Abstract-Previous studies have postulated an important role for the inwardly rectifying potassium current (I K1 ) in controlling the dynamics of electrophysiological spiral waves responsible for ventricular tachycardia and fibrillation. In this study, we developed a novel tissue model of cultured neonatal rat ventricular myocytes (NRVMs) with uniform or heterogeneous Kir2.1expression achieved by lentiviral transfer to elucidate the role of I K1 in cardiac arrhythmogenesis. Kir2.1-overexpressed NRVMs showed increased I K1 density, hyperpolarized resting membrane potential, and increased action potential upstroke velocity compared with green fluorescent protein-transduced NRVMs. Opposite results were observed in Kir2.1-suppressed NRVMs. Optical mapping of uniformly Kir2.1 gene-modified monolayers showed altered conduction velocity and action potential duration compared with nontransduced and empty vector-transduced monolayers, but functional reentrant waves could not be induced. In monolayers with an island of altered Kir2.1 expression, conduction velocity and action potential duration of the locally transduced and nontransduced regions were similar to those of the uniformly transduced and nontransduced monolayers, respectively, and functional reentrant waves could be induced. The waves were anchored to islands of Kir2.1 overexpression and remained stable but dropped in frequency and meandered away from islands of Kir2.1 suppression. In monolayers with an inverse pattern of I K1 heterogeneity, stable high frequency spiral waves were present with I K1 overexpression, whereas lower frequency, meandering spiral waves were observed with I K1 suppression. Our study provides direct evidence for the contribution of I K1 heterogeneity and level to the genesis and stability of spiral waves and highlights the potential importance of I K1 as an antiarrhythmia target. Key Words: Kir2.1 Ⅲ inwardly rectifying potassium current Ⅲ reentry Ⅲ spiral waves Ⅲ ventricular tachycardia Ⅲ ventricular fibrillation V entricular fibrillation (VF) is the leading cause of cardiac arrest and sudden cardiac death in the industrialized world. 1 Studies in the 1970s suggested that the heart could sustain electrical activity that rotated around a functional obstacle. 2,3 These reentrant waves are believed to be the unitary components of fibrillation. Several other studies that focused on understanding the mechanisms of initiation and maintenance of VF concluded that the stability of spiral waves (functional form of reentrant waves) depends on the abbreviation of action potential duration, as well as the reduction of wavefront-wavetail interactions, at fibrillation frequencies. 4 -6 In addition, ionic heterogeneity may be a key factor in the initiation of spiral waves and their transition to the irregular spatiotemporal pattern seen in VF. [7][8][9] Numerous studies pioneered mainly by Jalife and colleagues have indicated that I K1 plays an important role in determining cardiac excitability and arrhythmogenesis and that I K1 block has a significa...
Background: The sinoatrial node (SAN) has intricate architecture, which facilitates the spontaneous action potentials generated from the SAN to pace and drive the neighboring myocardium. We sought to create an engineered SAN that recapitulates the native SAN’s ability to overcome source-sink mismatch. We hypothesized spheroids consisting of induced pacemaker cells (iPM) can pace and drive surrounding quiescent myocardium. Methods: The iPMs were created by singular expression of a transcription factor, TBX18, to neonatal rat ventricular myocytes as we have previously demonstrated. The iPM or GFP-spheroids (control) were created by subjecting the NRVMs to the AggreWell™ plate at 1000 cells/well, and cultured for three days in suspension. Spheroids consisted of 90% NRVM and 10% cardiac fibroblast. Results: iPM-spheroids demonstrated spontaneous pacing at 145±26 bpm compared to 66±1 bpm (p=6.5770E-9) of control, GFP-spheroids. When one iPM-spheroid was surrounded by a monolayer of quiescent NRVMs (iPM:NRVM=1:100), the iPM-spheroids were able to pace and drive the quiescent ventricular myocardium at a capture rate of 48±7%. In contrast, the random activity of control spheroids captured the myocardium at 7±4% (p=0.0078). A monolayer of TBX18 cells (1:4=TBX18:NRVM) failed to pace and drive the neighboring sheet of ventricular myocytes. iPM-spheroids had a 17-fold increase in SAN-specific gap junction, Cx45, transcripts (p=0.0001) and a 2-fold decrease in myocardial gap junction, Cx43 (p=0.0030), compared to GFP-spheroids. The iPM-spheroids have superior viability compared to control GFP-spheroids, 87±1% and 72±5% respectively (p= 0.0463). TUNEL staining confirmed apoptotic fibroblast in the periphery. When cultured for >2 weeks, iPM-spheroids demonstrated small α-sarcomeric actinin positive cells organized as a mesh in the core, similar to the pacemaker cells in the native SAN. Conclusion: iPM-spheroids can pace and drive surrounding quiescent myocardium, overcoming the source-sink mismatch. The iPM-spheroids are viable in long-term and exhibit native SAN-like pacemaker cell organization. These data provide an in vitro platform on which the design principles of native SAN could be tested.
Introduction : An important role for the inwardly rectifying potassium current (I K1 ) has been postulated in controlling the stability and frequency of rotors responsible for ventricular tachycardia and fibrillation. We investigated the effects of Kir2.1 overexpression and Kir2.1AAA dominant-negative mutant suppression on the electrophysiology and inducibility, stability and frequency of spiral waves in an in vitro cardiac tissue model. Methods/Results : Neonatal rat ventricular myocytes (NRVMs) were transduced by lentiviral vectors encoding Kir2.1 or Kir2.1AAA. Immunostaining revealed Kir2.1 or mutant Kir2.1 protein overexpression and whole cell-clamp confirmed the predicted effects on I K1 , resting potential, and action potential duration (APD 80 ). Optical mapping was performed on confluent NRVM monolayers containing a 5 mm diameter central island of gene-modified NRVMs created by a stenciling technique. APs propagated with increased CV (25.1±2.7 cm/sec, n=7) and shortened APD 80 (73±11 msec, n=7) in islands of Kir2.1 overexpression, or decreased CV (13.1±1.1 cm/sec, n=7) and prolonged APD 80 (263±14 msec, n=7) in islands of Kir2.1AAA suppression, compared with normal CV and APD 80 of 19.2±0.4 cm/sec and 169±14 msec (n=7) in non-transduced islands. Reentry was initiated by rapid pacing. With Kir2.1 overexpression, reentrant waves anchored to the island and remained stable (89±15 minutes, n=3) with a frequency of 8±2 Hz. Superfusion with 0.5 mM BaCl 2 to block I K1 slowed reentry to 1 Hz and terminated it shortly after initiation. NRVM monolayers with islands of Kir2.1AAA suppression (n=3) displayed rapid spontaneous activity. Rapid pacing of these monolayers initiated an unstable figure-of-eight reentry (n=3) that degraded into single and multi-armed spiral waves, anchored to varying parts of the island with a maximum frequency of 2±1 Hz. Importantly, no reentry could be initiated in monolayers with non-transduced islands (n=3). Conclusion : Functional reentrant waves induced by rapid pacing are anchored to islands of localized Kir2.1 overexpression whereas they drop in frequency and meander in islands of dominant-negative suppression of Kir2.1, confirming the importance of I K1 for the stability of these waves in cardiac tissue.
Atrioventricular block (AVB), caused by impairment in the heart conduction system, presents extreme diversity and is associated with other complications. Only half of AVB patients require a permanent pacemaker, and the process determining the pacemaker implantation is associated with an increase in cost and patient morbidity and mortality. Thus, there is a need for models capable of accurately identifying transient or reversible causes for conduction disturbances and predicting the patient risks and the necessity of a pacemaker. Deep learning (DL) is brought to the forefront due to its prediction accuracy, and the DL-based electrocardiogram (ECG) analysis can be a breakthrough to analyze a massive amount of data. However, the current DL models are unsuitable for AVB-ECG, where the P waves are decoupled from the QRS/T waves, and a black-box nature of the DL-based model lowers the credibility of prediction models to physicians. Here, we present a real-time-capable DL-based algorithm that can identify AVB-ECG waves and automate AVB phenotyping for arrhythmogenic risk assessment. Our algorithm can analyze unformatted ECG records with abnormal patterns by integrating the two representative DL algorithms: convolutional neural networks (CNN) and recurrent neural networks (RNN). This hybrid CNN/RNN network can memorize local patterns, spatial hierarchies, and long-range temporal dependencies of ECG signals. Furthermore, by integrating parameters derived from dimension reduction analysis and heart rate variability into the hybrid layers, the algorithm can capture the P/QRS/T-specific morphological and temporal features in ECG waveforms. We evaluated the algorithm using the six AVB porcine models, where TBX18, a pacemaker transcription factor, was transduced into the ventricular myocardium to form a biological pacemaker, and an additional electronic pacemaker was transplanted as a backup pacemaker. We achieved high sensitivity (95% true positive rate) and quantified the potential risks of various pathological ECG patterns. This study may be a starting point in conducting both retrospective and prospective patient studies and will help physicians understand its decision-making workflow and find the incorrect recommendations for AVB patients.
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