Automatic detection of spatial and dynamic heterogeneity of repolarization

Time-Warping Analysis of the T-Wave Peak-to-End Interval to Quantify Ventricular Repolarization Dispersion During Ischemia

Variations in the dispersion of ventricular repolarization can be quantified by T-wave time-warping based index, dw. However, the early phase of the T-wave can be affected by ST-segment changes during ischemia. We hypothesized that restricting dw to the T-wave peak-to-end ( Tpe) would circumvent this limitation while still quantifying variations in repolarization dispersion. A total of 101 ECG recordings from patients undergoing coronary occlusion, together with their control recordings, were analyzed. A series of dw values was calculated by quantifying the Tpe morphological variations between the T-waves at different occlusion stages and a baseline T-wave. We introduced a normalized version of dw, Rd, reflecting variations of dw during occlusion relative to control recordings ( Rd = 1 corresponds to the same level of variation). The dw series followed a gradually increasing trend with occlusion time, reaching median [range] Rd values of 9.44 [1.01, 80.74] at the occlusion end. Rd at occlusion end was significantly higher than threshold values of 1, 2, 5, and 10 in 94.1%, 85.11%, 64.4% and 48.5% of patients, respectively. The spatial lead-wise analysis of dw showed distinct distributions depending on the occluded artery, suggesting a relation with the ischemia location. The relative variation R with ischemia of index dw (9.4) is greater than that of the T-wave amplitude (7.7), Tpe interval (2.7) and T-wave width (3.0). In conclusion, dw tracks ischemic-induced variations in repolarization dispersion in a more robust manner than classical indexes, avoiding the impact of ST segment elevation/depression or early T-wave distortions, thus warranting further clinical studies.

Weighted Time Warping Improves T-wave Morphology Markers Clinical Significance

Background: T-wave (TW) morphology indices based on time-warping (d_w) have shown significant cardiovascular risk stratification value. However, errors in the location of TW boundaries may impact their prognostic power. Our aim was to test the hypothesis that a weighted time-warping function (WF) would reduce the sensitivity of d_w to these errors and improve their clinical significance. Methods: The WFs were proportional to (i) the reference TW (T), and (ii) the absolute value of its derivative (D). The index d_w was recalculated using these WFs, and its performance was compared to the unweighted control case (C) in four different scenarios: 1) robustness against simulated TW boundaries location errors; 2) ability to retain physiological information in an electrophysiological cardiac model; 3) ability to monitor blood potassium concentration changes ([K⁺]) in 29 hemodialysis (HD) patients; 4) and the sudden cardiac death (SCD) risk stratification value of the TW morphology restitution (TMR) index, derived from d_w, in 651 chronic heart failure (CHF) patients. Results and Discussion: The WFs led to a reduced sensitivity (R) of d_w to TW boundary location errors as compared to C (median R=0.19 and 0.22 and 0.35 for T, D and C, respectively). They also preserved the physiological relationship between d_w and repolarization dispersion changes at ventricular level. No improvements in [K⁺] tracking were observed for the HD patients (Pearsons median correlation [r] between [K⁺] and d_w was 0.86r0.90 for T, D and C). In CHF patients, the SCD risk stratification value of TMR was improved by applying T (hazard ratio, HAR, of 2.80), followed by D (HAR=2.32) and C (HAR=2.23). Conclusions and Significance: The proposed WFs, with T showing the best performance, increased the robustness of time-warping based markers against TW location errors preserving their physiological information content and boosting their SCD risk stratification value. Results from this work support the use of T when deriving d_w for future clinical applications.

Comparison of cardiogoniometry and ECG at rest versus myocardial perfusion scintigraphy

BACKGROUND

Cardiogoniometry (CGM) is a novel resting electrocardiac method based on computer-generated three-dimensional data derived from cardiac potentials. The purpose of this study was to determine CGM's and electrocardiography's (ECG) accuracy for detecting myocardial ischemia and/or lesions in comparison with stress/rest myocardial perfusion scintigraphy (single photon emission computer tomography [SPECT]).

METHOD

A cohort of consecutively enrolled patients (n = 100) with suspected or known coronary artery disease (mean age 67.8 years, 52% female) were examined by CGM and resting ECG before stress/rest myocardial scintigraphy.

RESULTS

Pathological scintigraphy findings at adenosine stress perfusion (ASP) and/or rest were conclusively identified in 21 patients. Diagnostic sensitivity was 71% for CGM and 24% for ECG, specificity was 70% for CGM and 95% for ECG. Reversible ischemia was diagnosed in 16 of 21 patients with pathological scintigraphy results. In this subgroup, sensitivity was 67% for CGM and 25% for ECG.

CONCLUSIONS

At rest, the sensitivity of a CGM significantly surmounts that of a standard 12-lead ECG for detection of isolated myocardial ischemia or myocardial lesions revealed by scintigraphy/SPECT; specificity is in a reasonable range. CGM's ease of use and its considerable agreement with the results of myocardial scintigraphy, suggests a possible role for patient screening in the primary care setting.

Vectorcardiographic recordings of the Q-T interval in a pediatric long Q-T syndrome population

Measurements of the Q-T interval are less reliable in children than in adults. Identification of superior diagnostic tools is warranted. This study aimed to investigate whether a vectorcardiogram (VCG) recorded from three orthogonal leads (X, Y, Z) according to Frank is superior to a 12-lead electrocardiogram (ECG) in providing a correct long Q-T syndrome (LQTS) diagnosis in children. This LQTS group consisted of 35 genetically confirmed carriers of mutations in the KCNQ1 (n = 29) and KCNH2 (n = 6) genes. The control group consisted of 35 age- and gender-matched healthy children. The mean age was 7 years in the LQTS group and 6.7 years in the control group (range, 0.5-16 years). The corrected Q-T interval (QT(c)) was measured manually (QT(man)) by one author (A.W.). The 12-lead ECG automatic measurements (QT(ECG)) and interpretation (QT(Interpret)) of QT(c) were performed with the Mac5000 (GE Medical System), and the VCG automatic measurements (QT(VCG)) were performed with the Mida1000, CoroNet (Ortivus AB, Sweden). By either method, a QT(c) longer than 440 ms was considered prolonged and indicative of LQTS. Of the 35 children with genetically confirmed LQTS, 30 (86 %) received a correct diagnosis using QT(VCG), 29 (82 %) using QT(man), 24 (69 %) using QT(ECG), and 17 (49 %) using QT(Interpret). Specificity was 0.80 for QT(VCG), 0.83 for QT(man), 0.77 for QT(ECG), and 0.83 for QT(Interpret). The VCG automatic measurement of QT(c) seems to be a better predictor of LQTS than automatic measurement and interpretation of 12-lead ECG.

Cardiogoniometric parameters for detection of coronary artery disease at rest as a function of stenosis localization and distribution

Cardiogoniometry (CGM), a spatiotemporal electrocardiologic 5-lead method with automated analysis, may be useful in primary healthcare for detecting coronary artery disease (CAD) at rest. Our aim was to systematically develop a stenosis-specific parameter set for global CAD detection. In 793 consecutively admitted patients with presumed non-acute CAD, CGM data were collected prior to elective coronary angiography and analyzed retrospectively. 658 patients fulfilled the inclusion criteria, 405 had CAD verified by coronary angiography; the 253 patients with normal coronary angiograms served as the non-CAD controls. Study patients--matched for age, BMI, and gender--were angiographically assigned to 8 stenosis-specific CAD categories or to the controls. One CGM parameter possessing significance (P < .05) and the best diagnostic accuracy was matched to one CAD category. The area under the ROC curve was .80 (global CAD versus controls). A set containing 8 stenosis-specific CGM parameters described variability of R vectors and R-T angles, spatial position and potential distribution of R/T vectors, and ST/T segment alterations. Our parameter set systematically combines CAD categories into an algorithm that detects CAD globally. Prospective validation in clinical studies is ongoing.

Two automatic QT algorithms compared with manual measurement in identification of long QT syndrome

BACKGROUND

Long QT syndrome (LQTS) is an inherited disorder that increases the risk of syncope and malignant ventricular arrhythmias, which may result in sudden death.

METHODS

We compared manual measurement by 4 observers (QT(manual)) and 3 computerized measurements for QT interval accuracy in the diagnosis of LQTS: 1. QT measured from the vector magnitude calculated from the 3 averaged orthogonal leads X, Y, and Z (QTVCG) and classified using the same predefined QTc cut-points for classification of QT prolongation as in manual measurements; 2. QT measured by a 12-lead electrocardiogram (ECG) program (QTECG) and subsequently classified using the same cut-points as in (1) above; 3. The same QT value as in (2) above, automatically classified by a 12-lead ECG program with thresholds for QT prolongation adjusted for age and sex (QTinterpret). The population consisted of 94 genetically confirmed carriers of KCNQ1 (LQT1) and KCNH2 (LQT2) mutations and a combined control group of 28 genetically confirmed noncarriers and 66 unrelated healthy volunteers.

RESULTS

QT(VCG) provided the best combination of sensitivity (89%) and specificity (90%) in diagnosing LQTS, with 0.948 as the area under the receiver operating characteristic curve. The evaluation of QT measurement by the 4 observers revealed a high interreader variability, and only 1 of 4 observers showed acceptable level of agreement in LQTS mutation carrier identification (kappa coefficient >0.75).

CONCLUSION

Automatic QT measurement by the Mida1000/CoroNet system (Ortivus AB, Danderyd, Sweden) is an accurate, efficient, and easily applied method for initial screening for LQTS.

[Clinical significance of dynamic QT-interval-analyses]

Dynamic parameters of ventricular repolarization as Holter derived parameters expressed as QT-interval adaptation to heart rate changes (QT/RR-slope) and QT-interval-variability are being more and more frequently used to identify patients with increased risk for ventricular arrhythmias. Steep QT-RR-slopes, reflecting inadequate adaptation of ventricular repolarization to heart rate changes, as well as increased QT-interval-variability, reflecting temporal inhomogeneity of ventricular repolarization duration, are frequently observed in patients at risk for sudden cardiac death. Additionally, there is strong evidence for significant alterations in the dynamics of action potential duration restitution in patients with structural heart disease. This review gives an up-to-date overview about the current research in methods of assessment and clinical relevance of dynamic parameters of ventricular repolarization.

Late Complications of Repair of Tetralogy of Fallot and Indications for Pulmonary Valve Replacement

With increasing follow-up of patients after surgical repair of tetralogy of Fallot, the long-term complications of chronic pulmonary regurgitation (PR), ventricular dilation, electrical inhomogeneity and myocardial scarring are becoming apparent. In this article we review the existing literature regarding the deleterious effects of chronic PR in these patients and the current data regarding the timing and mode of intervention.

QT dynamics and variability

Repolarization dynamics and variability are of increasing interest as Holter-derived parameters reflecting changes in myocardial vulnerability and contributing to increased risk of arrhythmic events and sudden death. Repolarization dynamics is usually defined as phenomenon describing and quantifying QT adaptation to changing heart rate. The analysis of QT-R-R slopes in long ECG recordings is one of the ways to evaluate repolarization dynamics. Increased QT-R-R slopes are frequently observed in patients at risk for cardiac death and arrhythmic events: postinfarction patients, long QT syndrome patients, patients with nonischemic cardiomyopathy as well as in patients taking drugs affecting repolarization. QT variability reflects beat-to-beat changes in repolarization duration and morphology and such changes can be quantified using a number of algorithms currently in various phases of development and validation. Increased QT variability is observed in several conditions with increased risk of arrhythmias. Recent data from MADIT II indicate that increased QT variability is a powerful predictor of arrhythmic events in postinfarction patients with left ventricular dysfunction. More studies are needed to determine further the potential clinical usefulness for diagnosing patients and for risk stratification purposes using both QT dynamics and QT variability methods, and compare these methods with exercise-induced T wave alternans.

T wave spectral variance for noninvasive identification of patients with idiopathic dilated cardiomyopathy prone to ventricular fibrillation even in the presence of bundle branch block or atrial fibrillation

Conventional methods using Holter ECG recordings for noninvasive risk stratification are limited in patients with idiopathic dilated cardiomyopathy (IDC) prone to ventricular fibrillation (VF) having atrial fibrillation (AF) or bundle branch block (BBB). We therefore investigated, whether spectral assessment of beat-to-beat alternations of repolarization is associated with VF in these patients. Twenty-four-hour Holter ECG recordings in 462 patients with IDC were used. The VF group comprised of 64 consecutive patients who survived cardiac arrest, the no VF group consisted of 398 consecutive patients without a history of malignant ventricular arrhythmia. One hundred patients with ischemic cardiomyopathy (ICM) served as a control group. In each patient, 1,024 consecutive T waves were aligned using cross correlation methods. Two-dimensional Fourier transform (2D FFT) used the data matrix of 1,024 consecutive 200-ms segments centered to the T wave peak. Power spectra of the 2D FFT revealed the frequency content of the T wave in the first dimension and the periodicity of this frequency content in the second dimension. The ratio between periodic frequency contents and the sum of nonperiodic and periodic frequency contents between 0.5 and 50 Hz is equal to the T wave spectral variance (TWSV) index. Thus, TWSV index = 0 would mean that all 1,024 T waves are identical and TWSV index = 1 would mean that the 1,024 T waves are totally variable. The TWSV index was significantly higher in the VF group (0.93 +/- 0.14) than in the no VF group (0.53 +/- 0.13, P < 0.01). The best cutoff between the VF and the no VF group was achieved by using a TWSV index of 0.75 (sensitivity = 89%, specificity = 78%). No significant differences were observed between patients with and without AF or with and without BBB, and between patients with IDC and ICM. Even in the presence of BBB or AF spectral assessment of T wave alternations by TWSV index using 2D FFT in Holter ECG recordings, allows the identification of patients with IDC at risk for VF.

Catecholamine-induced T-wave lability in congenital long QT syndrome: a novel phenomenon associated with syncope and cardiac arrest

OBJECTIVE

To determine the effects of phenylephrine and dobutamine on repolarization lability in patients with genotyped long QT syndrome (LQTS).

PATIENTS AND METHODS

Between December 1998 and August 2000, 23 patients with genotyped LQTS (13 LQT1, 7 LQT2, and 3 LQT3) and 16 controls underwent electrocardiographic stress testing at the Mayo Clinic in Rochester, Minn. Aperiodic repolarization lability was quantified from digitized electrocardiograms recorded during catecholamine stress testing with phenylephrine and dobutamine. T-wave lability was quantified as a root-mean-square of the differences between corresponding signal values of subsequent beats. The magnitude of aperiodic T-wave lability was quantified by using a newly derived T-wave lability index (TWLI).

RESULTS

The TWLI was significantly greater in patients with LQTS than in controls (0.0945 +/- 0.0517 vs 0.0445 +/- 0.0123; P < .003). Marked T-wave lability (TWLI > or = 0.095) was detected in all 3 LQTS genotypes (10/23) but in no controls (P < .003). There was no correlation between the TWLI and the baseline corrected QT interval. All high-risk patients having either a history of out-of-hospital cardiac arrest or syncope had a TWLI of 0.095 or greater.

CONCLUSIONS

Beat-to-beat nonalternating T-wave lability occurs in LQT1, LQT2, and LQT3 patients during catecholamine provocation and is associated with a history of prior cardiac events. The quantification of this novel phenomenon may assist in identifying LQTS patients with increased risk of sudden cardiac death.

Significant difference in beat-to-beat QT interval variability among different leads

QT interval on the surface electrocardiogram (ECG) reflects the time for repolarization of myocardium, and QTc prolongation is strongly associated with sudden cardiac death. Findings of studies that have examined the dispersion of QT interval in 12-lead ECGs strongly suggest that an increased QT dispersion between different leads is associated with an increased risk of cardiac mortality. Recent studies using novel techniques on beat-to-beat QT interval variability have also shown that the interval is both influenced by the autonomic nervous system and a predictor of sudden cardiac death. However, in these studies the variability of QT was quantified in a single lead, and thus is different from QT dispersion. The present study examined whether there was a significant difference between QT variability (detrended QT variance [QTv]) and Qtvi (index of QT interval variability corrected for mean QT squared divided by heart rate variability corrected for mean heart rate squared) in 17 data sets of continuous ECGs recorded in three channels (leads V(5), V(1), and V(3)) during ambulatory monitoring of ECG digitized at 1,000 Hz. The results showed a highly significant difference between QT variability measures (QTv and QTvi) (P < 0.0001) in two of three lead configurations that were used (V(5) and V(1) versus V(3)). This finding underscores the importance of using the same lead while recording ECG for the calculation of QT variability, though further investigations are clearly warranted before any definitive conclusion can be made. These findings should be evaluated in relation to the issue of local versus global cardiac repolarization of ventricular myocardium.

Amiodarone reduces the prevalence of T wave alternans in a population with ventricular tachyarrhythmias

INTRODUCTION

Testing for the presence of microvolt T wave alternans (TWA) is useful for arrhythmic risk stratification. Whether antiarrhythmic pharmacotherapy affects the presence of TWA is unknown. We tested whether patients with known ventricular tachyarrhythmias who were receiving amiodarone were less likely to manifest TWA as compared with those not receiving amiodarone.

METHODS AND RESULTS

Forty-four patients with a history of ventricular tachyarrhythmias and an implantable cardioverter defibrillator (ICD) implanted at least 1 month earlier underwent TWA testing. In this group, 14 patients were receiving amiodarone and 30 were not. Indeterminate test results occurred in 13 patients without a significant difference in those receiving or not receiving amiodarone. In the 31 patients with determinate TWA testing, a positive test was less likely in those receiving amiodarone (1 of 9 [11%]) as compared with those not receiving amiodarone (14 of 22 [64%]; P = 0.04). During a follow-up period averaging 0.9 +/- 0.2 years, the presence of TWA (P = 0.04) and decreased left ventricular ejection fraction (P = 0.05) predicted appropriate ICD therapy for ventricular tachyarrhythmias.

CONCLUSION

The prevalence of TWA was decreased in a chronic ventricular tachyarrhythmic population receiving amiodarone as compared with a population not receiving amiodarone.

ECG features of microvolt T-wave alternans in coronary artery disease and long QT syndrome patients

T-wave alternans (TWA) is a marker of myocardial electrical instability. We compared ECG features of microvolt TWA in coronary artery disease (CAD) and long QT syndrome (LQTS) patients.

METHOD

The study populations consisted of 43 CAD and 39 LQTS patients. TWA was detected in resting Holter recordings using the new correlation method (CM). After preprocessing to adjust for RR variability and respiratory modulation, CM was used to quantify TWA amplitude (A(CM)), duration (N(CM)), and magnitude (MAG(CM); defined as the product of A(CM) and N(CM)).

RESULTS

TWA was detected in 19 (44%) CAD and 17 (44%) LQTS patients. TWA was associated with longer RR intervals (P = 0.006) and had larger magnitudes (P = 0.067) in LQTS than CAD patients. The TWA was identified as transient (nonstationary) in 15 of 19 (79%) TWA-positive CAD patients, and in 8 of 17 (47%) TWA-positive LQTS patients (P = 0.047).

CONCLUSIONS

The frequency of TWA detected with CM is similar in LQTS and CAD patients. TWA is larger in LQTS than in CAD patients, whereas TWA is more frequently transient (nonstationary) in LAD than LQTS patients. In LQTS patients, but not in CAD patients, a longer RR is associated with TWA, indicating different electrophysiologic mechanisms in the two pathologies.

T wave spectral variance: A new method to determine inhomogeneous repolarization by T wave beat-to-beat variability in patients prone to ventricular arrhythmias

Inhomogeneous repolarization is considered to be associated with increased risk of ventricular arrhythmias, but exact determination of the end of the T wave is difficult, and a single measurement of the QTc interval may be insufficient for risk stratification. A new algorithm was therefore developed to determine the beat-to-beat variability of the T wave in Holter electrocardiographic recordings. This algorithm, termed T Wave Spectral Variance (TWSV) uses the two-dimensional fast Fourier transform to determine the beat-to-beat variability of the T wave in Hotter ECG recordings. The two-dimensional fast Fourier transform was calculated by use of a data matrix with 1,024 consecutive single beats (first dimension) and a 200-ms segment centered on the T wave (second dimension). The power spectra of the 2D-FFT revealed the frequency content of the T wave in the first dimension and the periodicity of these frequencies in cycles per beat in the second dimension. A TWSV index of periodicity was calculated by dividing total spectral energy by spectral energy at 0 cycles per beat. A TWSV index of 0 means a constant T wave from beat to beat; a TWSV index of 1 means a completely irregular T wave. Of the 200 patients investigated, all of whom had had myocardial infarctions, 50 had documented sustained ventricular tachycardia (VT) (<230 beats/min) (group 1), 50 had ventricular fibrillation (VF) (group 2), and 100 were without VT or VF (group 3); 10 normal subjects were also investigated. The visually determined QTc was 442 +/- 62 ms in group 1, 402 +/- 13 ms in group 2, 411 +/- 26 ms in group 3, and 398 +/- 43 ms in normal subjects (differences not significant). The TWSV index was 0.95 +/- 0.14 in group 1, 0.90 +/- 0.16 in group 2, and 0.64 +/- 0.24 in group 3; it showed a highly constant T wave in normal subjects (0.52 +/- 0.23). Differences between patients with VT and VF as against patients without VT or VF were significant (P < .05), whereas no statistical differences between patients with VT and VF could be found. Thus, TWSV, a new method to assess beat-to-beat variability of the T wave, revealed increased heterogeneity of repolarization in patients prone to both VT and VF after myocardial infarction, whereas a single QTc interval measurement showed no significant differences.

Congestive heart failure and VVI pacing mode: dynamic behavior of the dispersion of ventricular repolarization

Dynamic Behavior of the Dispersion of Ventricular Repolarization. The aim of this study was to evaluate the circadian variation in the spatial dispersion of ventricular repolarization in continuously paced patients with congestive heart failure (CHF). Fourteen patients (10 males, 4 females, aged 65 +/- 8 years) with CHF due to dilated cardiomyopathy (DCM) and an echocardiographic ejection fraction of 28% +/- 3% were studied. All patients underwent AV junctional RF ablation and permanent pacemaker implantation for drug refractory chronic atrial fibrillation (AF). Patients were evaluated at 1 month postimplant with a three-channel 24-hour Holter monitor, using the three plane Frank orthogonal leads (X, Y, and Z), in VVI pacing mode at 70 beats/min. For each hour, the mean value of spike-T interval dispersion of the first five beats was measured. The control group consisted of 20 patients without structural heart disease, but with AF and complete AV block, continuously paced in VVI mode at 70 beats/min. The dispersion of the spike-T interval had a circadian behavior in the study population, with higher values at night and lower during the daytime. During the daytime, the mean value of spike-T interval dispersion was 39 +/- 5 ms and during the nighttime it was 45 +/- 7 ms (P = 0.003). Such a difference between day and night was not found in the control group (38 +/- 6 ms and 40 +/- 8 ms, respectively, P = NS). In the daytime period the mean value of spike-T interval dispersion of our study population was comparable to that of the control group (P = NS), while during the nighttime it was significantly higher (P = 0.0004). In conclusion, by evaluating the dispersion of ventricular repolarization in two dimensions, space and time, a circadian variation was found in paced patients with CHF due to DCM. The increased QT dispersion in these patients during the nighttime period was attributed to different effects of vagal activity in normal and abnormal myocardial areas.