Is there a significant transmural gradient in repolarization time in the intact heart? Cellular basis of the T wave: a century of controversy

Modeling ventricular repolarization gradients in normal cases using the equivalent dipole layer

Background Electrical activity underlying the T-wave is less well understood than the QRS-complex. This study investigated the relationship between normal T-wave morphology and the underlying ventricular repolarization gradients using the equivalent dipole layer (EDL). Methods Body-surface-potential-maps (BSPM, 67‑leads) were obtained in nine normal cases. Subject specific MRI-based anatomical heart/torso-models with electrode positions were created. The boundary element method was used to account for the volume conductor effects. To simulate the measured T-waves, the EDL was used to apply different ventricular repolarization gradients: a) transmural, b) interventricular c) apico-basal and d) all three gradients (a-c) combined. The combined gradient (d) was optimized using an inverse procedure (Levenberg-Marquardt). Correspondence between simulated and measured T-waves was assessed using correlation coefficient (CC) and relative difference (RD). Results Realistic T-waves were simulated if repolarization times of: (a) the epicardium were smaller than the endocardium; (b) the left ventricle were smaller than the right ventricle and (c) the apex increased towards the base. The apico-basal gradient resulted in the highest correspondence between measured and simulated T-waves (CC = 0.84(0.81-0.91);RD = 0.68(0.60-0.71)) compared to a transmural gradient (CC = 0.77(0.71-0.80);RD = 1.46(0.82-1.75)) and an interventricular gradient (CC = 0.71(0.67-0.80);RD = 0.85(0.75-0.87)). All three gradients combined further improved the correspondence between measured and simulated T-waves (CC = 0.83(0.82-0.89);RD = 0.60(0.51-0.63)), especially after optimization (CC = 0.96(0.94-0.98);RD = 0.27(0.22-0.34)). Conclusion The application of all repolarization gradients combined resulted in the largest agreement between simulated and measured T-waves, followed by the apico-basal repolarization gradient. With these findings, we will optimize our EDL-based inverse procedure to assess repolarization abnormalities.

Spatiotemporal repolarization dispersion before and after exercise in patients with long QT syndrome type 1 versus controls: probing into the arrhythmia substrate

Congenital long QT syndrome (LQTS) carries an increased risk for syncope and sudden death. QT prolongation promotes ventricular extrasystoles, which, in the presence of an arrhythmia substrate, might trigger ventricular tachycardia degenerating into fibrillation. Increased electrical heterogeneity (dispersion) is the suggested arrhythmia substrate in LQTS. In the most common subtype LQT1, physical exercise predisposes for arrhythmia and spatiotemporal dispersion was therefore studied in this context. Thirty-seven patients (57% on β-blockers) and 37 healthy controls (mean age, 31 vs. 35; range, 6-68 vs. 6-72 yr) performed an exercise test. Frank vectorcardiography was used to assess spatiotemporal dispersion as Tampl, Tarea, the ventricular gradient (VG), and the Tpeak-end interval from 10-s signal averages before and 7 ± 2 min after exercise; during exercise too much signal disturbance excluded analysis. Baseline and maximum heart rates as well as estimated exercise intensity were similar, but heart rate recovery was slower in patients. At baseline, QT and heart rate-corrected QT (QTcB) were significantly longer in patients (as expected), whereas dispersion parameters were numerically larger in controls. After exercise, QTpeakcB and Tpeak-endcB increased significantly more in patients (18 ± 23 vs. 7 ± 10 ms and 12 ± 17 vs. 2 ± 6 ms; P < 0.001 and P < 0.01). There was, however, no difference in the change in Tampl, Tarea, and VG between groups. In conclusion, although temporal dispersion of repolarization increased significantly more after exercise in patients with LQT1, there were no signs of exercise-induced increase in global dispersion of action potential duration and morphology. The arrhythmia substrate/mechanism in LQT1 warrants further study.NEW & NOTEWORTHY Physical activity increases the risk for life-threatening arrhythmias in LQTS type 1 (LQT1). The arrhythmia substrate is presumably altered electrical heterogeneity (a.k.a. dispersion). Spatiotemporal dispersion parameters were therefore compared before and after exercise in patients versus healthy controls using Frank vectorcardiography, a novelty. Physical exercise prolonged the time between the earliest and latest complete repolarization in patients versus controls, but did not increase parameters reflecting global dispersion of action potential duration and morphology, another novelty.

Left bundle branch area pacing reduces epicardial dispersion of repolarization compared with biventricular cardiac resynchronization therapy

BACKGROUND

Biventricular endocardial pacing (BiV-endo) and left bundle branch area pacing (LBBAP) are novel methods of delivering cardiac resynchronization therapy. These techniques are associated with improved activation times and acute hemodynamic response compared with conventional biventricular epicardial pacing (BiV-epi); however, the effects on repolarization and arrhythmic risk are unknown.

OBJECTIVE

The purpose of this study was to compare the effects of temporary BiV-epi, BiV-endo, and LBBAP on epicardial left ventricular (LV) repolarization using electrocardiographic imaging (ECGi).

METHODS

Eleven patients indicated for cardiac resynchronization therapy underwent a temporary pacing protocol with ECGi. BiV-endo was delivered via endocardial stimulation of the LV lateral wall. LBBAP was delivered by pacing the LV septum. Epicardial LV repolarization time (LVRT-95; time taken for 95% of the LV to repolarize), LV RT dispersion, mean LV activation recovery interval (ARI), LV ARI dispersion, and RT gradients were calculated.

RESULTS

The protocol was completed in 10 patients. During LBBAP, there were significant reductions in LVRT-95 (94.9 ± 17.4 ms vs 125.0 ± 29.4 ms; P = .03) and LV RT dispersion (29.4 ± 6.3 ms vs 40.8 ± 11.4 ms; P = .015) compared with BiV-epi. In contrast, there were no significant differences between baseline, BiV-epi, or BiV-endo. There was a nonsignificant reduction in mean RT gradients between LBBAP and baseline rhythm (0.74 ± 0.22 ms/mm vs 1.01 ± 0.31 ms/mm; P = .07). There were no significant differences in mean LV ARI or LV ARI dispersion between groups.

CONCLUSION

Temporary LBBAP reduces epicardial dispersion of repolarization compared with conventional BiV-epi. Further study is required to determine whether these repolarization changes on ECGi translate into a reduced risk of ventricular arrhythmia in clinical practice.

A Role of Thyroid Hormones in Acute Myocardial Infarction: An Update

The acute coronary syndrome is one of the commonest life-threatening illnesses. It encompasses the clinical spectrum of acute myocardial ischemia and includes unstable angina and acute myocardial infarction both with and without ST segment elevation. The acute coronary syndrome can be attributed to a significant hemodynamic insult that leads to atherosclerosis of the epicardial coronary arteries. The main causative risk factors, such as obesity, smoking, and alcohol intake, increase the burden of acute coronary syndrome. Owing to an increase in the utilization of antioxidants, the antioxidant capacity decreases concerning the scavenging of lipid peroxides. Moreover, the thyroid hormones are important regulators of the expression of cardiac genes, and many of the cardiac manifestations of thyroid dysfunction are associated with alterations in triiodothyronine- mediated gene expression. Cardiovascular signs and symptoms of thyroid disease are among the most acute clinically relevant findings that occur in combination with both hypothyroidism and hyperthyroidism. By understanding the cellular mechanism of the action of thyroid hormones on the heart and cardiovascular system, it is possible to explain rhythm disturbances and alterations in cardiac output, blood pressure, cardiac contractility, and vascular resistance that result from thyroid dysfunction. Oxidative stress is thereby induced, together with a decrease in antioxidant capacity for overcoming oxidative stress, which leads to endothelial dysfunction, subsequent atherosclerosis, and, ultimately, acute myocardial infarction. The implications for the identification of the effects of thyroid disease on acute myocardial infarction include the observation that restoration of normal thyroid function repeatedly reverses abnormalities in cardiovascular hemodynamics.

Alternating Early Afterdepolarizations Underlying Bradycardia-Dependent Macroscopic T Wave and Discordant Mechanical Alternans

BACKGROUND

Macroscopic T wave alternans (macro-TWA) often heralds the onset of Torsades de Pointes in patients with QT prolongation. However, the mechanisms underlying macro-TWA remain unclear. We examined the cellular and ionic basis for macro-TWA in rabbits with left ventricular hypertrophy (LVH).

METHODS

The renovascular hypertension model was used to induce LVH in rabbits. Action potentials were simultaneously recorded from epicardium and endocardium together with a transmural ECG and isometric contractility in arterially perfused left ventricular wedges. Late sodium current (I_Na-L) was recorded in single-isolated left ventricular myocytes with the whole cell patch-clamp technique.

RESULTS

Macro-TWA and accompanied mechanical alternans occurred spontaneously in 8 of 33 LVH rabbits (P<0.05, versus 0/15 in controls) and were induced by an I_Na-L enhancer ATX-II at 1 to 3 nM in additional 7. Macro-TWA and mechanical alternans occurred discordantly, that is, that longer QT interval and larger T wave were associated with weaker isometric contvractility. Alternating early afterdepolarizations in the endocardium caused macro-TWA in 12 of 15 LVH rabbits and, therefore, early afterdepolarization-dependent R-from-T extrasystoles and Torsades de Pointes always originated from the beats with longer QT and larger T wave during macro-TWA. I_Na-L density was significantly larger in LVH myocytes than that of control myocytes. Macro-TWA, mechanical alternans, R-from-T extrasystoles, and Torsades de Pointes were all abolished by I_Na-L blocker ranolazine or mexiletine.

CONCLUSIONS

LVH enhances I_Na-L density and promotes alternating early afterdepolarizations in the left ventricular endocardium that manifest as macro-TWA with discordant mechanical alternans. I_Na-L blockade abolishes macro-TWA, mechanical alternans, early afterdepolarization-dependent R-from-T extrasystoles, and Torsades de Pointes.

The association between QTc, QTd, TPE, and fragmented QRS before and after PPCI with hospital mortality in STEMI patients

INTRODUCTION

ST-elevation myocardial infarction (STEMI) is known to be associated with significant arrhythmia and consequent mortality. QT prolongation is a risk factor for arrhythmia in STEMI patients who underwent primary percutaneous coronary intervention (PPCI). The aim of this investigation was to evaluate the association of corrected QT interval (QTc), QT dispersion (QTd), T-wave peak to end (TPE), and fragmented QRS with mortality in these patients.

METHODS

Eligible patients with the characteristic symptoms of STEMI who underwent PPCI were included. QTc, QTd, TPE, and fragmented QRS were measured before and after the PPCI. These predictors were compared between patients who died during hospitalization and discharged patients.

RESULTS

After coronary angiography, 10 patients (4%) died during the hospitalization after PPCI. Comparing the non-survivers and discharged patients in terms of arrhythmia predictors showed that the mean QT dispersion and TPE before intervention were significantly higher in the non-survivors. Also, the number of patients who experienced fragmented QRS both before and after the intervention was significantly higher in the non-survivors.

CONCLUSION

These data suggested that evaluating such arrhythmia predictors, especially before PPCI, could be used as a predictor of mortality in STEMI patients who underwent PPCI.

Optimizing ECG lead selection for detection of prolongation of ventricular repolarization as measured by the Tpeak-end interval

Background

The Tpeak‐end(Tp‐e) has not been compared in all 12 ECG leads in healthy adults to determine if the Tp‐e varies across leads. If there is variation, it remains uncertain, which lead(s) are preferred for recording in order to capture the maximal Tp‐e value.

Objective

The purpose of the current study was to determine the optimal leads, if any, to capture the maximal Tp‐e interval in healthy young adults.

Methods

In 88 healthy adults (ages 21–38 years), including derivation (n = 21), validation (n = 20), and smoker/vaper (n = 47) cohorts, the Tp‐e was measured using commercial computer software (LabChart Pro 8 with ECG module, ADInstruments) in all 12 leads at rest and following a provocative maneuver, abrupt standing. Tp‐e was compared to determine which lead(s) most frequently captured the maximal Tp‐e interval.

Results

In the rest and abrupt standing positions, the Tp‐e was not uniform among the 12 leads; the maximal Tp‐e was most frequently captured in the precordial leads. At rest, grouping leads V2–V4 resulted in detection of the maximum Tp‐e in 85.7% of participants (CI 70.7, 99.9%) versus all other leads (p < .001). Upon abrupt standing, grouping leads V2‐V6 together, resulted in detection of the maximum Tp‐e 85.0% of participants (CI 69.4, 99.9% versus all other leads; p < .001). These findings were confirmed in the validation cohort, and extended to the smoking/vaping cohort.

Conclusion

If only a subset of ECG leads will be recorded or analyzed for the Tp‐e interval, selection of the precordial leads is preferred since these leads are most likely to capture the maximal Tp‐e value.

Critical Requirements for the Initiation of a Cardiac Arrhythmia in Rat Ventricle: How Many Myocytes?

Cardiovascular disease is the leading cause of death worldwide due in a large part to arrhythmia. In order to understand how calcium dynamics play a role in arrhythmogenesis, normal and dysfunctional Ca²⁺ signaling in a subcellular, cellular, and tissued level is examined using cardiac ventricular myocytes at a high temporal and spatial resolution using multiscale computational modeling. Ca²⁺ sparks underlie normal excitation-contraction coupling. However, under pathological conditions, Ca²⁺ sparks can combine to form Ca²⁺ waves. These propagating elevations of (Ca²⁺)_i can activate an inward Na⁺-Ca²⁺ exchanger current (I_NCX) that contributes to early after-depolarization (EADs) and delayed after-depolarizations (DADs). However, how cellular currents lead to full depolarization of the myocardium and how they initiate extra systoles is still not fully understood. This study explores how many myocytes must be entrained to initiate arrhythmogenic depolarizations in biophysically detailed computational models. The model presented here suggests that only a small number of myocytes must activate in order to trigger an arrhythmogenic propagating action potential. These conditions were examined in 1-D, 2-D, and 3-D considering heart geometry. The depolarization of only a few hundred ventricular myocytes is required to trigger an ectopic depolarization. The number decreases under disease conditions such as heart failure. Furthermore, in geometrically restricted parts of the heart such as the thin muscle strands found in the trabeculae and papillary muscle, the number of cells needed to trigger a propagating depolarization falls even further to less than ten myocytes.

Role of Purkinje-Muscle Junction in Early Ventricular Fibrillation in a Porcine Model: Beyond the Trigger Concept

BACKGROUND

The role of the Purkinje network in triggering ventricular fibrillation (VF) has been studied, however, its involvement after onset and in early maintenance of VF is controversial.

AIM

We studied the role of the Purkinje-muscle junctions (PMJ) on epicardial-endocardial activation gradients during early VF.

METHODS

In a healthy, porcine, beating-heart Langendorff model [control, n = 5; ablation, n = 5], simultaneous epicardial-endocardial dominant frequent mapping was used (224 unipolar electrograms) to calculate activation rate gradients during the onset and early phase of VF. Selective Purkinje ablation was performed using Lugol's solution, followed by VF re-induction and mapping and finally, histological evaluation.

RESULTS

Epicardial activation rates were faster than endocardial rates for both onset and early VF. After PMJ ablation, activation rates decreased epicardially and endocardially for both onset and early VF [Epi: 9.7±0.2 to 8.3±0.2 Hz (P<0.0001) and 10.9±0.4 to 8.8±0.3 Hz (P<0.0001), respectively; Endo: 8.2 ± 0.3 Hz to 7.4 ± 0.2 Hz (P<0.0001) and 7.0 ± 0.4 Hz to 6.6 ± 0.3 Hz (P = 0.0002), respectively]. In controls, epicardial-endocardial activation rate gradients during onset and early VF were 1.7±0.3 Hz and 4.5±0.4 Hz (P<0.001), respectively. After endocardial ablation of PMJs, these gradients were reduced to 0.9±0.3 Hz (onset VF, P<0.001) and to 2.2±0.3 Hz (early VF, P<0.001). Endocardial-epicardial Purkinje fibre arborization and selective Purkinje fibre extinction after only endocardial ablation (not with epicardial ablation) was confirmed on histological analysis.

CONCLUSIONS

Beyond the trigger paradigm, PMJs determine activation rate gradients during onset and during early maintenance of VF. This article is protected by copyright. All rights reserved.

Effect of dialysis and transplantation on myocardial repolarization parameters and P-wave dispersion in chronic kidney disease

BACKGROUND

Chronic kidney disease (CKD) patients are at higher risk for cardiac arrhythmias. The risk of arrhythmia may change with different treatment modalities. We proposed to compare the effects of varied therapy methods on myocardial repolarization parameters (Tp-e, QT, QTc intervals, Tp-e/QT, Tp-e/QTc ratios) and P-wave dispersion (PWD) in patients with CKD.

METHODS

Three groups were formed from the patients aged between 18 and 65 years, as Group 1 consisting of CKD patients receiving hemodialysis (HD) three times a week, Group 2 consisting of predialysis CKD patients and Group 3 consisting of CKD patients who underwent successful transplantation. All patients' basic demographic data, risk factors, and echocardiographic parameters were recorded, and electrocardiographic repolarization parameters and PWD were analyzed.

RESULTS

The PR, QT, and QTc intervals were significantly shorter in the transplantation group compared to the other groups (P = .020, P < .001, P = .035; respectively). Tp-e interval, Tp-e/QT, and Tp-e/QTc ratios were significantly higher in the predialysis group compared to the other groups (P < .001, P < .001, P = .001; respectively), while there was no significant variation between the HD and transplantation groups (P > .05). PWD was significantly increased in the predialysis group compared to other two groups (P < .001), while no significant variation between the HD and transplantation groups was observed.

CONCLUSION

We found that the Tp-e interval, Tp-e/QT, Tp-e/QTc, and PWD were significantly higher in the predialysis CKD group, but the PR, QT, and QTc intervals were significantly shorter in the transplantation group compared to the other groups. The prognostic significance and prediction of these parameters in arrhythmic events in CKD patients requires further evaluation with long-time follow-up.

The Link Between Sex Hormones and Susceptibility to Cardiac Arrhythmias: From Molecular Basis to Clinical Implications

It is well-known that gender is an independent risk factor for some types of cardiac arrhythmias. For example, males have a greater prevalence of atrial fibrillation and the Brugada Syndrome. In contrast, females are at increased risk for the Long QT Syndrome. However, the underlying mechanisms of these gender differences have not been fully identified. Recently, there has been accumulating evidence indicating that sex hormones may have a significant impact on the cardiac rhythm. In this review, we describe in-depth the molecular interactions between sex hormones and the cardiac ion channels, as well as the clinical implications of these interactions on the cardiac conduction system, in order to understand the link between these hormones and the susceptibility to arrhythmias.

Importance of electrocardiographic markers in predicting cardiac events in children

ECG is a common diagnostic tool in medical practice. Sudden cardiac death (SCD) is a rare but devastating event. The most common cause of SCD in the young is a primary arrhythmic event, which is often produced by malignant ventricular arrhythmia. Several electrocardiographic markers for ventricular repolarization and depolarization have been proposed to predict this arrhythmic risk and SCD in children. Although many of these parameters can easily be used in clinical practice, some of them need specific techniques for interpretation. In this review, we summarized the current knowledge regarding the clinical importance and the ability of these ECG parameters to predict adverse cardiac events in the pediatric population.

The prognostic value of dispersion of repolarization in stress cardiomyopathy

INTRODUCTION

Although abnormalities of ventricular repolarization are a hallmark of SC, their clinical impact on management remains to be determined. This study sought to evaluate the prognostic value of dispersion of repolarization in stress cardiomyopathy (SC) with regards to major cardiac events (MCE), recovery time, and recurrence.

METHODS

This study analyzed data from258 patients with SC, from January 2009 to January 2018. Standard 12 lead ECG recordings during the acute, subacute, and recovery phases were collected for each eligible patient. Logistic regression was used to identify independent predictors of MCE, a composite of 30 day all-cause mortality, cardiogenic shock, life-threatening ventricular arrhythmias, and stroke.

RESULTS

Among the 101 eligible patients (80.2% females, mean age 45.8 ± 11.5 years) in the study cohort, MCE occurred in 16 patients (15.8%). Cox regression analysis identified two independent predictors of MCE: increased ΔQT dispersion ≥ 40 ms (HR 1.31, 95% CI 1.05-9.77, p = 0.029) and increased Δnegative T wave amplitude dispersion ≤ -2.0 mV (HR 1.25, 95% CI 1.11-11.93, p = 0.018) during the subacute phase. The final regression model had good accuracy (sensitivity 81.3%, specificity 96.5%) and discriminative power (AUC 0.89, 95% CI 0.83-0.95). Kaplan-Meier analysis revealed that there was increasing MCE in patients with zero, one, or two predictors (log rank p < 0.001). In addition, patients with increased dispersion also had a significantly longer time to achieve complete recovery (21.4 ± 6.8 vs. 8.5 ± 4.3 days, p = 0.012) and a higher incidence of recurrence (31.3% vs. 2.4%, p = 0.011) of SC.

CONCLUSION

Evaluation of dynamic changes of dispersion of repolarization is a simple bed-side tool with high predictive accuracy for prognostication of short term adverse outcomes, delayed recovery, and recurrence in patients with SC.

Effect of Methadone on Cardiac Repolarization in Japanese Cancer Patients: A Longitudinal Study

INTRODUCTION

Methadone is known to prolong the QT interval, which could induce lethal arrhythmias such as torsades de pointes. To determine the risk of ventricular arrhythmias in cancer patients using methadone, we measured QT dispersion (QTD) and Tpeak-Tend (TpTe) before and after methadone administration and evaluated the correlations between methadone dosage and cardiac repolarization.

METHODS

We conducted a retrospective observational study with 19 patients undergoing follow-up for cancer pain with methadone. Electrocardiogram (ECG) recordings were obtained from the patients at methadone initiation and 1 week, 1 month, and 2 months later. The QT, corrected QT (QTc), QTD, QTc dispersion (QTcD), TpTe, TpTe/QT, and TpTe/QTc were measured manually via ECG records and analyzed using a repeated measures one-way ANOVA. The correlations between these ECG parameters and each methadone dosage were determined using Spearman's rank correlation coefficient.

RESULTS

The QTD, QTcD, TpTe/QT, and TpTe/QTc remained unchanged, while TpTe was prolonged significantly at 2 months (initiation: 82 ± 17 ms; 2 months: 106 ± 20 ms, p = 0.018). In addition, there was a positive correlation between TpTe and methadone dosage (rs = 0.4, p = 0.041).

CONCLUSIONS

The findings suggested that small or modest doses of methadone could exert dose-dependent effects on cardiac repolarization in cancer patients.

TRIAL REGISTRATION

UMIN Clinical Trials Registry, UMIN000034519.

Comparison of Cardiac Repolarization After Transcatheter Aortic Valve Implantation and Surgical Aortic Valve Replacement: A Longitudinal Study

INTRODUCTION

Transcatheter aortic valve implantation (TAVI) has been established as an alternative to surgical aortic valve replacement (SAVR) for high-risk patients. To assess the impact of TAVI on cardiac repolarization, we compared QT dispersion (QTD) and the interval from the peak to the end of the T wave (Tpeak-Tend: TpTe) between the patients who underwent TAVI and those who underwent SAVR and TpTe between the patients who underwent TAVI or SAVR.

METHODS

This retrospective study was approved by the ethics committee of Dokkyo Medical University Hospital. The study included 45 patients who underwent TAVI and 45 patients who underwent SAVR. The QT, corrected QT (QTc), QTD, QTc dispersion (QTcD), Tp-Te, Tp-Te/QT, and Tp-Te/QTc were manually measured in standard 12-lead electrocardiogram (ECG) recordings obtained before surgery, immediately after surgery, 1 month, 3 months, and 6 months after surgery and compared between the two groups.

RESULTS

No change was observed in RR, QT, QTc, Tp-Te, Tp-Te/QT, and Tp-Te/QTc in the two groups throughout the study. The QTD and QTcD significant decreased immediately after surgery in the TAVI group as compared to the SAVR group (P < 0.001). In contrast, QTD and QTcD in the SAVR group gradually, but not significantly declined 6 months after surgery.

CONCLUSIONS

QTD and QTcD immediately decreased after TAVI as compared to SAVR. Our findings indicate that TAVI more rapidly improved dispersion of spatial repolarization than SAVR.

Comparison of the integral indices of the vectorcardiogram with the data of echocardiography in patients with idiopathic and chronic thromboembolic pulmonary hypertension

AIM

The aim of the work is to compare vectorcardiographic (VCG) variables - spatial QRS-T angle and electrocardiographic ventricular gradient (VG) with echocardiography (EchoCG) data in patients with idiopathic pulmonary hypertension (IPH) and chronic thromboembolic pulmonary hypertension (CTEPH).

MATERIALS AND METHODS

In 40 patients with IPH and 40 patients with CTEPH at the age of 45±12 years, systolic pulmonary artery pressure (SPAP); the sizes of heart chambers, parameters of RV systolic and diastolic function were evaluated with EchoCG. The QRS-T and VG angles were calculated on the VCG, derived from 12-lead digital ECG.

RESULTS

In all patients SPAP was greater than 40 mm Hg (mean 83±18 mm Hg), EchoCG data indicated hypertrophy and dilatation of RV, its systolic and diastolic function; dilatation of the right atrium (RA). Prognostically unfavorable changes in EchoCG were observed: the presence of pericardial effusion in 35 (44%) patients, RA area greater than 26 cm2 in 18 (23%) patients; TAPSE less than 1.5 cm in 37 (46%) patients. EchoCG and VCG variables had statistically significant differences in patients with III-IV functional class in comparison with I-II functional class. Statistically significant moderate correlations between VCG and EchoCG variables were revealed. VCG variables allowed to separate patient groups with the presence and absence of prognostically unfavorable changes in EchoCG with sensitivity from 54 to 78% and specificity from 66 to 87%.

CONCLUSION

In patients with IPH and CTEPH, changes of QRS-T angle and VG correlate with SPAP, the size of RV and RA, parameters of RV systolic and diastolic function. The possibility of the use of QRS-T angle and VG for the detection of patients with prognostically unfavorable echocardiographic changes in the general group of patients with IPH and CTEPH has been shown.

Differences in the upslope of the precordial body surface ECG T wave reflect right to left dispersion of repolarization in the intact human heart

BACKGROUND

The relationship between the surface electrocardiogram (ECG) T wave to intracardiac repolarization is poorly understood.

OBJECTIVE

The purpose of this study was to examine the association between intracardiac ventricular repolarization and the T wave on the body surface ECG (SECGTW).

METHODS

Ten patients with a normal heart (age 35 ± 15 years; 6 men) were studied. Decapolar electrophysiological catheters were placed in the right ventricle (RV) and lateral left ventricle (LV) to record in an apicobasal orientation and in the lateral LV branch of the coronary sinus (CS) for transmural recording. Each catheter (CS, LV, RV) was sequentially paced using an S1-S2 restitution protocol. Intracardiac repolarization time and apicobasal, RV-LV, and transmural repolarization dispersion were correlated with the SECGTW, and a total of 23,946 T waves analyzed.

RESULTS

RV endocardial repolarization occurred on the upslope of lead V1, V2, and V3 SECGTW, with sensitivity of 0.89, 0.91, and 0.84 and specificity of 0.67, 0.68, and 0.65, respectively. LV basal endocardial, epicardial, and mid-endocardial repolarization occurred on the upslope of leads V6 and I, with sensitivity of 0.79 and 0.8 and specificity of 0.66 and 0.67, respectively. Differences between the end of the upslope in V1, V2, and V3 vs V6 strongly correlated with right to left dispersion of repolarization (intraclass correlation coefficient 0.81, 0.83, and 0.85, respectively; P <.001). Poor association between the T wave and apicobasal and transmural dispersion of repolarization was seen.

CONCLUSION

The precordial SECGTW reflects regional repolarization differences between right and left heart. These findings have important implications for accurately identifying biomarkers of arrhythmogenic risk in disease.

Transverse cardiac slicing and optical imaging for analysis of transmural gradients in membrane potential and Ca2+ transients in murine heart

Key points

A robust cardiac slicing approach was developed for optical mapping of transmural gradients in transmembrane potential (V_m) and intracellular Ca²⁺ transient (CaT) of murine heart.

Significant transmural gradients in V_m and CaT were observed in the left ventricle.

Frequency‐dependent action potentials and CaT alternans were observed in all ventricular regions with rapid pacing, with significantly greater incidence in the endocardium than epicardium.

The observations demonstrate the feasibility of our new approach to cardiac slicing for systematic analysis of intrinsic transmural and regional gradients in V_m and CaT.

Abstract

Transmural and regional gradients in membrane potential and Ca²⁺ transient in the murine heart are largely unexplored. Here, we developed and validated a robust approach which combines transverse ultra‐thin cardiac slices and high resolution optical mapping to enable systematic analysis of transmural and regional gradients in transmembrane potential (V_m) and intracellular Ca²⁺ transient (CaT) across the entire murine ventricles. The voltage dye RH237 or Ca²⁺ dye Rhod‐2 AM were loaded through the coronary circulation using a Langendorff perfusion system. Short‐axis slices (300 μm thick) were prepared from the entire ventricles (from the apex to the base) by using a high‐precision vibratome. Action potentials (APs) and CaTs were recorded with optical mapping during steady‐state baseline and rapid pacing. Significant transmural gradients in V_m and CaT were observed in the left ventricle, with longer AP duration (APD₅₀ and APD₇₅) and CaT duration (CaTD₅₀ and CaTD₇₅) in the endocardium compared with that in the epicardium. No significant regional gradients were observed along the apico‐basal axis of the left ventricle. Interventricular gradients were detected with significantly shorter APD₅₀, APD₇₅ and CaTD₅₀ in the right ventricle compared with left ventricle and ventricular septum. During rapid pacing, AP and CaT alternans were observed in most ventricular regions, with significantly greater incidence in the endocardium in comparison with epicardium. In conclusion, these observations demonstrate the feasibility of our new approach to cardiac slicing for systematic analysis of intrinsic transmural and regional gradients in V_m and CaT in murine ventricular tissue.

A robust cardiac slicing approach was developed for optical mapping of transmural gradients in transmembrane potential (V_m) and intracellular Ca²⁺ transient (CaT) of murine heart.

Significant transmural gradients in V_m and CaT were observed in the left ventricle.

Frequency‐dependent action potentials and CaT alternans were observed in all ventricular regions with rapid pacing, with significantly greater incidence in the endocardium than epicardium.

The observations demonstrate the feasibility of our new approach to cardiac slicing for systematic analysis of intrinsic transmural and regional gradients in V_m and CaT.

Local transmural action potential gradients are absent in the isolated, intact dog heart but present in the corresponding coronary-perfused wedge

The left ventricular (LV) coronary‐perfused canine wedge preparation is a model commonly used for studying cardiac repolarization. In wedge studies, transmembrane potentials typically are recorded; whereas, extracellular electrical recordings are commonly used in intact hearts. We compared electrically measured activation recovery interval (ARI) patterns in the intact heart with those recorded at the same location in the LV wedge preparation. We also compared electrically recorded and optically obtained ARIs in the LV wedge preparation. Five Langendorff‐perfused canine hearts were paced from the right atrium. Local activation and repolarization times were measured with eight transmural needle electrodes. Subsequently, left ventricular coronary‐perfused wedge preparations were prepared from these hearts while the electrodes remained in place. Three electrodes remained at identical positions as in the intact heart. Both electrograms and optical action potentials were recorded (pacing cycle length 400–4000 msec) and activation and repolarization patterns were analyzed. ARIs found in the subepicardium were shorter than in the subendocardium in the LV wedge preparation but not in the intact heart. The transmural ARI gradient recorded at the cut surface of the wedge was not different from that recorded internally. ARIs recorded internally and at the cut surface in the LV wedge preparation, both correlated with optically recorded action potentials. ARI and RT gradients in the LV wedge preparation differed from those in the intact canine heart, implying that those observations in human LV wedge preparations also should be extrapolated to the intact human heart with caution.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Hormones and sex differences: changes in cardiac electrophysiology with pregnancy

Disruption of cardiac electrical activity resulting in palpitations and syncope is often an early symptom of pregnancy. Pregnancy is a time of dramatic and dynamic physiological and hormonal changes during which numerous demands are placed on the heart. These changes result in electrical remodelling which can be detected as changes in the electrocardiogram (ECG). This gestational remodelling is a very under-researched area. There are no systematic large studies powered to determine changes in the ECG from pre-pregnancy, through gestation, and into the postpartum period. The large variability between patients and the dynamic nature of pregnancy hampers interpretation of smaller studies, but some facts are consistent. Gestational cardiac hypertrophy and a physical shift of the heart contribute to changes in the ECG. There are also electrical changes such as an increased heart rate and lengthening of the QT interval. There is an increased susceptibility to arrhythmias during pregnancy and the postpartum period. Some changes in the ECG are clearly the result of changes in ion channel expression and behaviour, but little is known about the ionic basis for this electrical remodelling. Most information comes from animal models, and implicates changes in the delayed-rectifier channels. However, it is likely that there are additional roles for sodium channels as well as changes in calcium homoeostasis. The changes in the electrical profile of the heart during pregnancy and the postpartum period have clear implications for the safety of pregnant women, but the field remains relatively undeveloped.

QRS duration and dispersion for predicting ventricular arrhythmias in early stage of acute myocardial infraction

OBJECTIVE

To determine the relationship between QRS duration and dispersion and the occurrence of ventricular arrhythmias in early stages of acute myocardial infarction (AMI).

DESIGN

A retrospective, longitudinal descriptive study was carried out.

SETTING

Hospital General Universitario "Camilo Cienfuegos", Sancti Spíritus, Cuba. Secondary health care.

PATIENTS OR PARTICIPANTS

A total of 209 patients diagnosed with ST-segment elevation AMI from January 2012 to June 2014.

MAIN VARIABLES OF INTEREST

The duration and dispersion of the QT interval, corrected QT interval, and QRS complex were measured in the first electrocardiogram performed at the hospital. The presence of ventricular tachycardia/fibrillation was assessed during follow-up (length of hospital stay).

RESULTS

Arrhythmias were found in 46 patients (22%); in 25 of them (15.9%), arrhythmias originated in ventricles, and were more common in those subjects with extensive anterior wall AMI, which was responsible for 81.8% of the ventricular fibrillations and more than half (57.1%) of the ventricular tachycardias. The widest QRS complexes (77.3±13.3 vs. 71.5±6.4ms; P=.029) and their greatest dispersion (24.1±16.2 vs. 16.5±4.8ms; P=.019) were found on those leads that explore the regions affected by ischemia. The highest values of all measurements were found in extensive anterior wall AMI, with significant differences: QRS 92.3±18.8ms, QRS dispersion 37.9±23.9ms, corrected QT 518.5±72.2ms, and corrected QT interval dispersion 94.9±26.8ms. Patients with higher QRS dispersion values were more likely to have ventricular arrhythmias, with cutoff points at 23.5ms and 24.5ms for tachycardia and ventricular fibrillation, respectively.

CONCLUSIONS

Increased QRS duration and dispersion implied a greater likelihood of ventricular arrhythmias in early stages of AMI than increased duration and dispersion of the corrected QT interval.

Species-Dependent Mechanisms of Cardiac Arrhythmia: A Cellular Focus

Although ventricular arrhythmia remains a leading cause of morbidity and mortality, available antiarrhythmic drugs have limited efficacy. Disappointing progress in the development of novel, clinically relevant antiarrhythmic agents may partly be attributed to discrepancies between humans and animal models used in preclinical testing. However, such differences are at present difficult to predict, requiring improved understanding of arrhythmia mechanisms across species. To this end, we presently review interspecies similarities and differences in fundamental cardiomyocyte electrophysiology and current understanding of the mechanisms underlying the generation of afterdepolarizations and reentry. We specifically highlight patent shortcomings in small rodents to reproduce cellular and tissue-level arrhythmia substrate believed to be critical in human ventricle. Despite greater ease of translation from larger animal models, discrepancies remain and interpretation can be complicated by incomplete knowledge of human ventricular physiology due to low availability of explanted tissue. We therefore point to the benefits of mathematical modeling as a translational bridge to understanding and treating human arrhythmia.

Automated Algorithm for J-Tpeak and Tpeak-Tend Assessment of Drug-Induced Proarrhythmia Risk

BACKGROUND

Prolongation of the heart rate corrected QT (QTc) interval is a sensitive marker of torsade de pointes risk; however it is not specific as QTc prolonging drugs that block inward currents are often not associated with torsade. Recent work demonstrated that separate analysis of the heart rate corrected J-Tpeakc (J-Tpeakc) and Tpeak-Tend intervals can identify QTc prolonging drugs with inward current block and is being proposed as a part of a new cardiac safety paradigm for new drugs (the "CiPA" initiative).

METHODS

In this work, we describe an automated measurement methodology for assessment of the J-Tpeakc and Tpeak-Tend intervals using the vector magnitude lead. The automated measurement methodology was developed using data from one clinical trial and was evaluated using independent data from a second clinical trial.

RESULTS

Comparison between the automated and the prior semi-automated measurements shows that the automated algorithm reproduces the semi-automated measurements with a mean difference of single-deltas <1 ms and no difference in intra-time point variability (p for all > 0.39). In addition, the time-profile of the baseline and placebo-adjusted changes are within 1 ms for 63% of the time-points (86% within 2 ms). Importantly, the automated results lead to the same conclusions about the electrophysiological mechanisms of the studied drugs.

CONCLUSIONS

We have developed an automated algorithm for assessment of J-Tpeakc and Tpeak-Tend intervals that can be applied in clinical drug trials. Under the CiPA initiative this ECG assessment would determine if there are unexpected ion channel effects in humans compared to preclinical studies. The algorithm is being released as open-source software.

TRIAL REGISTRATION

NCT02308748 and NCT01873950.

Joint influence of transmural heterogeneities and wall deformation on cardiac bioelectrical activity: A simulation study

The aim of this work is to investigate, by means of numerical simulations, the influence of myocardial deformation due to muscle contraction and relaxation on the cardiac repolarization process in presence of transmural intrinsic action potential duration (APD) heterogeneities. The three-dimensional electromechanical model considered consists of the following four coupled components: the quasi-static transversely isotropic finite elasticity equations for the deformation of the cardiac tissue; the active tension model for the intracellular calcium dynamics and cross-bridge binding; the anisotropic Bidomain model for the electrical current flow through the deforming cardiac tissue; the membrane model of ventricular myocytes, including stretch-activated channels. The numerical simulations are based on our finite element parallel solver, which employs Multilevel Additive Schwarz preconditioners for the solution of the discretized Bidomain equations and Newton-Krylov methods for the solution of the discretized non-linear finite elasticity equations. Our findings show that: (i) the presence of intrinsic transmural cellular APD heterogeneities is not fully masked by electrotonic current flow or by the presence of the mechanical deformation; (ii) despite the presence of transmural APD heterogeneities, the recovery process follows the activation sequence and there is no significant transmural repolarization gradient; (iii) with or without transmural APD heterogeneities, epicardial electrograms always display the same wave shape and discordance between the polarity of QRS complex and T-wave; (iv) the main effects of the mechanical deformation are an increase of the dispersion of repolarization time and APD, when computed over the total cardiac domain and over the endo- and epicardial surfaces, while there is a slight decrease along the transmural direction.

Global electrical heterogeneity: A review of the spatial ventricular gradient

The ventricular gradient, an electrocardiographic concept calculated by integrating the area under the QRS complex and T-wave, represents the degree and direction of myocardial electrical heterogeneity. Although the concept of the ventricular gradient was first introduced in the 1930s, it has not yet found a place in routine electrocardiography. In the modern era, it is relatively simple to calculate the ventricular gradient in three dimensions (the spatial ventricular gradient (SVG)), and there is now renewed interest in using the SVG as a tool for risk stratification of ventricular arrhythmias and sudden cardiac death. This manuscript will review the history of the ventricular gradient, describe its electrophysiological meaning and significance, and discuss its clinical utility.

Ventricular stimulus site influences dynamic dispersion of repolarization in the intact human heart

Spatial variation of restitution in relation to varying stimulus site is poorly defined in the intact human heart. Repolarization gradients were shown to be dependent on site of activation with epicardial stimulation promoting significant transmural gradients. Steep restitution slopes were predominant in the normal ventricle.

The spatial variation in restitution properties in relation to varying stimulus site is poorly defined. This study aimed to investigate the effect of varying stimulus site on apicobasal and transmural activation time (AT), action potential duration (APD) and repolarization time (RT) during restitution studies in the intact human heart. Ten patients with structurally normal hearts, undergoing clinical electrophysiology studies, were enrolled. Decapolar catheters were placed apex to base in the endocardial right ventricle (RV_endo) and left ventricle (LV_endo), and an LV branch of the coronary sinus (LV_epi) for transmural recording. S1–S2 restitution protocols were performed pacing RV_endo apex, LV_endo base, and LV_epi base. Overall, 725 restitution curves were analyzed, 74% of slopes had a maximum slope of activation recovery interval (ARI) restitution (S_max) > 1 (P < 0.001); mean S_max = 1.76. APD was shorter in the LV_epi compared with LV_endo, regardless of pacing site (30-ms difference during RV_endo pacing, 25-ms during LV_endo, and 48-ms during LV_epi; 50th quantile, P < 0.01). Basal LV_epi pacing resulted in a significant transmural gradient of RT (77 ms, 50th quantile: P < 0.01), due to loss of negative transmural AT-APD coupling (mean slope 0.63 ± 0.3). No significant transmural gradient in RT was demonstrated during endocardial RV or LV pacing, with preserved negative transmural AT-APD coupling (mean slope −1.36 ± 1.9 and −0.71 ± 0.4, respectively). Steep ARI restitution slopes predominate in the normal ventricle and dynamic ARI; RT gradients exist that are modulated by the site of activation. Epicardial stimulation to initiate ventricular activation promotes significant transmural gradients of repolarization that could be proarrhythmic.

Transmural, interventricular, apicobasal and anteroposterior action potential duration gradients are all essential to the genesis of the concordant and realistic T wave: A whole-heart model study

BACKGROUND

It has been reported that ventricular repolarization dispersion resulting from transmural, apicobasal and interventricular action potential duration (APD) gradients makes the T wave concordant with the QRS complex.

METHOD AND RESULTS

A whole-heart model integrating transmural, apicobasal, interventricular and anteroposterior APD gradients was used, and the corresponding electrocardiograms were simulated to study the influence of these APD gradients on the T-wave amplitudes. The simulation results showed that changing a single APD gradient (e.g., interventricular APD gradient alone) only made substantial changes to the T-wave amplitudes in a limited number of leads and was not able to generate T waves with amplitudes comparable with clinical findings in all leads. A combination of transmural, apicobasal and interventricular APD gradients could simulate T waves with amplitudes similar to clinical values in the limb leads only. Adding the anteroposterior APD gradient into the model greatly improved the consistency between the simulated T-wave amplitudes and the clinical values.

CONCLUSION

The simulation results support that the transmural, apicobasal, interventricular and the anteroposterior APD gradient are all essential to the genesis of the clinical T wave.

Ventricular repolarization markers for predicting malignant arrhythmias in clinical practice

Malignant cardiac arrhythmias which result in sudden cardiac death may be present in individuals apparently healthy or be associated with other medical conditions. The way to predict their appearance represents a challenge for the medical community due to the tragic outcomes in most cases. In the last two decades some ventricular repolarization (VR) markers have been found to be useful to predict malignant cardiac arrhythmias in several clinical conditions. The corrected QT, QT dispersion, Tpeak-Tend, Tpeak-Tend dispersion and Tp-e/QT have been studied and implemented in clinical practice for this purpose. These markers are obtained from 12 lead surface electrocardiogram. In this review we discuss how these markers have demonstrated to be effective to predict malignant arrhythmias in medical conditions such as long and short QT syndromes, Brugada syndrome, early repolarization syndrome, acute myocardial ischemia, heart failure, hypertension, diabetes mellitus, obesity and highly trained athletes. Also the main pathophysiological mechanisms that explain the arrhythmogenic predisposition in these diseases and the basis for the VR markers are discussed. However, the same results have not been found in all conditions. Further studies are needed to reach a global consensus in order to incorporate these VR parameters in risk stratification of these patients.

Action potential duration gradients in the heart ventricles and the cardiac electric field during ventricular repolarization (a model study)

BACKGROUND

We simulated contributions of transmural, apicobasal, anteroposterior and interventricular action potential duration (APD) gradients to the body surface potential distribution (BSPD) with constant or varied magnitudes of the transmural and apicobasal gradients.

METHODS

Simulations were done in the framework of the discrete computer model of the rabbit heart ventricles on the basis of realistic activation sequence and APDs. The APD gradients were set constant at 20 ms or varied in the range of ±80 ms.

RESULTS

The apicobasal, transmural and interventricular APD gradients of 20 ms produced similar BSPDs, whereas the BSPD inversion was caused by the inverted apicobasal or transmural 80 ms gradients. The transmural APD gradient produced transversal and mainly apicobasal T-wave vectors due to wall curvature and cancellation effects. The "normal" transversal and apicobasal repolarization gradients were decreased and increased by activation sequence, respectively.

CONCLUSION

The different APD gradients contributed consistently to the development of BSPD.

The multiple electrocardiographic manifestations of ventricular repolarization memory

T wave “memory” is a peculiar variety of cardiac remodeling caused by a transient change in the course of ventricular depolarization (due to ventricular pacing, rate-dependent intraventricular block, ventricular preexcitation or tachyarrhythmias with wide QRS complexes). It is usually manifested by inverted T waves that appears when normal ventricular activation is restored. This phenomenon is cumulative and occurs earlier if the ventricular myocardium has previously been exposed to the same conditioning stimuli. In this article the different conditions giving rise to “classical” T wave memory development are reviewed and also “another” type of T wave memory is described. It is also shown that cardiac memory may induce not only negative (pseudo-primary) T waves but also a reversal of primary and pseudo-primary T waves leading to “normalization” of ventricular repolarization. The knowledge of these dissimilar consequences of T wave memory is essential to assess the characteristics of ventricular repolarization.

The measurement of the QT interval

The evaluation of every electrocardiogram should also include an effort to interpret the QT interval to assess the risk of malignant arrhythmias and sudden death associated with an aberrant QT interval. The QT interval is measured from the beginning of the QRS complex to the end of the T-wave, and should be corrected for heart rate to enable comparison with reference values. However, the correct determination of the QT interval, and its value, appears to be a daunting task. Although computerized analysis and interpretation of the QT interval are widely available, these might well over- or underestimate the QT interval and may thus either result in unnecessary treatment or preclude appropriate measures to be taken. This is particularly evident with difficult T-wave morphologies and technically suboptimal ECGs. Similarly, also accurate manual assessment of the QT interval appears to be difficult for many physicians worldwide. In this review we delineate the history of the measurement of the QT interval, its underlying pathophysiological mechanisms and the current standards of the measurement of the QT interval, we provide a glimpse into the future and we discuss several issues troubling accurate measurement of the QT interval. These issues include the lead choice, U-waves, determination of the end of the T-wave, different heart rate correction formulas, arrhythmias and the definition of normal and aberrant QT intervals. Furthermore, we provide recommendations that may serve as guidance to address these complexities and which support accurate assessment of the QT interval and its interpretation.

Sex differences in the mechanisms underlying long QT syndrome

Sexual dimorphism is a well-established phenomenon, but its degree varies tremendously among species. Since the early days of Einthoven's development of the three-lead galvanometer ECG, we have known there are marked differences in QT intervals of men and women. It required over a century to appreciate the profound implications of sex-based electrophysiological differences in QT interval on the panoply of sex differences with respect to arrhythmia risk, drug sensitivity, and treatment modalities. Little is known about the fundamental mechanism responsible for sex differences in electrical substrate of the human heart, in large part due to the lack of tissue availability. Animal models are an important research tool, but species differences in the sexual dimorphism of the QT interval, the ionic currents underlying the cardiac repolarization, and effects of sex steroids make it difficult to interpolate animal to human sex differences. In addition, in some species, different strains of the same animal model yield conflicting data. Each model has its strengths, such as ease of genetic manipulation in mice or size in dogs. However, many animals do not reproduce the sexual dimorphism of QT seen in humans. To match sex linked prolongation of QT interval and arrhythmogenic phenotype, the current data suggest that the rabbit may be best suited to provide insight into sex differences in humans. In the future, emerging technologies such as induced pluripotent stem cell derived cardiac myocyte systems may offer the opportunity to study sex differences in a controlled hormonal situation in the context of a sex specific human model system.

Rare giant T-wave inversions associated with myocardial stunning: report of 2 cases

Prominent T-wave inversions are well recognized electrocardiographic signs that can occur in acute myocardial infarction (AMI). However, the giant negative T waves may be associated with myocardial stunning without AMI.This case report describes 2 patients without AMI who developed rare giant T-wave inversions measuring up to 35 mm in depth and QT prolongation after admission to hospital. While 1 patient presented with acute pulmonary edema, the other patient presented with severe chest pain at rest and transient ST elevation.The giant T-wave inversion with QT prolongation may be caused by myocardial stunning due to the triple vessel diseases and elevated wall stress, high-end diastolic pressure and decreased coronary arterial flow during pulmonary edema in the first patient. The giant T-wave inversion with QT prolongation in the second patient may be caused by myocardial stunning due to the left anterior descending artery spasm (transient ST elevation) leading to transient total occlusion of left anterior descending artery. Percutaneous coronary intervention was successfully undergone for both patients. The patients remained well.The electrophysiologic mechanism responsible for giant T-wave inversion with QT prolongation is presently unknown. The two cases demonstrate that the rare giant negative T waves may be associated with myocardial stunning without AMI.

Electrocardiographic T wave and its relation with ventricular repolarization along major anatomical axes

BACKGROUND

The genesis of the electrocardiographic T wave is incompletely understood and subject to controversy. We have correlated the ventricular repolarization sequence with simultaneously recorded T waves.

METHODS AND RESULTS

Nine pig hearts were Langendorff-perfused (atrial pacing, cycle length 650 ms). Local activation and repolarization times were derived from unipolar electrograms sampling the ventricular myocardium. Dispersion of repolarization time was determined along 4 anatomic axes: left ventricle (LV)-right ventricle (RV), LV:apico-basal, LV:anterior-posterior, and LV:transmural. The heart was immersed in a fluid-filled bucket containing 61 electrodes to determine Tp (Tpeak in lead of maximum integral), TpTe (Tp to Tend), and TpTe_total (first Tpeak in any lead to last Tend in any lead). Repolarization was nonlinearly distributed in time. RT25 (time at which 25% of sites were repolarized, 288±26 ms) concurred with Tp. TpTe was 38±8 ms, and TpTe_total was 75±9 ms. TpTe_total correlated with dispersion of repolarization time in the entire heart (73±18 ms), but not with dispersion of repolarization times along individual axes (LV-RV, 66±17 ms; LV:apico-basal, 51±18 ms; LV:anterior-posterior, 51±27 ms; mean LV:transmural, 14±7 ms; all n=9).

CONCLUSIONS

We provide a correlation between local repolarization and T wave in a pseudo-ECG. Repolarization differences along all anatomic axes contribute to the T wave. TpTe_total represents total dispersion of repolarization. At Tp, ≈25% of ventricular sites have been repolarized.

Influence of aging and chronic heart failure on temporal dispersion of myocardial repolarization

Background and purpose:

QT and T_peak-T_end (Te) intervals are associated with sudden cardiac death in patients with chronic heart failure (CHF). We studied age-dependent influence on short-term temporal dispersion of these two variables in patients with postischemic CHF.

Method:

We grouped 75 CHF and 53 healthy control subjects into three age subsets: ≤50 years, >50 years and ≤65 years, and >65 years. We then calculated the following indices: QT and Te variability index (QTVI and TeVI), the ratio between the short-term variability (STV) of QT or Te, and the STV of resting rate (RR) (QT/RR STV and Te/RR STV).

Results:

In all different age subgroups, patients with CHF showed a higher level of QTVI than age-matched control subjects (≤50 years: P < 0.0001; >50 years and ≤65 years: P < 0.05; >65 years: P < 0.05). Patients with CHF < 50 years old also had all repolarization variability indices higher than normal age-matched controls (TeVI, P < 0.05; QT/RR STV, P < 0.05; Te/RR STV, P < 0.05), whereas we did not find any difference between the two older classes of subjects. Both QTVI (r²: 0.178, P < 0.05) and TeVI (r²: 0.433, P < 0.001) were positively related to age in normal subjects, even if the first correlation was weaker than the second one.

Conclusion:

Our data showed that QTVI could be used in all ages to evaluate repolarization temporal liability, whereas the other indices are deeply influenced by age. Probably, the age-dependent increase in QTVI was more influenced by a reduction of RR variability reported in older normal subjects.

What does the T(peak)-T(end) interval reflect? An experimental and model study

BACKGROUND

It is unclear whether the Tpeak-Tend interval is an index of the transmural or the total dispersion of repolarization.

METHODS

We examined the Tpeak-Tend interval using a computer model of the rabbit heart ventricles based on experimentally measured transmural, apicobasal, and interventricular gradients of action potential duration.

RESULTS

Experimentally measured activation-recovery intervals increased from apex to base, from the left ventricle to the right ventricle, and in the apical portion of the left ventricle from epicardium to endocardium and from the right side of septum to the left side. The simulated Tpeak corresponded to the earliest end of repolarization, whereas the Tend corresponded to the latest end of repolarization. The different components of the global repolarization dispersion were discerned by simulation.

CONCLUSIONS

The Tpeak-Tend interval corresponds to the global dispersion of repolarization with distinct contributions of the apicobasal and transmural action potential duration gradients and apicobasal difference in activation times.

The role of HCN channels in ventricular repolarization

Hyperpolarization-activated cyclic nucleotide gated (HCN) channels pass a cationic current (I(h)/I(f)) that crucially contributes to the slow diastolic depolarization (SDD) of sinoatrial pacemaker cells and, hence, is a key determinant of cardiac automaticity and the generation of the heartbeat. However, there is growing evidence that HCN channels are not restricted to the spontaneously active cells of the sinoatrial node and the conduction system but are also present in ventricular cardiomyocytes that produce an action potential lacking SDD. This observation raises the question of the principal function(s) of HCN channels in working myocardium. Our recent analysis of an HCN3-deficient (HCN3-/-) mouse line has shed new light on this central question. We propose that HCN channels contribute to the ventricular action potential waveform, specifically during late repolarization. In this review, we outline this new concept.

Computational modelling of electrocardiograms: repolarisation and T-wave polarity in the human heart

For more than a century, electrophysiologists, cardiologists and engineers have studied the electrical activity of the human heart to better understand rhythm disorders and possible treatment options. Although the depolarisation sequence of the heart is relatively well characterised, the repolarisation sequence remains a subject of great controversy. Here, we study regional and temporal variations in both depolarisation and repolarisation using a finite element approach. We discretise the governing equations in time using an unconditionally stable implicit Euler backward scheme and in space using a consistently linearised Newton-Raphson-based finite element solver. Through systematic parameter-sensitivity studies, we establish a direct relation between a normal positive T-wave and the non-uniform distribution of the controlling parameter, which we have termed refractoriness. To establish a healthy baseline model, we calibrate the refractoriness using clinically measured action potential durations at different locations in the human heart. We demonstrate the potential of our model by comparing the computationally predicted and clinically measured depolarisation and repolarisation profiles across the left ventricle. The proposed framework allows us to explore how local action potential durations on the microscopic scale translate into global repolarisation sequences on the macroscopic scale. We anticipate that our calibrated human heart model can be widely used to explore cardiac excitation in health and disease. For example, our model can serve to identify optimal pacing sites in patients with heart failure and to localise optimal ablation sites in patients with cardiac fibrillation.

Apicobasal gradient of left ventricular myocardial edema underlies transient T-wave inversion and QT interval prolongation (Wellens' ECG pattern) in Tako-Tsubo cardiomyopathy

BACKGROUND

Tako-Tsubo cardiomyopathy (TTC) presents with chest pain, ST-segment elevation followed by T-wave inversion and QT interval prolongation (Wellens' electrocardiographic [ECG] pattern), and left ventricular dysfunction, which may mimic an acute coronary syndrome.

OBJECTIVE

To assess the pathophysiologic basis of the Wellens' ECG pattern in TTC by characterization of underlying myocardial changes by using cardiac magnetic resonance (CMR).

METHODS

The study population included 20 consecutive patients with TTC (95% women; mean age 65.3 ± 10.4 years) who underwent CMR studies both in the initial phase and after 3-month follow-up by using a protocol that included cine images, T2-weighted sequences for myocardial edema, and post-contrast sequences for late gadolinium enhancement. Quantitative ECG indices of repolarization, such as maximal amplitude of negative T waves, sum of the amplitudes of negative T waves, and maximum corrected QT interval (QTc max), were correlated to CMR findings.

RESULTS

At the time of initial CMR study, there was a significant linear correlation between the apicobasal ratio of T2-weighted signal intensity for myocardial edema and the maximal amplitude of negative T waves (ρ = 0.498; P = .02), sum of the amplitudes of negative T waves (ρ = 0.483; P = .03), and maximum corrected QT interval (ρ = 0.520; P = .02). Repolarization indices were unrelated to either late gadolinium enhancement or quantitative cine parameters. Wellens' ECG abnormalities and myocardial edema showed a parallel time course of development and resolution on initial and follow-up CMR studies.

CONCLUSIONS

Our study results show that the ischemic-like Wellens' ECG pattern in TTC coincides and quantitatively correlates with the apicobasal gradient of myocardial edema as evidenced by using CMR. Dynamic negative T waves and QTc prolongation are likely to reflect the edema-induced transient inhomogeneity and dispersion of repolarization between apical and basal left ventricular regions.

Autonomic nerve activity and the short-term variability of the Tpeak-Tend interval in dogs with pacing-induced heart failure

BACKGROUND

In congestive heart failure (CHF), autonomic nervous system (ANS) activity is known to modulate arrhythmic risk through its effects on myocardial repolarization. An increased interval between the peak and the end of the T wave (T(peak)-T(end)) has been reported to increase the incidence of sudden cardiac death. However, the ANS influence on the T(peak)-T(end) interval remains unclear.

OBJECTIVE

We directly measured ANS nerve activity in ambulatory dogs with pacing-induced CHF to test the hypothesis that ANS activity modulates the T(peak)-T(end) variability index (T(peak)-T(end)VI), the short-term variability of the T(peak)-T(end) interval obtained on 30 beats (T(peak)-T(end)STV(30)), and the short-term variability of the T(peak)-T(end) interval obtained on 5-minute ECG recording (T(peak)-T(end)STV(T)).

METHODS

By using data previously recorded in 6 ambulatory dogs before and after pacing-induced CHF, we assessed ANS activity recorded with an implanted radiotransmitter that monitored integrated left stellate ganglion nervous activity (iSGNA), integrated vagus nerve activity (iVNA), and electrocardiogram (ECG). We selected for analysis 36 segments recorded at baseline and 36 after pacing-induced CHF with similar iSGNA.

RESULTS

During CHF, T(peak)-T(end)STV(30) (P<.001) and T(peak)-T(end)STV(T) (P<.05) were significantly higher than those at baseline. The multiple linear mixed regression analysis disclosed a significant positive correlation between iSGNA and T(peak)-T(end)STV(T) (baseline: β 2.92, P<.001; CHF: β 1.13, P<.001) and a significant negative correlation between iVNA and T(peak)-T(end)STV(T) (baseline: β-6.74, P<.001; CHF: β-1.42, P< .001).

CONCLUSIONS

In a canine model of pacing-induced CHF, iSGNA correlates positively while iVNA correlates negatively with T(peak)-T(end)STV(T). These findings suggest that SGNA increases while VNA decreases the dispersion of ventricular repolarization in ambulatory dogs with CHF.