Background The site of origin and pattern of excitation within the human sinoatrial node (SAN) has not been directly mapped. Objective We hypothesized that the human SAN is functionally insulated from the surrounding atrial myocardium except for several exit pathways which electrically bridge the nodal tissue and atrial myocardium. Methods The SAN was optically mapped in coronary perfused preparations from non-failing human hearts (n=4, age 54±15 years) using dye Di-4-ANBDQBS and Blebbistatin. SAN 3D structure was reconstructed using histology. Results Optical recordings from the SAN had diastolic depolarization and multiple upstroke components, which corresponded to the separate excitations of the SAN and atrial layers. Excitation originated in the middle of the SAN (66±17 BPM), then slowly (1–18 cm/s) and anisotropically spread. After a 82±17 ms conduction delay within the SAN, the atrial myocardium was excited via superior, middle, and/or inferior sinoatrial conduction pathways. Atrial excitation was initiated 9.4±4.2 mm from the leading pacemaker site. The oval 14.3±1.5 × 6.7±1.6 × 1.0±0.2 mm SAN structure was functionally insulated from the atrium by connective tissue, fat, and coronary arteries, except for these pathways. Conclusion These data demonstrated for the first time the location of the leading SAN pacemaker site, the pattern of excitation within the human SAN, and the conduction pathways into the right atrium. The existence of these pathways explained why, even during normal sinus rhythm, atrial breakthroughs could arise from a region parallel to the CT that is significantly larger (26.0±7.8 mm) than the area of the anatomically defined SAN.

INTRODUCTION This study compared the effects of ATP-regulated potassium channel (KATP) openers, diazoxide and pinacidil, on diseased and normal human atria and ventricles. METHODS We optically mapped the endocardium of coronary-perfused right (n=11) or left (n=2) posterior atrial-ventricular free wall preparations from human hearts with congestive heart failure (CHF, n=8) and non-failing human hearts without (NF, n=3) or with (INF, n=2) infarction. We also analyzed the mRNA expression of the KATP targets Kir6.1, Kir6.2, SUR1, and SUR2 in the left atria and ventricles of NF (n=8) and CHF (n=4) hearts. RESULTS In both CHF and INF hearts, diazoxide significantly decreased action potential durations (APDs) in atria (by −21±3% and −27±13%, p<0.01) and ventricles (by −28±7% and −28±4%, p<0.01). Diazoxide did not change APD (0±5%) in NF atria. Pinacidil significantly decreased APDs in both atria (−46 to - 80%, p<0.01) and ventricles (−65 to −93%, p<0.01) in all hearts studied. The effect of pinacidil on APD was significantly higher than that of diazoxide in both atria and ventricles of all groups (p<0.05). During pinacidil perfusion, burst pacing induced flutter/fibrillation in all atrial and ventricular preparations with dominant frequencies of 14.4±6.1 Hz and 17.5 ±5.1 Hz, respectively. Glibenclamide (10 μM) terminated these arrhythmias and restored APDs to control values. Relative mRNA expression levels of KATP targets were correlated to functional observations. CONCLUSION Remodeling in response to CHF and/or previous infarct potentiated diazoxide-induced APD shortening. The activation of atrial and ventricular KATP channels enhances arrhythmogenicity, suggesting that such activation may contribute to reentrant arrhythmias in ischemic hearts.

Background-Numerous studies implicate the sinoatrial node (SAN) as a participant in atrial arrhythmias, including atrial flutter (AFL) and atrial fibrillation (AF). However, the direct role of the SAN has never been described. Methods and Results-The SAN was optically mapped in coronary perfused preparations from normal canine hearts (nϭ17).Optical action potentials were recorded during spontaneous rhythm, overdrive atrial pacing, and AF/AFL induced by acetylcholine (ACh; 0.3 to 3 mol/L) and/or isoproterenol (Iso; 0.2 to 1 mol/L). An optical action potential multiple component algorithm and dominant frequency analysis were used to reconstruct SAN activation and to identify specialized sinoatrial conduction pathways. Both ACh and Iso facilitated pacing-induced AF/AFL by shortening atrial repolarization. The entire SAN structure created a substrate for macroreentry with 9.6Ϯ1.7 Hz (69 episodes in all preparations). Atrial excitation waves could enter the SAN through the sinoatrial conduction pathways and overdrive suppress the node. The sinoatrial conduction pathways acted as a filter for atrial waves by slowing conduction and creating entrance block. ACh/Iso modulated filtering properties of the sinoatrial conduction pathways by increasing/decreasing the degree of the entrance block, respectively. Thus, the SAN could beat independently from AF/AFL reentrant activity during ACh (49Ϯ39%) and ACh/Iso (62Ϯ25%) (Pϭ0.38). Without ACh, the AF/AFL waves captured the SAN and overdrive suppressed it. Spontaneous SAN activity could terminate or convert AFL to AF during cholinergic withdrawal. Conclusions-The specialized structure of the SAN can be a substrate for AF/AFL. Cholinergic stimulation not only can slow sinus rhythm and facilitate AF/AFL but also protects the intrinsic SAN function from the fast AF/AFL rhythm. (Circulation. 2010;122:782-789.)Key Words: acetylcholine Ⅲ atrial fibrillation Ⅲ atrial flutter Ⅲ sinoatrial node Ⅲ mapping Ⅲ isoproterenol B oth atrial flutter (AFL) and atrial fibrillation (AF) are often associated with sinus node dysfunction. 1-3 Sinoatrial node (SAN) structural and functional abnormalities can play an important role in the initiation and maintenance of AF. 1,2,4 -9 On the other hand, the fast AF/AFL rate can lead to SAN dysfunction. 10 -12 However, because of the lack of mapping data from the human SAN, no one has directly shown how the SAN participates in AF or what the SAN activation is during AF. Do fibrillating waves overdrive suppress the SAN, or does the intrinsic SAN activity remain present and even participate in AF 13 ? What role does the autonomic nervous system play in the interactions between SAN and AF reentrant waves 13 ? Although these subjects have been widely discussed, no measurements have been published, leaving these questions unanswered until now. 14 Clinical Perspective on p 789The SAN is a specialized, complex anatomic structure. [15][16][17] Anatomic 16,18 -21 and functional 22,23 studies suggest that the canine SAN is a more realistic model for the human SAN 24...

Background The structure-function relationship in the atrioventricular junction (AVJ) of various animal species has been investigated in detail, however less is known about the human AVJ. In this study, we performed high-resolution optical mapping of the human AVJ (n=6) to define its pacemaker properties and response to autonomic stimulation. Methods and Results Isolated, coronary-perfused AVJ preparations from failing human hearts (n=6, 53±6 years) were optically mapped using the near-infrared, voltage-sensitive dye, di-4-ANBDQBS, with isoproterenol (Iso, 1 μM) and acetylcholine (ACh, 1μM). An algorithm detecting multiple components of optical action potentials was used to reconstruct multi-layered intramural AVJ activation and to identify specialized slow and fast conduction pathways (SP and FP). The anatomical origin and propagation of pacemaker activity was verified via histology. Spontaneous AVJ rhythms of 29±11 bpm (n=6) originated in the nodal-His region (NH, n=3) and/or the proximal His bundle (H, n=4). Iso accelerated the AVJ rhythm to 69±12 bpm (n=5); shifted the leading pacemaker to the transitional cell (TC) regions near the FP and SP (n=4) and/or coronary sinus (n=2); and triggered reentrant arrhythmias (n=2). ACh (n=4) decreased the AVJ rhythm to 18±4 bpm; slowed FP/SP conduction leading to block between the AVJ and atrium; and shifted the pacemaker to either the TC or TC/NH (bifocal activation). Conclusions We have demonstrated that the AVJ pacemaker in failing human hearts is located in the NH or H-regions and can be modified with autonomic stimulation. Moreover, we found that both the FP and SP are involved in anterograde and retrograde conduction.

Background - Pulmonary vein isolation (PVI) is a common catheter ablation technique used to treat atrial fibrillation originating from the pulmonary veins. However, incomplete lesion formation, pulmonary vein reconnection, and collateral damage to the phrenic nerve and esophagus can occur. Electroporation is a new modality to ablate and has the potential for permanent PVI and selective efficacy on cardiac tissue, however strong evidence of selective myocardial injury using electroporation is lacking. Methods - Monolayers of neonatal rat ventricular cardiomyocytes (NRVM), rat cortical neurons (RCN), and esophageal smooth muscle cells (SMC) were stained with propidium iodide to measure shock-induced cell death. Biphasic shocks (10 ms) were delivered from line electrodes (1 mm separation). NRVMs were optically mapped to evaluate post-electroporation electrical conduction. Results - Conduction block occurred when 50-80% of the cells near the electrode were killed, and required 400 & 50 V/cm with the electrodes in contact vs. 690 & 70 V/cm with the electrodes 1 mm above the cells (p<0.01). For 400 V/cm shocks applied in contact with cells, NRVM cultures yielded the highest degree of cell death (~60%) compared to RCN (~40%) and SMCs (~20%). When the electrode was raised 1 mm SMCs were nearly unaffected by the shock. Conclusions - Cell type alone yielded selective efficacy to electroporation without the confounding influences present in clinical studies, but electrode proximity to the target tissue remains important for efficacy. This exciting result suggests that electroporation may be a more selective modality for PVI.

Background The mechanisms of the electrocardiographic changes and arrhythmias in Brugada syndrome (BrS) remain controversial. Mutations in the sodium channel gene, SCN5A and regulatory proteins that reduce or eliminate sodium current (INa) have been linked to BrS. We studied the properties of a BS-associated SCN5A mutation in a protein kinase A (PKA) consensus phosphorylation site, R526H. Methods and Results In-vitro PKA phosphorylation was detected in the I-II linker peptide of wild type (WT) channels but not R526H or S528A (phosphorylation site) mutants. Cell surface expression of R526H and S528A channels were reduced compared with WT. Whole-cell INa through all channel variants revealed no significant differences in the steady-state activation, inactivation and recovery from inactivation. Peak current densities of the mutants were significantly reduced compared to WT. Infection of 2D cultures of neonatal rat ventricular myocytes with WT and mutant channels increased conduction velocity (CV) compared with non-infected cells. PKA stimulation significantly increased peak INa and CV of WT but not mutant channels. Oxidant stress inhibits cardiac Na currents; WT and mutant INa decreases with the intracellular application of NADH, an effect that is reversed by PKA stimulation in WT but not in R526H or S528A channels. Conclusions We identified a family with BrS and an SCN5A mutation in a PKA consensus phosphorylation site. The BrS mutation R526H is associated with a reduction in the basal level of INa and a failure of PKA stimulation to augment the current which may contribute to the predisposition to arrhythmias in patients with BrS, independent of the precipitants.

A major consideration in the design of engineered cardiac tissues for the faithful representation of physiological behavior is the recapitulation of the complex topography and biochemistry of native tissue. In this study we present engineered heart slices (EHS), which seed neonatal rat ventricular cells (NRVCs) onto thin slices of decellularized cardiac tissue that retain important aspects of native extracellular matrix (ECM). To form EHS, rat or pig ventricular tissue was sectioned into 300 µm-thick, 5 to 16 mm-diameter disks, which were subsequently decellularized using detergents, spread on coverslips, and seeded with NRVCs. The organized fiber structure of the ECM remained after decellularization and promoted cell elongation and alignment, resulting in an anisotropic, functional tissue that could be electrically paced. Contraction decreased at higher pacing rates, and optical mapping revealed electrical conduction that was anisotropic with a ratio of approximately 2.0, rate-dependent shortening of the action potential and slowing of conduction, and lidocaine (sodium channel blocker)-induced slowing of conduction. Reentrant arrhythmias could also be pace-induced and terminated. EHS constitute an attractive in vitro cardiac tissue in which cardiac cells are cultured on thin slices of decellularized cardiac ECM that provide important biochemical, structural, and mechanical cues absent in traditional cell cultures.

BACKGROUND-Defibrillation shock is known to induce atrial stunning, which is electrical and mechanical dysfunction.