Mitsunori Maruyama

Continuous Low-Level Vagus Nerve Stimulation Reduces Stellate Ganglion Nerve Activity and Paroxysmal Atrial Tachyarrhythmias in Ambulatory Canines

Background— We hypothesize that left-sided low-level vagus nerve stimulation (LL-VNS) can suppress sympathetic outflow and reduce atrial tachyarrhythmias in ambulatory dogs. Methods and Results— We implanted a neurostimulator in 12 dogs to stimulate the left cervical vagus nerve and a radiotransmitter for continuous recording of left stellate ganglion nerve activity, vagal nerve activities, and ECGs. Group 1 dogs (N=6) underwent 1 week of continuous LL-VNS. Group 2 dogs (N=6) underwent intermittent rapid atrial pacing followed by active or sham LL-VNS on alternate weeks. Integrated stellate ganglion nerve activity was significantly reduced during LL-VNS (7.8 mV/s; 95% confidence interval [CI] 6.94 to 8.66 versus 9.4 mV/s [95% CI, 8.5 to 10.3] at baseline; P=0.033) in group 1. The reduction was most apparent at 8 AM, along with a significantly reduced heart rate (P=0.008). Left-sided low-level vagus nerve stimulation did not change vagal nerve activity. The density of tyrosine hydroxylase–positive nerves in the left stellate ganglion 1 week after cessation of LL-VNS were 99 684 &mgr;m2/mm2 (95% CI, 28 850 to 170 517) in LL-VNS dogs and 186 561 &mgr;m2/mm2 (95% CI, 154 956 to 218 166; P=0.008) in normal dogs. In group 2, the frequencies of paroxysmal atrial fibrillation and tachycardia during active LL-VNS were 1.4/d (95% CI, 0.5 to 5.1) and 8.0/d (95% CI, 5.3 to 12.0), respectively, significantly lower than during sham stimulation (9.2/d [95% CI, 5.3 to 13.1]; P=0.001 and 22.0/d [95% CI, 19.1 to 25.5], P<0.001, respectively). Conclusions— Left-sided low-level vagus nerve stimulation suppresses stellate ganglion nerve activities and reduces the incidences of paroxysmal atrial tachyarrhythmias in ambulatory dogs. Significant neural remodeling of the left stellate ganglion is evident 1 week after cessation of continuous LL-VNS.

Small-Conductance Calcium-Activated Potassium Channel and Recurrent Ventricular Fibrillation in Failing Rabbit Ventricles

Rationale: Fibrillation/defibrillation episodes in failing ventricles may be followed by action potential duration (APD) shortening and recurrent spontaneous ventricular fibrillation (SVF). Objective: We hypothesized that activation of apamin-sensitive small-conductance Ca2+-activated K+ (SK) channels is responsible for the postshock APD shortening in failing ventricles. Methods and Results: A rabbit model of tachycardia-induced heart failure was used. Simultaneous optical mapping of intracellular Ca2+ and membrane potential (Vm) was performed in failing and nonfailing ventricles. Three failing ventricles developed SVF (SVF group); 9 did not (no-SVF group). None of the 10 nonfailing ventricles developed SVF. Increased pacing rate and duration augmented the magnitude of APD shortening. Apamin (1 &mgr;mol/L) eliminated recurrent SVF and increased postshock APD80 in the SVF group from 126±5 to 153±4 ms (P<0.05) and from 147±2 to 162±3 ms (P<0.05) in the no-SVF group but did not change APD80 in nonfailing group. Whole cell patch-clamp studies at 36°C showed that the apamin-sensitive K+ current (IKAS) density was significantly larger in the failing than in the normal ventricular epicardial myocytes, and epicardial IKAS density was significantly higher than midmyocardial and endocardial myocytes. Steady-state Ca2+ response of IKAS was leftward-shifted in the failing cells compared with the normal control cells, indicating increased Ca2+ sensitivity of IKAS in failing ventricles. The Kd was 232±5 nmol/L for failing myocytes and 553±78 nmol/L for normal myocytes (P=0.002). Conclusions: Heart failure heterogeneously increases the sensitivity of IKAS to intracellular Ca2+, leading to upregulation of IKAS, postshock APD shortening, and recurrent SVF.

Intracellular Calcium Dynamics and Acceleration of Sinus Rhythm by β-Adrenergic Stimulation

Background— Recent evidence indicates that membrane voltage and Ca2+ clocks jointly regulate sinoatrial node (SAN) automaticity. Here we test the hypothesis that sinus rate acceleration by &bgr;-adrenergic stimulation involves synergistic interactions between these clock mechanisms. Methods and Results— We simultaneously mapped intracellular calcium (Cai) and membrane potential in 25 isolated canine right atrium, using previously described criteria of the timing of late diastolic Cai elevation (LDCAE) relative to the action potential upstroke to detect the Ca2+ clock. Before isoproterenol, the earliest pacemaking site occurred in the inferior SAN, and LDCAE was observed in only 4 of 25 preparations. Isoproterenol infusion (1 &mgr;mol/L) increased sinus rate and shifted pacemaking site to superior SAN, concomitant with the appearance of LDCAE preceding the action potential upstroke by 98±31 ms. Caffeine had similar effects, whereas sarcoplasmic reticulum Ca2+ depletion with ryanodine and thapsigargin prevented isoproterenol-induced LDCAE and blunted sinus rate acceleration. Cai transient relaxation time during isoproterenol was shorter in superior SAN (124±34 ms) than inferior SAN (138±24 ms; P=0.01) or right atrium (164±33 ms; P=0.001) and was associated with a lower sarcoplasmic reticulum Ca2+ ATPase pump to phospholamban protein ratio in SAN than in right atrium. Hyperpolarization-activated pacemaker current (If) blockade with ZD 7288 modestly blunted but did not prevent LDCAE or sinus rate acceleration by isoproterenol. Conclusions— Acceleration of the Ca2+ clock in the superior SAN plays an important role in sinus acceleration during &bgr;-adrenergic stimulation, interacting synergistically with the voltage clock to increase sinus rate.

Diastolic Intracellular Calcium-Membrane Voltage Coupling Gain and Postshock Arrhythmias Role of Purkinje Fibers and Triggered Activity

Rationale: Recurrent ventricular arrhythmias after initial successful defibrillation are associated with poor clinical outcome. Objective: We tested the hypothesis that postshock arrhythmias occur because of spontaneous sarcoplasmic reticulum Ca release, delayed afterdepolarization (DAD), and triggered activity (TA) from tissues with high sensitivity of resting membrane voltage (Vm) to elevated intracellular calcium (Cai) (high diastolic Cai–voltage coupling gains). Methods and Results: We simultaneously mapped Cai and Vm on epicardial (n=14) or endocardial (n=14) surfaces of Langendorff-perfused rabbit ventricles. Spontaneous Cai elevation (SCaE) was noted after defibrillation in 32% of ventricular tachycardia/ventricular fibrillation at baseline and in 81% during isoproterenol infusion (0.01 to 1 &mgr;mol/L). SCaE was reproducibly induced by rapid ventricular pacing and inhibited by 3 &mgr;mol/L of ryanodine. The SCaE amplitude and slope increased with increasing pacing rate, duration, and dose of isoproterenol. We found TAs originating from 6 of 14 endocardial surfaces but none from epicardial surfaces, despite similar amplitudes and slopes of SCaEs between epicardial and endocardial surfaces. This was because DADs were larger on endocardial surfaces as a result of higher diastolic Cai–voltage coupling gain, compared to those of epicardial surfaces. Purkinje-like potentials preceded TAs in all hearts studied (n=7). IK1 suppression with CsCl (5 mmol/L, n=3), BaCl2 (3 &mgr;mol/L, n=3), and low extracellular potassium (1 mmol/L, n=2) enhanced diastolic Cai–voltage coupling gain and enabled epicardium to also generate TAs. Conclusions: Higher diastolic Cai–voltage coupling gain is essential for genesis of TAs and may underlie postshock arrhythmias arising from Purkinje fibers. IK1 is a major factor that determines the diastolic Cai–voltage coupling gain.

Demonstration of the Reentrant Circuit of Verapamil-Sensitive Idiopathic Left Ventricular Tachycardia: Direct Evidence for Macroreentry as the Underlying Mechanism

Reentrant Circuit of Idiopathic LV Tachycardia. The exact reentrant circuit of verapamilsensitive idiopathic left ventricular tachycardia (ILVT) remains unclear. This case report demonstrates the reentrant circuit of ILVT. A 20‐pole electrode catheter was placed along the left posterior fascicle during electrophysiologic study. ILVT was reproducibly induced by programmed ventricular stimulation. During the tachycardia, sequential diastolic potentials bridging the entire diastolic period were observed in the recordings from the electrodes positioned from left ventricular mid‐septum to inferoapical septum. The slow conduction zone appeared to be composed of a false tendon in this patient. Entrainment of the ILVT from the right ventricular outflow tract at a different pacing cycle length revealed that a dominant conduction delay occurred at the proximal site of the slow conduction zone. Entrainment studies from several sites on the left ventricular septum confirmed that these sites where sequential electrical activity was recorded were included within the reentrant circuit. However, the left posterior fascicle itself seemed to be a bystander. This report provides the direct evidence of macroreentry as the underlying mechanism of this ILVT, adjacent to the left posterior fascicle.

Electroanatomic Remodeling of the Left Stellate Ganglion After Myocardial Infarction

OBJECTIVES The purpose of this study was to evaluate the changes of left stellate ganglionic nerve activity (SGNA) and left thoracic vagal nerve activity (VNA) after acute myocardial infarction (MI). BACKGROUND Whether MI results in remodeling of extracardiac nerve activity remains unclear. METHODS We implanted radiotransmitters to record the SGNA, VNA, and electrocardiogram in 9 ambulatory dogs. After baseline monitoring, MI was created by 1-h balloon occlusion of the coronary arteries. The dogs were then continuously monitored for 2 months. Both stellate ganglia were stained for growth-associated protein 43 and synaptophysin. The stellate ganglia from 5 normal dogs were used as control. RESULTS MI increased 24-h integrated SGNA from 7.44 ± 7.19 Ln(Vs)/day at baseline to 8.09 ± 7.75 Ln(Vs)/day after the MI (p < 0.05). The 24-h integrated VNA before and after the MI was 5.29 ± 5.04 Ln(Vs)/day and 5.58 ± 5.15 Ln(Vs)/day, respectively (p < 0.05). A significant 24-h circadian variation was noted for the SGNA (p < 0.05) but not the VNA. The SGNA/VNA ratio also showed significant circadian variation. The nerve densities from the left SG were 63,218 ± 34,719 μm(2)/mm(2) and 20,623 ± 4,926 μm(2)/mm(2) for growth-associated protein 43 (p < 0.05) and were 32,116 ± 8,190 μm(2)/mm(2)and 16,326 ± 4,679 μm(2)/mm(2) for synaptophysin (p < 0.05) in MI and control groups, respectively. The right SG also showed increased nerve density after MI (p < 0.05). CONCLUSIONS MI results in persistent increase in the synaptic density of bilateral stellate ganglia and is associated with increased SGNA and VNA. There is a circadian variation of the SGNA/VNA ratio. These data indicate significant remodeling of the extracardiac autonomic nerve activity and structures after MI.

Genesis of phase 3 early afterdepolarizations and triggered activity in acquired long-QT syndrome.

Background—Both phase 2 and phase 3 early afterdepolarizations (EADs) occur in long-QT syndromes, but their respective roles in generating arrhythmias in intact cardiac tissue are incompletely understood. Methods and Results—Intracellular Ca (Cai) and membrane voltage (Vm) were optically mapped in a quasi 2-dimensional model of cryoablated Langendorff-perfused rabbit ventricles (n=16). E-4031 (an IKr blocker) combined with reduced extracellular K ([K+]o) and Mg ([Mg2+]o) prolonged action potential duration heterogeneously and induced phase 2 and phase 3 EADs. Whereas phase 2 EADs were Cai-dependent, phase 3 EADs were not. The origins of 47 triggered activity episodes were attributed to phase 2 EADs in 12 episodes (26%) and phase 3 EADs in 35 episodes (74%). When phase 2 EADs accompanied phase 3 EADs, they accentuated action potential duration heterogeneity, creating a large Vm gradient across the boundary between long and short action potential duration regions from which triggered activity emerged. The amplitude of phase 3 EADs correlated with the Vm gradient (r=0.898, P<0.001). Computer simulation studies showed that coupling of cells with heterogeneous repolarization could extrinsically generate phase 3 EADs via electrotonic current flow. Alternatively, reduced IK1 caused by low [K+]o could generate intrinsic phase 3 EADs capable of inducing triggered activity at the boundary zone. Conclusions—Phase 3 EADs can be extrinsic as the result of electrotonic current across steep repolarization gradients or intrinsic as the result of low IK1 and do not require spontaneous sarcoplasmic reticulum Ca release. Reduction of IK1 by low [K+]o strongly promotes ventricular arrhythmias mediated by phase 3 EADs in acquired long-QT syndrome caused by IKr blockade.

Osborn Waves Associated with Ventricular Fibrillation in a Patient with Vasospastic Angina

Osborn Waves in Ischemic Heart Disease. A 52‐year‐old man with a history of vasospastic angina experienced a severe ischemic episode accompanied by a non‐Q wave myocardial infarction. Two episodes of ventricular fibrillation (VF) occurred during the acute phase of the event. Osborn waves were observed on the ECG preceding each episode of VF. In the second episode, the gradual development of Osborn waves until VF occurred was confirmed on ECG. The Osborn waves appeared to be related to the occurrence of VF. This case may provide clinical evidence that a prominent Ito participates in the development of VF during myocardial ischemia.

Restrictive loss of Plakoglobin in cardiomyocytes leads to Arrhythmogenic Cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inheritable myocardial disorder associated with fibrofatty replacement of myocardium and ventricular arrhythmia. A subset of ARVC is categorized as Naxos disease, which is characterized by ARVC and a cutaneous disorder. A homozygous loss-of-function mutation of the Plakoglobin (Jup) gene, which encodes a major component of the desmosome and the adherens junction, had been identified in Naxos patients, although the underlying mechanism remained elusive. We generated Jup mutant mice by ablating Jup in cardiomyocytes. Jup mutant mice largely recapitulated the clinical manifestation of human ARVC: ventricular dilation and aneurysm, cardiac fibrosis, cardiac dysfunction and spontaneous ventricular arrhythmias. Ultra-structural analyses revealed that desmosomes were absent in Jup mutant myocardia, whereas adherens junctions and gap junctions were preserved. We found that ventricular arrhythmias were associated with progressive cardiomyopathy and fibrosis in Jup mutant hearts. Massive cell death contributed to the cardiomyocyte dropout in Jup mutant hearts. Despite the increase of β-catenin at adherens junctions in Jup mutant cardiomyoicytes, the Wnt/β-catenin-mediated signaling was not altered. Transforming growth factor-beta-mediated signaling was found significantly elevated in Jup mutant cardiomyocytes at the early stage of cardiomyopathy, suggesting an important pathogenic pathway for Jup-related ARVC. These findings have provided further insights for the pathogenesis of ARVC and potential therapeutic interventions.

Patterns of baseline autonomic nerve activity and the development of pacing-induced sustained atrial fibrillation

BACKGROUND Whether autonomic nerve activity is important in the development of pacing-induced sustained atrial fibrillation (AF) is unclear. OBJECTIVE The purpose of this study was to test the hypothesis that patterns of baseline autonomic nerve activity are important in the development of pacing-induced sustained AF. METHODS Radiotransmitters were implanted in 12 ambulatory dogs to record left stellate ganglion nerve activity (SGNA) and vagal nerve activity (VNA). Sustained (>48 hours) AF was induced with intermittent rapid atrial pacing. RESULTS At baseline (before pacing), 1-minute integrated nerve activity between SGNA and VNA demonstrated either a single linear relationship with excellent correlation (group 1, N = 3, r = 0.816 ± 0.105) or nonlinear relationships with poor correlation (group 2, N = 9, r = 0.316 ± 0.162, P <.05 vs group 1). Group 1 dogs had higher VNA (97.0 ± 11.5 mV-s) compared to group 2 (33.4 ± 21.7 mV-s, P <.001). Group 1 dogs had more frequent sympathovagal co-activation episodes than did group 2 (50 ± 19 per day vs 15 ± 6 per day, P <.05) and more paroxysmal atrial tachycardia (PAT; 5 ± 1 per day vs 2 ± 1 per day, P <.05) at baseline. Sustained AF occurred after 16 ± 4 days (range 13-20 days) of pacing in group 1 and after 46 ± 18 days (range 23-72 days) of pacing in group 2 (P <.05). In the week before development of sustained AF, VNA of group 2 dogs was significantly increased compared to baseline (P <.05). CONCLUSION Ambulatory dogs with good linear sympathovagal correlation and higher vagal tone at baseline have more PAT episodes at baseline and faster induction of sustained AF by rapid pacing. Rapid atrial pacing increased the VNA of the remaining dogs before induction of sustained AF.