Comparison of automatic repolarization measurement techniques in the normal magnetocardiogram

Multichannel MCG noninvasively measures cardiac magnetic field strength from many sites at the body surface, potentially providing useful regional information about ventricular repolarization. Previous work on ECGs has shown that automatic techniques for repolarization measurement are better than manual measurement at discriminating patients with cardiac conditions from normal subjects. Although automatic repolarization measurement techniques have been quantified for ECGs, no comparative data exists for the MCG. In this study four different automatic repolarization (QT) interval techniques for detecting T wave end in the MCG were compared. The influence of MCG filtering on the automatic algorithms was also quantified. MCGs were obtained at 49 sites over the heart from 23 normal subjects. Automatic measurements of the repolarization (QT) interval were made following the addition of different high pass (0.25, 0.5, 1 Hz) and low pass (100, 60, 40, 30 Hz) filters. There were consistent differences between automatic techniques in the unfiltered data amounting to greatest mean difference of 52.3 ms. Low pass filtering significantly increased the automatic repolarization (QT) interval relative to unfiltered measurement by 6.5 (3.2) ms (mean SD) for 100 Hz, 6.0 (3.0) ms for 60 Hz, 8.1 (3.2) ms for 40 Hz, and 8.8 (3.1) ms for 30 Hz across all techniques. High pass filtering significantly decreased the value by -2.6 (6.0) ms for 0.25 Hz, -5.5 (5.3) ms for 0.5 Hz, and -17.1 (7.8) ms for 1 Hz. Automatic measurements of repolarization (QT) in the MCG differ between techniques and are influenced by filtering. These effects should be considered when comparing results.     

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.     

Stability of ventricular repolarization in conscious dogs with chronic atrioventricular dissociation and his-bundle pacing

Following AVN ablation, eight dogs were surgically instrumented for chronic (continuous) HIS-bundle pacing. For data collection, implanted pacemakers were transiently programmed to pace in stepwise ascending ramps at rates from 50 to 200 beats/min in 30-beat/min steps. Each rate was held for 60 seconds. At each rate, ECG signals were collected from conscious dogs for measurements of QT intervals during the last 10 seconds of each paced rate to construct a QT-HR ramp. This QT-HR ramp was repeated twice on each day of study 10 minutes apart. Dogs were randomly assigned to two groups and studied weekly for a minimum of 6 weeks. Group 1 dogs had pacemakers programmed to a rate of 80 beats/min for the duration of the study. Group 2 dogs were paced at 80 beats/min for weeks 1-3, then increased to 140 beats/min for weeks 4-6. The difference between paired QT-HR ramps within 1 day was <3 ms. QT-HR ramps were statistically indistinguishable over the 6-week study for group 1 dogs. Group 2 dogs experienced a slight flattening in the slope of the QT-HR ramps from week 3 to week 6 due to a reduction in QT interval at low HRs (50 and 80 beats/min) only. This conscious HIS-pacing model in dogs is a sensitive, stable, and reproducible method to define ventricular repolarization characteristics over a range of programmable HRs and experimental conditions.     

Spatial distribution of repolarization times in patients with coronary artery disease

The potential clinical value of QT dispersion (QTd), a measure of the interlead range of QT interval duration in the surface 12-lead ECG, remains ambiguous. The aim of the study was the temporal and spatial analysis of the QT interval in healthy subjects and in patients with coronary artery disease (CAD) using magnetocardiography (MCG) and surface ECG. Standard 12-lead ECG and 37-channel MCG were performed in 20 healthy subjects, 23 patients with CAD without prior myocardial infarction (MI), 31 MI patients and 11 MI patients with ventricular tachycardia (VT). QTd was increased in CAD without MI compared to normals (ECG 46.1 +/- 6.0 vs 42.8 +/- 5.0, P < 0.05; MCG 66.8 +/- 20.3 vs 49.7 +/- 10.8, P < 0.01) and in VT compared to MI (ECG 66.8 +/- 16.5 vs 51.9 +/- 16.6, P < 0.05; MCG 93.6 +/- 29.6 vs 66.8 +/- 20.8, P < 0.005). In MCG, spatial distribution of QT intervals in patient groups differed from those in healthy subjects in three ways: (1) greater dispersion, (2) greater local variability, and (3) a change in overall pattern. This was quantified on the basis of smoothness indexes (SI). Normalized SI was higher in CAD without MI compared to normals (3.8 +/- 1.1 vs 2.7 +/- 0.6, P < 0.001) and in VT compared to MI (6.4 +/- 1.6 vs 4.2 +/- 1.4, P < 0.0005). For the normal-CAD comparison a sensitivity of 74% and a specificity of 80% was obtained, for MI-VT, 100% and 77%, respectively. The results suggest that examining the spatial interlead variability in multichannel MCG may aid in the initial identification of CAD patients with unimpaired left ventricular function and the identification of post-MI patients with augmented risk for VT.     

Cardiac repolarization properties during standardized exercise test as studied by QT,QT peak and terminated T-wave intervals

Summary. Changes in QT, QT peak (QT_p) and terminal T-wave, T_p–T_e (QT–QT_p) were studied in 11 apparently healthy subjects during and after a standardized exercise test. ECG was recorded at scalar lead positions. Averaged complexes were later analysed by computer for the different time intervals. QT and QT_p decreased in parallel with increasing heart rate with a ratio QT_p/QT of 0·80 ± 0.02 at rest and 0·74 ± 0·02 at maximal heart rate around 170. After exercise QT and QT_p prolonged disproportionately slower than heart rate, reaching the relation observed during exercise only 9·5 min post exercise. T_p–T_e was 75 ± 10 ms at rest and 65 ± 8 ms at maximal heart rate. The decrease was significant (P<0·001). The main part of the rate-associated shortening of the QT interval occurred in the QT_p interval where it was about six to seven times larger than in the T_p–T_e interval. In conclusion, QT and QT_p decreased similarly with heart rate during exercise. Post exercise there was an initial slower return of these intervals to the resting state than for heart rate. T_p–T_e changes were minimal.     

Temporal complexity of repolarization and mortality in patients with implantable cardioverter defibrillators

Increased repolarization variability has been observed in various cardiac conditions. However, data on its relation to heart rate variability and on its value in predicting adverse outcomes in high risk patients are limited. Forty-seven patients with decreased left ventricular function and ICDs had high resolution 10-minute ECG recordings and were followed for 781 +/- 258 days (mean +/- SD) on average. The interval from the R peak to the T wave peak with maximum amplitude (RTmax) and from the R peak to the T wave offset (RToff) were determined automatically on a beat-to-beat basis. Temporal beat-to-beat RTmax and RToff variability were analyzed using traditional summary statistics, a complexity measure (approximate entropy [ApEn]), and the short-term scaling exponent (alpha1). Eight (17%) patients died and 16 (34%) patients experienced death/appropriate ICD shock during follow-up. RTmax-ApEn was significantly higher in patients who died compared with patients who survived (1.24 +/- 0.13 vs 1.01 +/- 0.21, respectively, P=0.008). When RTmax-ApEn was tested together with the alpha1 of the RR intervals, occurrence of ventricular arrhythmias before ICD implantation, and beta-blockers usage in the Cox regression analysis, it still independently predicted mortality; hazard ratio=3.36 (1.28-8.83, 95% CI, P=0.014) for every 0.10-increase in RTmax-ApEn. None of the repolarization variability parameters independently predicted death/appropriate ICD shocks. Increased temporal complexity of repolarization (RTmax-ApEn) independently predicts mortality in ICD patients.     

Fetal diagnosis of KCNQ1-variant long QT syndrome using fetal echocardiography and magnetocardiography

A pregnant woman with KCNQ1 variant long QT syndrome (LQTS) underwent fetal magnetocardiography (fMCG) after atrioventricular (AV) block was noted during fetal echocardiogram—atypical for LQTS type 1. Concern for fetal LQTS on fMCG prompted monitoring of maternal labs, change of maternal beta blocker therapy, and frequent fetal echocardiograms. Collaboration between obstetricians, neonatologists, and pediatric cardiologists ensured safe delivery. Beta blocker therapy was initiated after birth, and postnatal evaluation confirmed genotype and phenotype positive LQTS in the infant. Our experience suggests diagnosis and evaluation of fetal LQTS can alter antenatal management to reduce risk of poor fetal and postnatal outcomes.     

Three-dimensional recovery time dispersion map by 64-channel magnetocardiography may demonstrate the location of a myocardial injury and heterogeneity of repolarization

Background QT dispersion reveals heterogeneities in the repolarization time in the three-dimensional (3D) structure of the ventricular myocardium. In this study, we report on a 3D function map of recovery time (RT) dispersions as measured by 64-channel magnetocardiography (MCG).Methods MCG were simultaneously recorded in 29 controls and 21 patients with previous myocardial infarction (MI). The 3D current density was calculated from 64-channel MCG data in the B _z component using a space filter. The heart outline, reconstructed from the integrated the current density, revealed both the atrium and ventricle. The RT for the intervals between QRS onset and the time of the maximum dT/dt of T wave, and the peak to the end of the T wave (T_peak-negative dT/dt) were automatically measured by means of a computer from 3D MCG data. The corrected RT (RT_c) and corrected T_peak-negative dT/dt were then calculated using Bazett’s formula. The 3D RT_c and the corrected T_peak-negative dT/dt dispersion map were superimposed on the heart outline generated by MCG.Results The RT_c was significantly longer for the MI group than in the control group (67±25 ms^1/2 vs. 16±6 ms^1/2) (p<0.0001). The corrected T_peak-negative dT/dt dispersions in each patient was also significantly longer for the MI group than in the control group (35±27 ms^1/2 vs. 10±5 ms^1/2) (p<0.0001). Furthermore, the 3D RT_c and T_peak-negative dT/dt dispersion maps corresponded with the space location of MI, as defined by Tc-99m tetrofosmin myocardial imagingConclusions 3D RT_c and T_peak-negative dT/dt dispersion maps in the ST segment, obtained by 64-channel MCG may be used demonstrate the location of a myocardial injury and heterogeneities of repolarization.     

Impact of electrocardiogram recording format on QT interval measurement and QT dispersion assessment

The aim of this study was to determine the effect of recording conditions on the operator dependent measures of QT dispersion in patients with known and/or suspected repolarization abnormalities. Among several methods for risk stratification, QT dispersion has been suggested as a simple estimate of repolarization abnormalities. In a cohort of high and low risk patients, different components of the repolarization process were assessed in the 12-lead ECG using three different paper speeds and amplifier gains. To assess measurement error and reproducibility, a straight line was repeatedly measured. The operator error was 0.675 +/- 0.02 mm and the repeatability of the measurement error was 31 +/- 6%. The QT interval was most frequently measurable in V2-V5. Depending on the lead selected for analysis, the incidence of visible U waves was greatest in the precordial leads with high amplifier gain and low paper speed, strongly affecting QT interval measurement. The timing of the onset of the QRS complex (QRS onset dispersion) or offset of the T wave was strongly dependent on the paper speed. Paper speed, but not amplifier gain, had a significant shortening effect on the measurement of the maximum QT interval. As QT interval measurement in each ECG lead incorporates QRS onset and T wave offset (depending on the number of visible U waves), the dispersion of each of these parameters significantly affected QT dispersion. Thus, QT dispersion appears to reflect merely the presence of more complex repolarization patterns in patients at risk of arrhythmias.     

Association between ventricular repolarization variables and cardiac diastolic function: A cross-sectional study of a healthy Chinese population

BACKGROUND Diastolic electromechanical couple, a well-described phenomenon in symptomatic heart failure, has not been well studied in healthy people. We hypothesized that ventricular repolarization variables, such as the QT interval,Tpeak-to-Tend(Tpe) interval and Tpe/QT ratio, are associated with cardiac diastolic function in the healthy Chinese population.AIM To assess the relationship between ventricular repolarization variables and cardiac diastolic function in apparently healthy Chinese individuals.METHODS This was a community-based cross-sectional study conducted in Shenyang,China. A total of 414 healthy subjects aged 35-91 years were enrolled. All subjects underwent standard 12-lead electrocardiography(ECG) and comprehensive echocardiography. ECG enabled the measurement of QT and Tpe intervals and Tpe/QT ratio. echocardiographic parameters, such as the ratio of mitral early diastolic inflow velocity(E) and late diastolic inflow velocity(A), E-wave deceleration time, left atrial volume(LAV) and LAV index, were measured to assess diastolic function. E/A < 0.75 was considered to indicate reduced diastolic function. ECG and echocardiography results were analyzed separately and in a blinded fashion. Correlation and regression analyses were applied to determine associations.RESULTS Ventricular repolarization variables, such as the QTc interval(393.59 ± 26.74 vs403.86 ± 33.56; P < 0.001), Tpe interval(72.68 ± 12.41 vs 77.26 ± 17.86; P < 0.01),Tpec interval(76.36 ± 13.53 vs 83.32 ± 21.25; P < 0.001) and Tpe/QT ratio(0.19 ±0.03 vs 0.20 ± 0.04; P < 0.01), were significantly different between the normal diastolic function group and the reduced diastolic function group. Significant associations were found between repolarization variables and diastolic function.After adjusting for all other possible confounders, the QTc and Tpe_c intervals were significantly associated with the E/A ratio(P = 0.008; P = 0.010). In men, the QTc interval was associated with abnormal diastolic function, and compared to the third QTc tertile, in the second QTc tertile, the odds ratio was 0.257(95%CI:0.102–0.649; P = 0.004).CONCLUSION Repolarization variables are associated with cardiac diastolic function even in healthy people. Moderate levels of the QTc interval exert a protective effect on diastolic dysfunction in men.     

Cumulative effect of preceding pacing cycle length on ventricular repolarization after a pause

This study was designed to investigate if the components of the QT interval after a pause are influenced by the preceding pacing cycle length. Ten patients (seven women and three men; age 79 +/- 9 years, means +/- SD) with complete atrioventricular block or sick sinus syndrome whose own heart rate was < 40 beats/min were examined. All patients had already undergone implantation of a permanent pacemaker. Ventricular pacing protocol was performed with simultaneous recording of a 12-lead electrocardiogram. One set of regular stimuli for 30 seconds (S1) with a variable cycle length (1,000, 700, and 400 ms) was followed by a single stimulus (S2) with a fixed coupling interval of 1,500 ms. QT intervals in response to the last S1 (S1-QT) and S2 (S2-QT) were measured. The QT interval was divided into two components, the interval from start of Q wave to the peak of T wave (QaT) and that from the peak to end of T wave (Tae). The S2-QT and S1-QT interval shortened in association with a decrease in the S1S1 interval. The abbreviation of S2-QT interval was not associated with a significant change in the Tae interval. The results demonstrated that the QT interval after a pause shortened by reducing the preceding pacing cycle length. This shortening is probably due to a homogenous abbreviation of action potential duration across the ventricular wall.     

Comparative Reproducibility of QT, QT Peak, and T Peak-T End Intervals and Dispersion in Normal Subjects, Patients with Myocardial Infarction, and Patients with Hypertrophic Cardiomyopathy

Abnormal repolarizaiion is associated with arrhythmogenesis. Because of controversies in existing methodology, new computerized methods may provide more reliable tools for the noninvasive assessment of myocardial repolarization from the surface electrocardiogram (ECC). Measurement of the interval between the peak and the end of the T wave (TpTe interval) has been suggested for the detection of repolarization abnormalities, but its clinical value has not been fully studied. The intrasubject reproducibility and reliability of automatic measurements of QT, QT peak, and TpTe interval and dispersion were assessed in 70 normal subjects, 49 patients with acute myocardial infarction (5th day; MI), and 37 patients with hypertrophic cardiomyopathy (HC). Measurements were performed automatically in a set of 10 ECCs obtained from each subject using a commercial software package (Marquette Medical Systems, Milwaukee, WI, U.S.A.). Compared to normal subjects, all intervals were significantly longer in HC patients (P < 0.001 for QT and QTp; p < 0.05 for TpTe); in MI patients, this difference was only significant for the maximum QT and QTp intervals (P < 0.05). In both patient groups, the QT and QTp dispersion was significantly greater compared to normal subjects (P < 0.05) but no consistent difference was observed in the TpTe dispersion among all three groups. In all subjects, the reproducibility of automatic measurement of QT and QTp intervals was high (coefficient of variation, CV, 1%-2%) and slightly lower for that of TpTe interval (2%–5%; p < 0.05). The reproducibility of QT, QTp, and TpTe dispersion was lower (12%–24%, 18%–28%, 16%–23% in normal subjects, MI and HC patients, respectively). The reliability of automatic measurement of QT, QTp, and TpTe intervals is high but the reproducibility of the repeated measurements of QT, QTp and TpTe dispersion is comparatively low.     

Prenatal diagnosis of QT prolongation by magnetocardiography

Magnetocardiography constitutes a new tool for monitoring fetal cardiac activity. The fetal magnetocardiogram (FMCG) recorded noninvasively over the maternal abdomen is detectable with high temporal resolution and permits analysis of all parts of the PQRST waveform. In this way measurements of cardiac time intervals, including the QT interval, become possible. The following article constitutes the first report of antenatal detection of QT prolongation in two fetuses by FMCG.     

Recommended standards for fetal magnetocardiography

Fetal magnetocardiography (FMCG) is increasingly being used in research and diagnostics of fetal heart function. Currently, FMCG is the only noninvasive procedure available, comparable to postnatal ECG, which can be used to assess cardiac electrophysiology during the second and third trimester of pregnancy. For a reliable evaluation and full clinical acceptance of this new technique, large numbers of patient investigations are required which can only be obtained in multicenter studies. An international standard protocol is needed to allow pooling of sufficient data and to permit the comparison of studies performed in different centers. This article provides recommended standards for FMCG in the fields of data acquisition and data analysis.     

Effects of intravenous magnesium in a prolonged QT interval model of polymorphic ventricular tachycardia focus on transmural ventricular repolarization

BACKGROUND: This study was performed to clarify the antiarrhythmic effects of magnesium sulfate (Mg(++)) in a prolonged QT interval canine model of polymorphic ventricular tachyarrhythmia (VTA). METHODS: In six experiments in a canine model of prolonged QT by anthopleurin-A, Mg(++) was administered in boluses of 0.2 mL/kg during repetitive episodes of self-terminating polymorphic VTA or frequent premature ventricular complexes (PVCs). The distribution of ventricular repolarization across the left ventricular(LV) wall and dispersion of transmural repolarization were analyzed before, and 30 and 120 seconds after Mg(++) administration, during ventricular pacing at 100 bpm. Transmural unipolar electrograms were recorded from multipolar needle electrodes, and local activation-recovery intervals (ARI) were measured. RESULTS: Mg(++) rapidly eliminated self-terminating polymorphic VTA and all isolated PVCs. During ventricular pacing at 100 bpm, Mg(++) caused modest shortening of ARI at all recording sites. Since the magnitude of ARI shortening was greater at mid-myocardial sites than at other ventricular sites, mean transmural ARI dispersion decreased from 80 +/- 22 to 45 +/- 18 ms within 30 seconds after Mg(++) injection. However, this effect was transient, and, at 120 seconds after Mg(++) administration, ARI had increased all sites and transmural ARI dispersion lengthened to 65 +/- 18 ms. Besides suppression of triggered premature activity, homogenization of transmural ventricular repolarization was associated with the antiarrhythmic effects of intravenous Mg(++) in this model. CONCLUSION: Since these effects were transient, a continuous intravenous infusion of Mg(++) is preferred to prevent recurrences of VTA.     

Dispersion of QT Intervals: A Measure of Dispersion of Repolarization or Simply a Projection Effect?

QT interval dispersion may provide little information about repolarization dispersion. Some clinical measurements demonstrate an association between high QT interval dispersion and high morbidity and mortality, but what is being measured is not clear. This study was designed to help resolve this dilemma. We compared the association between different clinical measures of QT interval dispersion and the ECG lead amplitudes derived from a heart vector model of repolarization with no repolarization dispersion whatsoever. We compared our clinical QT interval dispersion data obtained from 25 subjects without cardiac disease with similar data from published studies, and correlated these QT dispersion results with the distribution of lead amplitudes derived from the projection of the heart vector onto the body surface during repolarization. Published results were available for mean relative QT intervals and mean differences from the maximum QT interval. The leads were derived from Uijen and Dower lead vector data. Using the Uijen lead vector data, the correlation between measurements of dispersion and derived lead amplitudes ranged from 0.78 to 0.99 for limb leads, and using the Dower values ranged from 0.81 to 0.94 for the precordial leads. These results show a clear association between the measured QT interval dispersion and the variation in ECG lead amplitudes derived from a simple heart vector model of repolarization with no regional information. Therefore, measured QT dispersion is related mostly to a projection effect and is not a true measure of repolarization dispersion. Our existing interpretation of QT dispersion must be reexamined, and other measurements that provide true repolarization dispersion data investigated.     

QT Interval Dispersion and its Clinical Utility

QT dispersion as a measure ofin-terlead variations of QT interval duration in the surface 12-lead ECG is believed to reflect regional differences in repolarization heterogeneity and thus, may provide an indirect marker of arrhythmogenicity. Methodology for determining QT dispersion and reproducibility of this parameter vary significantly between studies and, together with some other unresolved problems witb QT dispersion assessment, often lead to contradictory suggestions about potential clinical utility of this parameter. The results of our own study in 213 survivors of myocardial infarction, together with a comprehensive review of the literature, suggest that most of these inconsistencies reflect incomplete understanding of electrocardiographic correlates of both normal and abnormal ventricular repolarization. The application of more objective techniques, such as spectral analysis or combined assessment of different parameters (e.g., area beneath the T wave and its symmetricity) may add a new dimension to the noninvasive assessment of ventricular repolarization.     

Heart rate,blood pressure and repolarization effects of an energy drink as compared to coffee

The goal of this study was to investigate the impact of energy drinks on haemodynamic and cardiac physiology. Comparisons were made to coffee as well as water consumption. In Protocol #1 the caffeine content was normalized to body weight to represent a controlled environment. Heart rate, blood pressure and cardiac QTc interval were assessed in 15 participants, on 4 days, prior to and for 6·5 h postconsumption of (i) energy drink (2 mg caffeine per kg body weight; low dose), (ii) energy drink (3 mg caffeine per kg body weight; medium dose), (iii) coffee (2 mg caffeine per kg body weight) and (iv) 250 ml water. In Protocol #2, the beverages were consumed in volumes that they are purchased to represent real‐life conditions. The aforementioned measurements were repeated in 15 participants following (i) 1 16 oz can of energy drink (16 oz Monster), (ii) 1 24 oz can of energy drink (24 oz Monster), (iii) 1 packet of Keurig K‐Cup Starbucks coffee (coffee) and (iv) 250 ml water. The order of the beverages was performed in a randomized double‐blinded fashion. For both protocols, QTc interval, heart rate and systolic blood pressure were unchanged in any condition (P>0·05). Diastolic blood pressure and mean blood pressure were slightly elevated in Protocol #1 (P<0·05, main effect of time) with no difference between beverages (P<0·05, interaction of beverage × time); however, they were unaffected in Protocol #2 (P>0·05). These findings suggest that acute consumption of these commonly consumed beverages has no negative effect on cardiac QTc interval.     

Gender Effects on Novel Time Domain Parameters of Ventricular Repolarization Inhomogeneity

Introduction: Parameters of ventricular repolarization variability are increasingly being used in an attempt to understand better and predict the occurrence of ventricular tachycardia. Nevertheless, some of the measures used have thus far not been analyzed regarding gender differences in a large group of healthy subjects. Furthermore, new parameters might give further insight.
Methods and Results: We investigated 139 healthy volunteers (mean age 41.6 ± 15.3 years, range 20–77, median 40.0 years, 76 women) without evidence of organic cardiac disease. Mean RR interval and established time domain parameters of heart rate variability (rMSSD; SDNN) were measured for each subject. Beat-to-beat QT interval and time-domain QT interval variability were analyzed. Characteristics of the QT interval and QT interval variability were determined as hourly mean values. The standard deviation of all QT intervals/hour (SDQT) and the standard deviation of all QTc intervals/hour (SDQTc) were used to measure QT interval variability. Four novel ratios of repolarization inhomogeneity (VRI: SDQT/SDNN; VR II: SDQT/rMSSD; VR III: SDQTc/SDNN; VR IV: SDQTc/rMSSD) were introduced. Female subjects exhibited significantly higher values in all four ratios of variability.
Conclusion: The obvious gender differences in repolarization inhomogeneity found in this study might be valuable in better understanding differences between men and women in the genesis of ventricular tachycardia.