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Initiation of reentry requires 2 factors: (1) a triggering event, most commonly focal excitations such as premature ventricular complexes (PVCs); and (2) a vulnerable substrate with regional dispersion of refractoriness and/or excitability, such as occurs during the T wave of the electrocardiogram when some areas of the ventricle have repolarized and recovered excitability but others have not. When the R wave of a PVC coincides in time with the T wave of the previous beat, this timing can lead to unidirectional block and initiation of reentry, known as the R-on-T phenomenon. Classically, the PVC triggering reentry has been viewed as arising focally from 1 region and propagating into another region whose recovery is delayed, resulting in unidirectional conduction block and reentry initiation. However, more recent evidence indicates that PVCs also can arise from the T wave itself. In the latter case, the PVC initiating reentry is not a separate event from the T wave but rather is causally generated from the repolarization gradient that manifests as the T wave. We call the former an "R-to-T" mechanism and the latter an "R-from-T" mechanism, which are initiation mechanisms distinct from each other. Both are important components of the R-on-T phenomenon and need to be taken into account when designing antiarrhythmic strategies. Strategies targeting suppression of triggers alone or vulnerable substrate alone may be appropriate in some instances but not in others. Preventing R-from-T arrhythmias requires suppressing the underlying dynamic tissue instabilities responsible for producing both triggers and substrate vulnerability simultaneously. The same principles are likely to apply to supraventricular arrhythmias.

Postural orthostatic tachycardia syndrome (POTS) has been previously described after SARS-CoV-2 infection; however, limited data is available on the relation of POTS with COVID-19 vaccination. Here we show in a cohort of 284,592 COVID-19 vaccinated individuals using a sequence-symmetry analysis, that the odds of POTS are higher 90 days after vaccine exposure than 90 days prior to exposure, and that the odds for POTS are higher than referent conventional primary care diagnoses, but lower than the odds of new POTS diagnosis after SARS-CoV-2 infection. Our results identify a possible association between COVID-19 vaccination and incidence of POTS. Notwithstanding the probable low incidence of POTS after COVID-19 vaccination, particularly when compared to SARS-Cov-2 post-infection odds which were five times higher, our results suggest that further studies, are needed to investigate the incidence and etiology of POTS occurring after COVID-19 vaccination.

Background Female sex is a known risk factor for drug-induced long QT syndrome (diLQTS). We recently demonstrated a sex difference in apamin-sensitive small-conductance Ca2+-activated K+ (SK) current (IKAS) activation during β-adrenergic stimulation. Objective To test the hypothesis that there is a sex difference of IKAS in the rabbit models of diLQTS. Methods We evaluated the sex differences in ventricular repolarization from 15 male and 22 female Langendorff-perfused rabbit hearts with optical mapping techniques during atrial pacing. HMR1556 (IKs blocker), E4031 (IKr blocker) and sea anemone toxin (ATX-II, INaL activator) were used to simulate types 1-3 LQTS, respectively. Apamin, an IKAS blocker was then added to determine the magnitude of further QT prolongation. Results HMR1556, E4031 and ATX-II led to APD80 prolongation in both male and female ventricles at pacing cycle lengths (PCLs) of 300-400 ms. Apamin further lengthened APD80 (in PCL350 ms) from 187.8±4.3 to 206.9±7.1 (p=0.014) in HMR1556 treated, from 209.9±7.8 to 224.9±7.8 (p=0.003) in E4031 treated, and from 174.3±3.3 to 188.1±3.0 (p=0.0002) in ATX-II treated female hearts. In contrast, apamin did not further lengthen the APD80 in male hearts. Compared with the baseline, the Cai transient duration (CaiTD) was significantly increased in diLQTS but without sex differences. There were no significant effects of apamin on CaiTD. Conclusion We conclude that IKAS is abundantly increased in female but not in male ventricles with diLQTS. Increased IKAS helps preserve the repolarization reserve in female ventricles treated with IKs and IKr blockers or INaL activators.

Background Obstructive sleep apnea (OSA) is associated with increased cardiac arrhythmia and sudden cardiac death. We recently developed a new method (neuECG) to noninvasively measure electrocardiogram and skin sympathetic nerve activity (SKNA). Objective The purpose of this study was to test the hypothesis that SKNA measured during sleep study is higher in patients with OSA than in those without OSA. Methods We prospectively recorded neuECG and polysomnography in 26 patients undergoing a sleep study. Sleep stages were scored into rapid eye movement (REM), and non-REM sleep stages 1 (N1), 2 (N2), and 3 (N3). Average voltage of skin sympathetic nerve activity (aSKNA) and SKNA burst area were calculated for quantification. Apnea/hypopnea index (AHI) >5 per hour was used to diagnose OSA. Results There was a positive correlation (r = 0.549; P = .018) between SKNA burst area and the arousal index in OSA but not in the control group. aSKNA during sleep was 0.61 ± 0.09 μV in OSA patients (n = 18) and 0.53 ± 0.04 μV in control patients (n = 8; P = .025). Burst area was 3.26 (1.90–4.47) μV·s/min in OSA patients and 1.31 (0.67–1.94) μV·s/min in control (P = .047). More apparent differences were found during N2, when the burst area in OSA (3.06 [1.46–5.52] μV·s/min) was much higher than that of the control (0.89 [0.79–1.65] μV·s/min; P = .03). Conclusion OSA patients have higher SKNA activity than control patients, with the most pronounced differences observed during N2. Arousal at the end of apnea episodes is associated with large SKNA bursts. Overlaps of aSKNA and SKNA burst area between groups suggest that not all OSA patients have increased sympathetic tone.

Although left cardiac sympathetic denervation (LCSD) is an effective antiarrhythmic therapy for patients with long QT syndrome (LQTS), direct evidence of reduced sympathetic activity after LCSD in humans is limited.The purpose of this study was to assess skin sympathetic nerve activity (SKNA) in patients with LQTS undergoing LCSD.We prospectively enrolled 17 patients with LQTS who underwent LCSD between 2017 and 2019. SKNA recordings from the left arm (L-SKNA) and chest (C-SKNA) leads were performed before and after LCSD. Mean SKNA, burst activity, and nonburst activity of L-SKNA and C-SKNA were analyzed.The mean patient age was 21 ± 9 years (8 men 47%). The longest baseline corrected QT value was 497 ± 55 ms at rest and 531 ± 38 ms on exercise stress testing. Five patients (29.4%) had previous LQTS-triggered cardiac events including syncope, documented torsades de pointes, and ventricular fibrillation. In the 24 hours after LCSD, mean L-SKNA decreased from 1.25 ± 0.64 to 0.85 ± 0.33 μV (P = .005) and mean C-SKNA from 1.36 ± 0.67 to 1.05 ± 0.49 μV (P = .11). The frequency of episodes of SKNA bursts recorded from the left-arm lead (2.87 ± 1.61 bursts per minute vs 1.13 ± 0.99 bursts per minute; P.001) and mean L-SKNA during burst (1.82 ± 0.79 μV vs 1.15 ± 0.44 μV; P.001) and nonburst (1.09 ± 0.60 μV vs 0.75 ± 0.32 μV; P = .03) periods significantly decreased after LCSD, while the frequency of episodes of SKNA bursts recorded from the chest lead (P = .57) and mean C-SKNA during burst (P = .44) and nonburst (P = .10) periods did not change significantly. No arrhythmic events were documented after 11.9 months (range 3.0-22.2 months) of follow-up.LCSD provides an inhibitory effect on cardiac sympathetic activity by suppressing burst discharge as measured by SKNA.

Chronic orthostatic intolerance (OI) is characterized by the development of tachycardia and other symptoms when assuming an upright body position.The purpose of this study was to test the hypothesis that skin sympathetic nerve activity (SKNA) bursts are specific symptomatic biomarkers in patients with chronic OI.We used an electrocardiogram monitor with a built-in triaxial accelerometer to simultaneously record SKNA and posture in ambulatory participants. Study 1 compared chronic OI (14 women and 2 men; mean age 35 ± 10 years) with reference control participants (14 women; mean age 31 ± 6 years). Study 2 included 17 participants with chronic OI (15 women and 2 men; mean age 39 ± 12 years) not yet treated with ivabradine, pyridostigmine, or β-blockers.In study 1, there were 124 episodes (8 ± 4 per participant) of postural changes, with 11 episodes (8.9%) associated with symptoms. In comparison, 0 of 104 postural changes (7 ± 3 per participant) in controls were symptomatic (P = .0011). In participants with chronic OI, the SKNA bursts associated with symptoms had higher burst frequencies, longer burst durations, and larger mean burst areas than did bursts during asymptomatic periods. However, SKNA bursts and tachycardia were asymptomatic in controls. We analyzed 110 symptomatic episodes in study 2 (6 ± 5 per participant). Among them, 98 (89.1%) followed at least 1 SKNA burst. In comparison, only 41 (37.3%) had heart rate exceed 100 beats/min 1 minute before symptom onset (P.0001).SKNA bursts are a highly specific, albeit insensitive, symptomatic biomarker for chronic OI.

Background: Three types of characteristic ST-segment elevation are associated with Brugada syndrome but only type 1 is diagnostic. Why only type 1 ECG is diagnostic remains unanswered. Methods: Computer simulations were performed in single cells, 1-dimensional cables, and 2-dimensional tissues to investigate the effects of the peak and late components of the transient outward potassium current (I to ), sodium current, and L-type calcium current (I Ca,L ) as well as other potassium currents on the genesis of ECG morphologies and phase 2 reentry (P2R). Results: Although a sufficiently large peak I to was required to result in the type 1 ECG pattern and P2R, increasing the late component of I to converted type 1 ECG to type 2 ECG and suppressed P2R. Increasing the peak I to promoted spiral wave breakup, potentiating the transition from tachycardia to fibrillation, but increasing the late I to prevented spiral wave breakup by flattening the action potential duration restitution and preventing P2R. A sufficiently large I Ca,L conductance was needed for P2R to occur, but once above the critical conductance, blocking I Ca,L promoted P2R. However, selectively blocking the window and late components of I Ca,L suppressed P2R, countering the effect of the late I to . Blocking either the peak or late components of sodium current promoted P2R, with the late sodium current blockade having the larger effect. As expected, increasing other potassium currents potentiated P2R, with ATP-sensitive potassium current exhibiting a larger effect than rapid and slow component of the delayed rectifier potassium current. Conclusions: The peak I to promotes type 1 ECG and P2R, whereas the late I to converts type 1 ECG to type 2 ECG and suppresses P2R. Blocking the peak I Ca,L and either the peak or the late sodium current promotes P2R, whereas blocking the window and late I Ca,L suppresses P2R. These results provide important insights into the mechanisms of arrhythmogenesis and potential therapeutic targets for treatment of Brugada syndrome. Graphic Abstract: A graphic abstract is available for this article.

Background:Telethonin (TCAP) is a Z-disk protein that maintains cytoskeletal integrity and various signaling pathways in cardiomyocytes. TCAP is shown to modulate α-subunit of the human cardiac sodium channel (hNav 1.5) by direct interactions. Several TCAP variants are found in cardiomyopathies. We sought to investigate whether TCAP variants are associated with arrhythmia syndromes., Methods:Mutational analyses for TCAP were performed in 303 Japanese patients with Brugada syndrome, arrhythmogenic right ventricular cardiomyopathy, and J-wave pattern ECG. Using patch-clamp techniques, electrophysiological characteristics of hNav 1.5 were studied in HEK-293 cells stably expressing hNav 1.5 and transiently transfected with wild-type (WT) or variant TCAP., Results:We identified two TCAP variants, c.145G>A:p.E49K and c.458G>A:p.R153H, in four individuals. p.E49K was found in two patients with ARVC or BrS. p.R153H was found in two patients with BrS or J-wave pattern ECG. No patient had variant hNav 1.5. Patch-clamp experiments demonstrated that peak sodium currents were significantly reduced in cells expressing p.R153H and p.E49K compared with WT-TCAP (66%, p.R153H; 72%, p.E49K). Voltage dependency of peak IV curve was rightward-shifted by 5 mV in cells expressing p.E49K compared with WT-TCAP. Voltage dependency of activation was not leftward-shifted by p.R153H, while voltage dependency of steady-state inactivation was leftward-shifted by p.E49K., Conclusions:We found two TCAP variants in the patients with BrS, J-wave pattern ECG, and ARVC that can cause loss-of-function of the hNav 1.5 in heterologous expression systems. Our observation suggests that these variants might impair INa and be associated with the patients' electrophysiological phenotypes. Further studies linking our experimental data to clinical phenotypes are warranted.