Estimates of repolarization and its dispersion from electrocardiographic measurements: direct epicardial assessment in the canine heart

Age-related increases in cardiac excitability, refractoriness and impulse conduction favor arrhythmogenesis in male rats

The effects of excitability, refractoriness, and impulse conduction have been independently related to enhanced arrhythmias in the aged myocardium in experimental and clinical studies. However, their combined arrhythmic effects in the elderly are not yet completely understood. Hence, the aim of the present work is to relate relevant cardiac electrophysiological parameters to enhanced arrhythmia vulnerability in the in vivo senescent heart. We used multiple-lead epicardial potential mapping in control (9-month-old) and aged (24-month-old) rat hearts. Cardiac excitability and refractoriness were evaluated at numerous epicardial test sites by means of the strength-duration curve and effective refractory period, respectively. During sinus rhythm, durations of electrogram intervals and waves were prolonged in the senescent heart, compared with control, demonstrating a latency in tissue activation and recovery. During ventricular pacing, cardiac excitability, effective refractory period, and dispersion of refractoriness increased in the aged animal. This scenario was accompanied by impairment of impulse propagation. Moreover, both spontaneous and induced arrhythmias were increased in senescent cardiac tissue. Histopathological evaluation of aged heart specimens revealed connective tissue deposition and perinuclear myocytolysis in the atria, while scattered microfoci of interstitial fibrosis were mostly present in the ventricular subendocardium. This work suggests that enhanced arrhythmogenesis in the elderly is a multifactorial process due to the joint increase in excitability and dispersion of refractoriness in association with enhanced conduction inhomogeneity. The knowledge of these electrophysiological changes will possibly contribute to improved prevention of the age-associated increase in cardiac arrhythmias.

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.

Functional and pharmacological analysis of cardiomyocytes differentiated from human peripheral blood mononuclear-derived pluripotent stem cells

Advances in induced pluripotent stem cell (iPSC) technology have set the stage for routine derivation of patient- and disease-specific human iPSC-cardiomyocyte (CM) models for preclinical drug screening and personalized medicine approaches. Peripheral blood mononuclear cells (PBMCs) are an advantageous source of somatic cells because they are easily obtained and readily amenable to transduction. Here, we report that the electrophysiological properties and pharmacological responses of PBMC-derived iPSC CM are generally similar to those of iPSC CM derived from other somatic cells, using patch-clamp, calcium transient, and multielectrode array (MEA) analyses. Distinct iPSC lines derived from a single patient display similar electrophysiological features and pharmacological responses. Finally, we demonstrate that human iPSC CMs undergo acute changes in calcium-handling properties and gene expression in response to rapid electrical stimulation, laying the foundation for an in-vitro-tachypacing model system for the study of human tachyarrhythmias.

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.

The application of root mean square electrocardiography (RMS ECG) for the detection of acquired and congenital long QT syndrome

BACKGROUND

Precise measurement of the QT interval is often hampered by difficulty determining the end of the low amplitude T wave. Root mean square electrocardiography (RMS ECG) provides a novel alternative measure of ventricular repolarization. Experimental data have shown that the interval between the RMS ECG QRS and T wave peaks (RTPK) closely reflects the mean ventricular action potential duration while the RMS T wave width (TW) tracks the dispersion of repolarization timing. Here, we tested the precision of RMS ECG to assess ventricular repolarization in humans in the setting of drug-induced and congenital Long QT Syndrome (LQTS).

METHODS

RMS ECG signals were derived from high-resolution 24 hour Holter monitor recordings from 68 subjects after receiving placebo and moxifloxacin and from standard 12 lead ECGs obtained in 97 subjects with LQTS and 97 age- and sex-matched controls. RTPK, QTRMS and RMS TW intervals were automatically measured using custom software and compared to traditional QT measures using lead II.

RESULTS

All measures of repolarization were prolonged during moxifloxacin administration and in LQTS subjects, but the variance of RMS intervals was significantly smaller than traditional lead II measurements. TW was prolonged during moxifloxacin and in subjects with LQT-2, but not LQT-1 or LQT-3.

CONCLUSION

These data validate the application of RMS ECG for the detection of drug-induced and congenital LQTS. RMS ECG measurements are more precise than the current standard of care lead II measurements.

Repolarization heterogeneity and rate dependency in a canine rapid pacing model of heart failure

BACKGROUND

Repolarization heterogeneity and rate dependency have long been established as factors contributing to arrhythmogenic risk. However, there are conflicting observations regarding the nature and extent of ventricular repolarization heterogeneity that complicate understanding of arrhythmogenic mechanisms. To explore these disparate findings, we studied ventricular repolarization heterogeneity and rate dependency in a canine, rapid pacing model of heart failure.

METHODS AND RESULTS

We studied ventricular repolarization heterogeneity and rate dependency in 10 canine hearts (5 normal and 5 after 1 month of rapid pacing at 240 beats per minute) by analyzing 64 body surface electrocardiograms, 64 epicardial, and 190 intramural plunge electrograms. We estimated mean ventricular depolarization and repolarization times from R- and T-wave peaks of the root-mean-square electrocardiogram (body surface) and local depolarization and repolarization times using activation-recovery interval (ARI) methods from recordings obtained during a range of fixed rate pacing. In addition, we estimated local epicardial and transmural gradients of ARIs to assess cardiac locations of greatest spatial repolarization heterogeneity. We compared changes in repolarization at different rates between normal and heart failure hearts. Findings documented prolongation of repolarization, repolarization rate dependency, and increased repolarization gradients in the heart failure hearts compared with control as observed from body surface, epicardial, and transmural measurements. Maximum local epicardial and intramural ARI gradients were comparable both in heart failure and control hearts. Intramural ARI distributions tended to be more irregular in the heart failure hearts compared with the systematic epicardium to endocardium ARI increase observed in control animals.

CONCLUSIONS

This study documented prolongation of repolarization, increase in both epicardial and transmural repolarization gradients, and irregularity of transmural distribution in a rapid pacing canine model of heart failure compared with control animals. The findings support previously published results of increased repolarization heterogeneity and repolarization prolongation observed in rapid pacing models of heart failure. New findings are the irregularity of transmural heterogeneity and the ability of noninvasive root-mean-square electrocardiogram R-T intervals to estimate mean ventricular repolarization duration in the setting of rapid pacing models of heart failure. These findings suggest increased arrhythmogenic risk in this model and potentially in patients with heart failure.

A reliability analysis of cardiac repolarization time markers

Only a limited number of studies have addressed the reliability of extracellular markers of cardiac repolarization time, such as the classical marker RT(eg) defined as the time of maximum upslope of the electrogram T wave. This work presents an extensive three-dimensional simulation study of cardiac repolarization time, extending the previous one-dimensional simulation study of a myocardial strand by Steinhaus [B.M. Steinhaus, Estimating cardiac transmembrane activation and recovery times from unipolar and bipolar extracellular electrograms: a simulation study, Circ. Res. 64 (3) (1989) 449]. The simulations are based on the bidomain - Luo-Rudy phase I system with rotational fiber anisotropy and homogeneous or heterogeneous transmural intrinsic membrane properties. The classical extracellular marker RT(eg) is compared with the gold standard of fastest repolarization time RT(tap), defined as the time of minimum derivative during the downstroke of the transmembrane action potential (TAP). Additionally, a new extracellular marker RT90(eg) is compared with the gold standard of late repolarization time RT90(tap), defined as the time when the TAP reaches 90% of its resting value. The results show a good global match between the extracellular and transmembrane repolarization markers, with small relative mean discrepancy (<or=1.6%) and high correlation coefficients (>or=0.92), ensuring a reasonably good global match between the associated repolarization sequences. However, large local discrepancies of the extracellular versus transmembrane markers may ensue in regions where the curvature of the repolarization front changes abruptly (e.g. near front collisions) or is negligible (e.g. where repolarization proceeds almost uniformly across fiber). As a consequence, the spatial distribution of activation-recovery intervals (ARI) may provide an inaccurate estimate of (and weakly correlated with) the spatial distribution of action potential durations (APD).

Experimental measures of ventricular activation and synchrony

BACKGROUND

A widened QRS complex as a primary indication for cardiac resynchronization therapy (CRT) for heart failure patients has been reported to be an inconsistent indicator for dyssynchronous ventricular activation. The purpose of this study was to conduct a detailed experimental investigation of total ventricular activation time (TVAT), determine how to measure it accurately, and compare it to the commonly used measure of QRS width. In addition, we investigated a measure of electrical synchrony and determined its relationship to the duration of ventricular activation.

METHODS

Unipolar electrograms (EGs) were recorded from the myocardial volume using plunge needle electrodes, from the epicardial surface using "sock" electrode arrays, and from the surface of an electrolytic torso-shaped tank. EGs were analyzed to determine a root mean square (RMS)-based measure of ventricular activation and electrical ventricular synchrony.

RESULTS

The RMS-based technique provided an accurate means of measuring TVAT from unipolar EGs recorded from the heart, the entire tank surface, or the precordial leads. In normal canine hearts, a quantification of ventricular electrical synchrony (VES) for normal ventricular activation showed that the ventricles activate, on average, within 3 ms of each other with the left typically activating first.

CONCLUSION

Conclusions from this study are: (1) ventricular activation was reflected accurately by the RMS width obtained from direct cardiac measurements and from precordial leads on the tank surface and (2) VES was not strongly correlated with TVAT.

Methods for accurate measures of total ventricular activation time

The purpose of this study was to determine improved measures of total ventricular activation time for the diagnosis and treatment of patients undergoing cardiac resynchronization therapy (CRT). This work investigates the accuracy of a root mean square (RMS) based QRS width computed from unipolar electrograms (EGs) measured in heart for representing true total ventricular activation time (TVAT). The study also investigated the use subsets of EGs obtained from the endocardial and epicardial surfaces as indicators of TVAT. Transmural needle electrodes (96) were used to obtain 960 EGs from six normal isolated canine hearts. RMS-based QRS-widths from the endocardial and epicardial surfaces and volume were compared to the TVAT measured from all 960 EGs. No statistically significant differences were found in RMS-based QRS-widths obtained from all three sets of electrograms when compared with true TVAT. Activation times obtained from endocardial and epicardial surfaces were found to be poor indicators of true TVAT. The results support the use of RMS techniques for providing more accurate measures of TVAT.

Scatter in repolarization timing predicts clinical events in post-myocardial infarction patients

BACKGROUND

Increased spatial and temporal dispersion of repolarization contributes to ventricular arrhythmogenesis. Beat-to-beat fluctuations in T-wave timing are thought to represent such dispersion and may predict clinical events.

OBJECTIVE

The purpose of this study was to assess whether a novel noninvasive measure of beat-to-beat instability in T-wave timing would provide additive prognostic information in post-myocardial infarction patients.

METHODS

We studied 678 patients from 12 hospitals with 32-lead 5-minute electrocardiogram recordings 6-8 weeks after myocardial infarction. Custom software identified R wave-to-T wave intervals (RTIs) and diastolic intervals (DIs). Repolarization scatter (RTI:DI(StdErr)) was then calculated as the standard error about the RTI:DI regression line. In addition, left ventricular ejection fraction (LVEF), short-term heart rate variability (HRV) parameters, and QT variability index were measured. Patients were followed for the composite endpoint of death or life-threatening ventricular arrhythmia.

RESULTS

After a mean follow-up of 63 months, 134 patients met the composite endpoint. An RTI:DI(StdErr) >5.50 ms was associated with a 210% increase in arrhythmias or deaths (P <.001). After adjusting for LVEF, RTI:DI(StdErr) remained an independent predictor (P <.001). RTI:DI(StdErr) was also independent of short-term HRV parameters and the QT variability index.

CONCLUSIONS

Increased repolarization scatter, a measure of high-frequency, cycle-length-dependent repolarization instability, predicts poor outcomes in patients after myocardial infarction.

The dominant T wave

This contribution discusses the dominant T wave, a concept providing an overall view on the repolarization currents of the ventricular myocardium. The dominant T wave is a weighted mean of the T waves on the body surface. It can be estimated from the matrix of sampled observed potentials. The timing of its peak reveals the mean of the repolarization times of the involved transmembrane potentials. Its amplitude is independent of the volume conductor properties of the tissues surrounding the heart.

QT and JT dispersion in the drug-induced long QT syndrome in anaesthetized rabbits is accurately detected by a three-lead surface ECG measurement

INTRODUCTION

QT dispersion (QTd) can be measured from three leads of the ECG in patients with myocardial ischemia. However, whether QT and JT dispersion (QTd, JTd) can be calculated from a three-lead of the ECG in drug-induced long QT syndrome (LQTS) in animals remains elusive. Therefore, we determined to what extent a three-lead measurement of the surface ECG accurately detects dispersion of QT and JT in comparison with multi-lead assessments in anaesthetized rabbits, challenged with methoxamine and additionally infused intravenously with solvent or dofetilide.

METHODS

Using several ECG leads in anaesthetized rabbits challenged intravenously with an alpha(1)-adrenoceptor agonist methoxamine, we assessed the QT and JT interval, as well as QT and JT dispersion, at baseline and in response to solvent or dofetilide (0.02 or 0.04 mg/kg/min iv for 60 min), an I(Kr) blocker. For that purpose, we recorded and analyzed the surface ECG and assessed QT and JT dispersion by four methods: (1) 12-lead ECG; (2) six precordial leads (V1-V6); (3) three leads most likely to contribute to the dispersion (aVF, V1, and V4); (4) three quasi-orthogonal leads (aVF, I, and V2). QT and JT dispersion were significantly lower in 6- and 3-lead measurements than in 12-lead measurement, both at baseline and during infusion of solvent or dofetilide. At 5 and 10 min of infusion, dofetilide at 0.02 or 0.04 mg/kg/min iv markedly increased QT and JT dispersion by 100% to 500% in all four ECG lead combinations. This dose regimen of dofetilide markedly prolonged QT and JT intervals in lead II, and was associated with high incidences of polymorphous ventricular tachycardia (PVT: 30% at 0.02 mg/kg/min; 100% at 0.04 mg/kg/min) and of ventricular fibrillation (VF: 17% with 0.02 mg/kg/min; 58% with 0.04 mg/kg/min).

CONCLUSIONS

Our present study shows that the measurement of QT and JT dispersion in three surface ECG leads only (aVF, I, V2 or aVF, V1 V4), instead of 12 ECG leads, is an appropriate approach to assess drug-induced heterogeneity or dispersion of ventricular repolarization in anaesthetized rabbits, both at baseline and during arrhythmogenic sensitization with methoxamine and challenged with dofetilide.

Cycle length sequence dependent repolarization dynamics

Cardiac repolarization, particularly its heterogeneity, is known to play a significant role in arrhythmogenesis. Steepness of cardiac restitution, or the cycle length dependency of repolarization, has also been implicated as a condition that favors occurrence of reentrant arrhythmias. However, most assessments of heterogeneity and restitution are based on static observations and do not directly account for the extent or heterogeneity of dynamic changes. The uncertainty and unpredictability of arrhythmias and the difficulty of identifying patients most at risk may possibly be explained by the lack of consideration of dynamic changes of repolarization, its heterogeneity and time varying restitution. In this brief article, we show the global changes in repolarization that occur in normal canine hearts in response to programmed cycle length sequences. Specifically, we show the beat-to-beat tracking of repolarization during rapid (step) changes in cycle length as well as linear up and down (sawtooth) changes, and random cycle length sequences. The measurement and robust characterization of the dynamic repolarization response to specific cycle length sequences may offer an opportunity to characterize the substrate for arrhythmias to a greater extent than has been possible to date. Although there is no guarantee that characterization of repolarization dynamics will provide definitive means to identify patients at risk, such assessment will, at a minimum, put into perspective the role that repolarization dynamics may play in detecting states of increased arrhythmia risk. Another potential use of these techniques is in the assessment of repolarization in patients undergoing EP testing, pharmacological therapies or during other provocative testing.

The Dominant T Wave and Its Significance

INTRODUCTION

The shapes of the T waves as observed in different leads placed on the thorax are very similar. The dominant T wave is introduced as a means to characterize this general signal shape. Its relationship to the transmembrane potentials of cardiac myocytes is discussed.

METHODS AND RESULTS

The source description of a biophysical model that previously was shown to yield realistic T waveforms was analyzed in order to exploit its relation to the transmembrane potentials of the cardiac myocytes at the surface bounding the myocardium. The product of this analysis is the dominant T wave: a waveform that describes the slope of the transmembrane potential. It is shown that the dominant T wave can be estimated easily from the matrix of sampled lead potentials. The timing of its peak reveals the mean of the repolarization times of the involved transmembrane potentials. The amplitude of the peak is the maximum downward slope of the transmembrane potential. This amplitude is independent of the volume conductor effects of the tissues surrounding the heart. The estimate of the dominant T wave retains this property.

CONCLUSION

The dominant T wave reflects the derivative of the recovery phase of a generalized transmembrane potential. Its amplitude is independent of the volume conductor properties of the tissues surrounding the heart. This is a unique feature that greatly facilitates the interpretation and application of the other signal features of the dominant T wave.

Noninvasive indices of repolarization and its dispersion

In experimental studies using Langendorff perfused, isolated canine hearts immersed in a torso-shaped electrolytic tank we studied repolarization and its dispersion using direct epicardial measurements and newly derived, noninvasive body surface indices. Activation recovery intervals (ARIs) measured from 64 epicardial sites based on differences between activation times (ATs) and recovery times (RTs) provided direct measures of repolarization. The indirect, torso surface indices were derived from inflections of the root-mean-square (RMS) voltage of the torso tank surface electrocardiograms recorded simultaneously with the epicardial data. For cycle lengths ranging from 300 to 900 ms, and electrolyte temperatures ranging from 32 degrees C to 40 degrees C we calculated mean, variance, and range of ATs, RTs, and ARIs from the epicardium. From epicardial and torso surface RMS waveforms, we used times of R and T peaks and their differences to estimate mean ATs, RTs, and ARIs, respectively. The RMS T wave width as determined from the second derivative inflections on either side of the T peak served as an estimate of the dispersion of RTs. In parallel studies, we showed that the direct measures of repolarization and its dispersion were reflected in RMS waveforms generated from the epicardial electrograms themselves. In this study, we confirm that the torso and epicardial RMS waveforms reflect comparable information for estimating repolarization and its dispersion. Furthermore, the derived measures provide a method to assess mean ARIs and dispersion of RTs on a beat-to-beat basis and during abnormal (ectopic ventricular) activation sequences.