions between extrinsic and intrinsic cardiac ANS on AV nodal function.

Spatially divergent cardiac responses to nicotinic stimulation of ganglionated plexus neurons in the canine heart

Ganglionated plexuses (GPs) are major constituents of the intrinsic cardiac nervous system, the final common integrator of regional cardiac control. We hypothesized that nicotinic stimulation of individual GPs exerts divergent regional influences, affecting atrial as well as ventricular functions. In 22 anesthetized canines, unipolar electrograms were recorded from 127 atrial and 127 ventricular epicardial loci during nicotine injection (100 mcg in 0.1 ml) into either the 1) right atrial (RA), 2) dorsal atrial, 3) left atrial, 4) inferior vena cava-inferior left atrial, 5) right ventricular, 6) ventral septal ventricular or 7) cranial medial ventricular (CMV) GP. In addition to sinus and AV nodal function, neural effects on atrial and ventricular repolarization were identified as changes in the area subtended by unipolar recordings under basal conditions and at maximum neurally-induced effects. Animals were studied with intact AV node or following ablation to achieve ventricular rate control. Atrial rate was affected in response to stimulation of all 7 GPs with an incidence of 50-95% of the animals among the different GPs. AV conduction was affected following stimulation of 6/7 GP with an incidence of 22-75% among GPs. Atrial and ventricular repolarization properties were affected by atrial as well as ventricular GP stimulation. Distinct regional patterns of repolarization changes were identified in response to stimulation of individual GPs. RAGP predominantly affected the RA and posterior right ventricular walls whereas CMVGP elicited biatrial and biventricular repolarization changes. Spatially divergent and overlapping cardiac regions are affected in response to nicotinic stimulation of neurons in individual GPs.

Post-operative atrial fibrillation management by selective epicardial vagal fat pad stimulation

PURPOSE

Post-operative atrial fibrillation (POAF) is a common complication after cardiac surgery and often leads to poorly tolerated fast ventricular rates. Negative dromotropic drugs are not always effective and may not be well tolerated in heart failure patients. Aim of this study is to verify if high-frequency stimulation of the right inferior fat pad (RIFPS) allows an effective decrease in ventricular rate (VR) during POAF.

METHODS

We enrolled 32 consecutive patients submitted to bypass; during surgery, a temporary heart wire was implanted in a site where RIFPS evoked a functional AV block. During POAF, RIFPS was delivered from the heart wire to decrease VR.

RESULTS

Intra-operative RIFPS evoked complete AV block in 29 patients (91%). Fourteen patients (44%) developed POAF (mean VR 127 +/- 12 bpm). In these patients, RIFPS achieved a 25% reduction of VR and complete AV block with 6.0 +/- 1.9 and 7.5 +/- 1.8 V (duration 0.2 ms, frequency 50 Hz), respectively.

CONCLUSION

Epicardial RIFPS represents an effective and feasible technique to decrease VR during POAF.

Atrial fibrillation begets atrial fibrillation: autonomic mechanism for atrial electrical remodeling induced by short-term rapid atrial pacing

BACKGROUND

The mechanism(s) for acute changes in electrophysiological properties of the atria during rapid pacing induced atrial fibrillation (AF) is not completely understood. We sought to evaluate the contribution of the intrinsic cardiac autonomic nervous system in acute atrial electrical remodeling and AF induced by 6-hour rapid atrial pacing.

METHODS AND RESULTS

Continuous rapid pacing (1200 bpm, 2x threshold [TH]) was performed at the left atrial appendage. Group 1 (n=7) underwent 6-hour pacing immediately followed by ganglionated plexi (GP) ablation; group 2 (n=7) underwent GP ablation immediately followed by 6-hour pacing; and group 3 (n=4) underwent administration of autonomic blockers, atropine (1 mg/kg), and propranolol (0.6 mg/kg) immediately followed by 6-hour pacing. The effective refractory period (ERP) and window of vulnerability (WOV, in milliseconds), ie, the difference between the longest and the shortest coupling interval of the premature stimulus that induced AF, were measured at 2xTH and 10xTH at the left atrium, right atrium, and pulmonary veins every hour before and after GP ablation or autonomic blockade. In group 1, ERP was markedly shortened in the first 2 hours and then stabilized both at 2xTH and 10xTH; however, WOV was progressively widened throughout the 6-hour period. After GP ablation, ERP was significantly longer than before ablation and AF could not be induced (WOV=0) at either 2xTH or 10xTH. In groups 2 and 3, rapid atrial pacing failed to shorten the ERP. AF could not be induced in 6 of 7 dogs in group 2 and all 4 dogs in group 3 during the 6-hour pacing period.

CONCLUSIONS

The intrinsic cardiac autonomic nervous system plays a crucial role in the acute stages of atrial electrical remodeling induced by rapid atrial pacing.

Pulmonary vein region ablation in experimental vagal atrial fibrillation: role of pulmonary veins versus autonomic ganglia

BACKGROUND

Pulmonary vein (PV) -encircling radiofrequency ablation frequently is effective in vagal atrial fibrillation (AF), and there is evidence that PVs may be particularly prone to cholinergically induced arrhythmia mechanisms. However, PV ablation procedures also can affect intracardiac autonomic ganglia. The present study examined the relative role of PVs versus peri-PV autonomic ganglia in an experimental vagal AF model.

METHODS AND RESULTS

Cholinergic AF was studied under carbachol infusion in coronary perfused canine left atrial PV preparations in vitro and with cervical vagal stimulation in vivo. Carbachol caused dose-dependent AF promotion in vitro, which was not affected by excision of all PVs. Sustained AF could be induced easily in all dogs during vagal nerve stimulation in vivo both before and after isolation of all PVs with encircling lesions created by a bipolar radiofrequency ablation clamp device. PV elimination had no effect on atrial effective refractory period or its responses to cholinergic stimulation. Autonomic ganglia were identified by bradycardic and/or tachycardic responses to high-frequency subthreshold local stimulation. Ablation of the autonomic ganglia overlying all PV ostia suppressed the effective refractory period-abbreviating and AF-promoting effects of cervical vagal stimulation, whereas ablation of only left- or right-sided PV ostial ganglia failed to suppress AF. Dominant-frequency analysis suggested that the success of ablation in suppressing vagal AF depended on the elimination of high-frequency driver regions.

CONCLUSIONS

Intact PVs are not needed for maintenance of experimental cholinergic AF. Ablation of the autonomic ganglia at the base of the PVs suppresses vagal responses and may contribute to the effectiveness of PV-directed ablation procedures in vagal AF.

How Does Successful Off-Pump Pulmonary Vein Isolation for Paroxysmal Atrial Fibrillation Influence Heart Rate Variability and Autonomic Activity?

Objective

Surgical pulmonary vein isolation (PVI) for paroxysmal atrial fibrillation (PAF) blocks trigger stimulation from PVs and partially disconnects the atria from sympathetic and parasympathetic neural stimulation. This study describes long-term changes in heart rate variability (HRV) and autonomic activity (AA) after successful bipolar radiofrequency PVI.

Methods

Twenty-seven patients who underwent coronary artery bypass grafting and successful (defined as stable sinus rhythm for 1 year) off-pump bipolar radiofrequency PVI for PAF were prospectively followed 3, 6, and 12 months after surgery including 24 hours Holter electrocardiogram. The following HRV and AA parameters were calculated: mean NN-interval, SD of NN-intervals, SD of averaged NN-intervals, root mean square of successive differences, low frequency (LF) power (0.04–0.15 Hz; a parameter specific for sympathetic activity), high frequency (HF) power (0.15–0.4 Hz; a parameter specific for parasympathetic activity), and the LF:HF ratio.

Results

Preoperatively, high HRV and AA parameters were recorded. In 3-, 6-, and 12-month time, a progressive reduction of HRV and AA was observed, reaching significance after 12 months. Respective rates before surgery and 12 months after it were: for SD of averaged NN-intervals (122.4 ± 113; 80.5 ± 42 milliseconds; P = 0.046), for root mean square of successive differences (79.2 ± 93; 45 ± 20 milliseconds; P = 0.04). The LF:HF ratios were 1.22 and 0.73 before and 12 months after surgery, respectively. The statistically significant continuous reduction in LF:HF ratio (P = 0.02) is suggestive of a progressive parasympathetic dominance 12 months after surgery.

Conclusions

Successful PVI for PAF results in HRV and sympathetic activity reduction with preoperative sympathetic dominance and oncoming vagal dominance after 1 year from surgery. Despite preoperative sympathetic dominance, successful PVI for PAF results in HRV and a reduction in sympathetic activity with emerging parasympathetic dominance 12 months after surgery.

Neural substrate for atrial fibrillation: implications for targeted parasympathetic blockade in the posterior left atrium

The parasympathetic (P) nervous system is thought to contribute significantly to focal atrial fibrillation (AF). Thus we hypothesized that P nerve fibers [and related muscarinic (M2) receptors] are preferentially located in the posterior left atrium (PLA) and that selective cholinergic blockade in the PLA can be successfully performed to alter vagal AF substrate. The PLA, pulmonary veins (PVs), and left atrial appendage (LAA) from six dogs were immunostained for sympathetic (S) nerves, P nerves, and M2 receptors. Epicardial electrophysiological mapping was performed in seven additional dogs. The PLA was the most richly innervated, with nerve bundles containing P and S fibers (0.9 ± 1, 3.2 ± 2.5, and 0.17 ± 0.3/cm2 in the PV, PLA, and LAA, respectively, P < 0.001); nerve bundles were located in fibrofatty tissue as well as in surrounding myocardium. P fibers predominated over S fibers within bundles (P-to-S ratio = 4.4, 7.2, and 5.8 in PV, PLA, and LAA, respectively). M2 distribution was also most pronounced in the PLA (17.8 ± 8.3, 14.3 ± 7.3, and 14.5 ± 8 M2-stained cells/cm2 in the PLA, PV, and LAA, respectively, P = 0.012). Left cervical vagal stimulation (VS) caused significant effective refractory period shortening in all regions, with easily inducible AF. Topical application of 1% tropicamide to the PLA significantly attenuated VS-induced effective refractory period shortening in the PLA, PV, and LAA and decreased AF inducibility by 92% ( P < 0.001). We conclude that 1) P fibers and M2 receptors are preferentially located in the PLA, suggesting an important role for this region in creation of vagal AF substrate and 2) targeted P blockade in the PLA is feasible and results in attenuation of vagal responses in the entire left atrium and, consequently, a change in AF substrate.

Autonomic mechanism to explain complex fractionated atrial electrograms (CFAE)

OBJECTIVE

To simulate complex fractionated atrial electrograms (CFAE) during sustained atrial fibrillation (AF) in experimental animals.

BACKGROUND

The mechanism(s) underlying CFAE has not been fully elucidated.

METHODS

Twenty-two dogs were subjected to a right and/or left thoracotomy. A gauze patch soaked with acetylcholine (ACh) was placed on the right atrial appendage (RAA) to induce sustained AF. During AF, varying concentrations of ACh (1, 10, 100 mM) were "painted" on the RA where electrograms showed regular organized activity. In another six dogs, anterior right ganglionated plexi (ARGP) near the sino-atrial node and inferior right GP (IRGP) at the junction of inferior vena cava and atria were sequentially ablated. In five dogs, ACh was injected into ARGP to induce CFAE.

RESULTS

During sustained AF, local "painting" with ACh 1 mM and 10 mM induced intermittent CFAE in 1 of 11 and 10 of 11 dogs, respectively. With 100 mM ACh, all 11 showed CFAE (two intermittent, nine continuous). In six other dogs, continuous CFAE induced by topical application of 100 mM ACh were markedly attenuated by ARGP + IRGP ablation. In another five of five dogs, ACh injection into ARGP induced a gradient of CFAE with the continuous CFAE always occurring near the ARGP and CFAE also occurring at left pulmonary vein-atrial junctions. During ARGP ablation, AF was terminated in all five dogs immediately after regularization of the rotor-like electrograms or continuous CFAE.

CONCLUSIONS

This study demonstrates an autonomic basis for CFAE formation, suggesting that graded hyperactive states of the autonomic nervous system (ANS) may induce various types of CFAE observed clinically.

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.