Cardiac repolarization analysis using the surface electrocardiogram

QT interval instability and QRS interval dispersion in healthy cats and cats with a hypertrophic cardiomyopathy phenotype

OBJECTIVES

Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats. Electrocardiographic (ECG) analysis can help with the diagnosis of HCM and also in the investigation of the secondary consequences of the disease. This study investigated ECG markers of QT interval variability (total instability [TI], short-term instability [STI], long-term instability [LTI], QT variance [QTv]), mean QT interval (QTa) and QT interval corrected for heart rate (QTac), as well as the duration (QRSd) and dispersion (QRSv) of the QRS interval in healthy cats and in those with HCM.

METHODS

Data were collected from 63 domestic cats: 40 in the control group and 23 in the HCM group. Fifty consecutive QT intervals were recorded for all cats and then QTa, QTac, QTv, TI, LTI and STI were calculated. QRSd and QRSv were also obtained for all animals. A Mann-Whitney U-test was used for group comparison. Receiver operating characteristic curves were plotted to evaluate the sensitivity and specificity of all markers for HCM. Logistic regression analysis was performed to assess the risks of cats having HCM, based on the studied indexes.

RESULTS

QTa (P <0.01), QTac (P <0.01), QRSd (P <0.01) and STI (P = 0.02) were higher in the HCM group. QTa >158.8 ms, QTac >27.4 ms and QRSd >0.045 s had an accuracy of 77.4%, 68.2% and 80.9%, respectively, in detecting HCM. Logistic regression showed that cats with QTa >158 ms, QTac >27.4 ms and QRSd >0.045 s had a 1.58-, 1,23- and 6.5-fold higher risk, respectively, of developing HCM.

CONCLUSIONS AND RELEVANCE

Cats with HCM had higher ventricular instability as assessed by STI and showed a prolongation of the QT and QRS intervals via the QTa, QTac and QRSd markers. These markers show potential as ancillary screening tools for identifying the presence of HCM.

Repolarization variability independent of heart rate during sympathetic activation elicited by head-up tilt

The fraction of repolarization variability independent of RR interval variability is of clinical interest. It has been linked to direct autonomic nervous system (ANS) regulation of the ventricles in healthy subjects and seems to reflect the instability of the ventricular repolarization process in heart disease. In this study, we sought to identify repolarization measures that best reflect the sympathetic influences on the ventricles independent of the RR interval. ECG was recorded in 46 young subjects during supine and then following 45 degrees head-up tilt. RR intervals and five repolarization features (QTend, QTpeak, RTend, RTpeak, and TpTe) were extracted from the ECG recordings. Repolarization variability was separated into RR-dependent and RR-independent variability using parametric spectral analysis. Results show that LF power of TpTe is independent of RR in both supine and tilt, while the LF power of QTend and RTend independent of RR and respiration increases following tilt. We conclude that TpTe is independent of RR and is highly affected by respiration. QTend and RTend LF power might reflect the sympathetic influences on the ventricles elicited by tilt. Graphical abstract.

Clinical applications of QT/RR hysteresis assessment: A systematic review

BACKGROUND

QT/RR hysteresis (QT-hys) is an index of the time accommodation of ventricular repolarization to heart rate changes. This report comprehensively reviews studies addressing QT-hys as a biomarker of medical conditions.

METHODS

This is a secondary analysis of data from a recent systematic review pertaining to methods of assessment of QT-hys. Articles included in the former review were filtered in order to select original articles investigating the association of QT-hys with medical conditions in humans.

RESULTS

Nineteen articles fulfilled our inclusion criteria. Given the heterogeneity of the methods and investigated conditions, no pooled analysis of data could be implemented. QT-hys was mostly studied as a risk marker of severe arrhythmias, as a predictor of the long QT syndrome (LQTS) phenotypes and genotypes and as a marker of exercise-induced ischemia. An increased QT-hys appears to be implicated in arrhythmogenesis, although the evidence in this regard relies on few human studies. An augmented QT-hys was reported in the LQTS, predominantly in the LQT2 genotype, but conflicting results were obtained between studies using different methods of assessment. In addition, QT-hys appears to be a useful marker of stress-induced myocardial ischemia in patients suspected of coronary artery disease.

CONCLUSIONS

QT-hys evaluation has potential clinical utility in at least some clinical conditions. Further studies of the clinical validity of QT-hys assessment are warranted, particularly condition specific studies based on QT-hys evaluation methods that provide separate estimates of QT-hys and QT/RR dependency.

Does endoscopic thoracic sympathectomy through clipping procedure have early effects on electrocardiographic parameters?

Background & objectives:

Hyperhidrosis is a pathologic condition of excessive sweating in amounts greater than physiologic needs. Endoscopic thoracic sympathectomy (ETS) is a globally accepted treatment modality for primary palmar and axillary hyperhidrosis. ETS also has distinctive effects on the heart, circulatory and respiratory systems. In this study early effects of ETS on electrocardiographic (ECG) parameters of hyperhidrosis patients were evaluated.

Methods:

Twelve-lead ECGs were performed on 72 patients who were free from cardiovascular, metabolic, neurological and pulmonary diseases and underwent planned ETS because of hyperhidrosis, before and after the procedure within the first 24 h. Heart rate (HR), PR, QT, corrected QT (QTc), QTc/Tpeak-Tend (TpTe) intervals, P-wave and QTc/TpTe dispersions were compared by ECG.

Results:

A total of 72 patients (24.1±6.0 yr, 17 female) were included in the study. The pre-operative HR of patients was significantly higher than post-operative HR of patients (73.8±12.8 vs. 68.1±12.6 beats/ min; P<0.001). The QTc dispersion (QTcd) durations of pre-operative patients were significantly longer than those of post-operative patients (51.5±6.3 vs. 44.9±5.6 msec; P<0.01). The TpTe dispersion value of pre-operative patients was significantly (P<0.001) higher than that of post-operative patients.

Interpretation & conclusions:

Our study showed that ETS through clipping procedure had positive effects on the mechanisms of arrhythmia by reducing HR, QTcd, TpTe and TpTe dispersion parameters of ECG in early periods in hyperhidrosis patients.

Comparison of QT-interval and variability index methodologies in individuals with spinal cord injury

STUDY DESIGN

Within-group comparison.

OBJECTIVES

Individuals with spinal cord injury (SCI) demonstrate an elevated risk for cardiac arrhythmias as indicated by an elevated QT-variability index (QTVI). The methodology measuring the QTVI, however, is not standardized, and therefore the purpose of this investigation is to determine whether the threshold and tangent methods of QT-interval measurement, as well as the electrocardiographic (ECG) epoch duration, influence the QTVI in individuals with SCI.

SETTING

Ontario, Canada.

METHODS

Ten minutes of resting ECG data were collected and analyzed from 14 individuals with SCI (C2-L3; ASIA Impairment Scale (AIS) A-D; 11.5±9.4 years post injury). The QTVI was analyzed via the threshold and tangent methods of QT-interval measurement, as well as from 1-, 5- and 10-min ECG epochs.

RESULTS

The threshold method produced significantly higher QTVI values compared with the tangent method. The QTVI from a 1-min epoch was significantly higher compared with that from 5- and 10-min epochs. The QTVI values acquired from the threshold method were shown to have higher reproducibility compared with those from the tangent method. There were no differences in QTVI values between participants with lesions above and below T1, as shown by both methods of QT-interval measurement.

CONCLUSION

The method of QT-interval measurement and the length of ECG epoch influence QTVI values in individuals with SCI. The methodology of QTVI analysis must be standardized in studies involving SCI individuals to reduce the variability accounted by methodological inconsistency.

Transverse false tendons in the left ventricular cavity are associated with early repolarization

BACKGROUND

Left ventricular false tendons (LVFTs) are related to precordial murmurs, ventricular arrhythmias and some repolarization abnormalities. Early repolarization (ER) is a specific type of repolarization abnormality.

OBJECTIVE

The aim of the present study was to investigate the relationship between LVFTs and ER.

METHODS

This study retrospectively included 99 consecutive healthy subjects and 33 patients with ER. Early repolarization was defined as an elevation of the QRS-ST junction of >0.1 mV from baseline in at least 2 inferior or lateral leads, manifested as QRS slurring or notching. Each participant was examined using echocardiography with second harmonic imaging, and the attachments of the LVFTs were recorded.

RESULTS

A total of 93 LVFTs were present in 82 (83%) of the 99 healthy subjects. Of these 93 LVFTs, the majority (79/93, or 84.9%) were longitudinal-type LVFTs, which originated from the basal interventricular septum (IVS) and progressed toward the apical segment of the left ventricular free wall. There were significant differences in the positioning of the LVFTs between the ER patients and control (P < 0.0001). LVFTs between mid-IVS to the middle of the LV free wall were found more common in patients with ER compared with control subjects (47.5% vs. 6.5%, P < 0.0001). In the ER group, LVFTs between the basal IVS to the apical segment of LV free wall were only identified in 21% of the LVFTs, compared to a value of 84.9% for the control group (P < 0.0001). The distribution of LVFT trends in the ER group was also significantly different from that in the control group (P < 0.05).

CONCLUSIONS

LVFTs are commonly visualized using echocardiography. An LVFT from the basal IVS to the apical segment of the left ventricular free wall may be a normal anatomical structure in the left ventricular cavity. On the contrary, transverse false tendons in the left ventricular cavity may be associated with ER.

Cardiac autonomic changes in middle-aged women: identification based on principal component analysis

The purpose of this study was to investigate the application of the principal component analysis (PCA) technique on power spectral density function (PSD) of consecutive normal RR intervals (iRR) aiming at assessing its ability to discriminate healthy women according to age groups: young group (20-25 year-old) and middle-aged group (40-60 year-old). Thirty healthy and non-smoking female volunteers were investigated (13 young [mean ± SD (median): 22·8 ± 0·9 years (23·0)] and 17 Middle-aged [51·7 ± 5·3 years (50·0)]). The iRR sequence was collected during ten minutes, breathing spontaneously, in supine position and in the morning, using a heart rate monitor. After selecting an iRR segment (5 min) with the smallest variance, an auto regressive model was used to estimate the PSD. Five principal component coefficients, extracted from PSD signals, were retained for analysis according to the Mahalanobis distance classifier. A threshold established by logistic regression allowed the separation of the groups with 100% specificity, 83·2% sensitivity and 93·3% total accuracy. The PCA appropriately classified two groups of women in relation to age (young and Middle-aged) based on PSD analysis of consecutive normal RR intervals.

A simulation of T-wave alternans vectocardiographic representation performed by changing the ventricular heart cells action potential duration

The presence of T wave alternans (TWA) in the surface ECG signals has been recognized as a marker of electrical instability, and is hypothesized to be related to patients at increased risk for ventricular arrhythmias. In this paper we present a TWA simulation study. The TWA phenomenon was simulated by changing the duration of the ventricular heart cells action potential. The magnitude was calculated in the surface ECG with the use of the time domain method. The spatially concordant TWA, where during one heart beat all ventricular cells display a short-duration action potential and during the next beat they exhibit a long-duration action potential, as well as the discordant TWA, where at least one region is out of phase, was simulated. The vectocardiographic representation was employed. The obtained results showed a high level of T-loop pattern and location disturbances connected to the discordant TWA simulation in contrast to the concordant one. This result may be explained by the spatial heterogeneity of the ventricular repolarization process, which could be higher for the discordant TWA than for the concordant TWA.

Some theoretical results on the observability of repolarization heterogeneity on surface ECG

Assessing repolarization heterogeneity (RH) from surface ECG recording is an open issue in modern electrocardiography, despite the fact that several indexes measured on the T-wave have been proposed and tested. To understand how RH occurring at myocite level is reflected on T-wave shapes, in this paper we propose a mathematical framework that combines a simple statistical model of cardiac repolarization times with the dominant T-wave formalism. Within this framework we compare different T-wave features such as T-wave amplitude, T-wave amplitude variability or QT intervals and we describe mathematically how they are linked to the spatial and temporal components of repolarization heterogeneity.

Acquired long QT syndrome: frequency, onset, and risk factors in intensive care patients

BACKGROUND

Acquired long QT syndrome is a reversible condition that can lead to torsades de pointes and sudden cardiac death.

OBJECTIVE

To determine the frequency, onset, frequency of medications, and risk factors for the syndrome in intensive care patients.

METHODS

In a retrospective chart review of 88 subjects, hourly corrected QT intervals calculated by using the Bazett formula were collected. Acquired long QT syndrome was defined as a corrected QT of 500 milliseconds or longer or an increase in corrected QT of 60 milliseconds or greater from baseline level. Risk factors and medications administered were collected from patients' medical records.

RESULTS

The syndrome occurred in 46 patients (52%); mean time of onset was 7.4 hours (SD, 9.4) from time of admission. Among the 88 patients, 52 (59%) received a known QTc-prolonging medication. Among the 46 with the syndrome, 23 (50%) received a known QT-prolonging medication. No other risk factor studied was significantly predictive of the syndrome.

CONCLUSIONS

Acquired long QT syndrome occurs in patients not treated with a known QT-prolonging medication, indicating the importance of frequent QT monitoring of all intensive care patients.

Relation of B-type natriuretic peptide level in heart failure to sudden cardiac death in patients with and without QT interval prolongation

Increased levels of B-type natriuretic peptide (BNP) are associated with prolongation of the action potential in ventricular myocardium. We investigated the relation of a BNP increase, QT interval, and sudden cardiac death (SCD) in the presence of heart failure (HF). We enrolled 398 patients with HF, New York Heart Association class III or IV, and left ventricular ejection fraction <40%. At baseline and after 3 months, we measured BNP and the QT interval. A BNP increase was defined as a change in BNP of ≥+10%. The QTc interval was calculated using the Bazett formula. QTc interval prolongation was defined as a change in QTc of ≥+10%. The patients were followed up for 1 year. During a 3-month period, BNP increased significantly in 53% of the patients (group 1) and did not in 47% (group 2). During the same period, the QTc interval was more prolonged in group 1 (+44 ± 12 ms) than in group 2 (+7 ± 6 ms; p = 0.01). During 1 year of follow-up, 20 patients died suddenly (SCD), 16 from pump failure. Although the SCD rates did not differ between the 2 groups (5.7% in group 1 vs 4.2% in group 2, p = 0.53), they were significantly greater in the patients in group 1 with QTc interval prolongation ≥+10% (13.8%, p <0.001). The Kaplan-Meier-derived SCD-free survival rates were 2.9 times greater in patients without QTc interval prolongation than in those with prolonged QTc (p <0.001). QTc interval prolongation was an independent correlate of SCD (p = 0.006), but BNP increase was not (p = 0.32). In conclusion, a BNP increase in patients with HF was associated with an increased risk of SCD only in patients with QTc interval prolongation.

Effect of using ECG derived respiration (EDR) signal in linear parametric QT-RR modeling

Linear parametric modeling techniques are widely used for modeling the short term QT-RR interaction to explore the factors (i.e. heart rate variability, Autonomic Nervous system) controlling ventricular repolarisation variability. Recent studies established that respiration also has important effect on the ventricular repolarisation process like it has on the heart rate variability. So for the clear understanding of cardiac regulations, respiration signal should be considered for modeling the QT-RR dynamics. Due to several problems in collecting original respiration signal using the traditional recording devices that measure the nasal air flow or abdominal or chest pressure, a lot of research has been done to extract respiration information from the ECG signal called the ECG derived respiration (EDR). In this study we verify the use of EDR signal as a surrogate of original respiratory signal in modeling QT-RR interaction. We collect 10 young subjects' ECG and respiration signal from Fantasia database. We developed linear parametric autoregressive model with multiple exogenous inputs with an autoregressive noise term and check the model performance by using original respiration and EDR signal and found statistically similar result. Our findings showed that EDR can be used as a surrogate of original respiration in QT-RR model for the better understanding of cardiac regulations in young healthy subjects.

Indexes of temporal myocardial repolarization dispersion and sudden cardiac death in heart failure: any difference?

BACKGROUND

The QT variability index, calculated between Q- and the T-wave end (QTend VI), is an index of temporal myocardial repolarization lability associated with sudden cardiac death (SCD) in chronic heart failure (CHF). Little is known about temporal variability in the other two temporal myocardial repolarization descriptors obtained from Q-Tpeak and Tpeak -Tend intervals. We therefore investigated differences between these indexes in patients with CHF who died suddenly and in those who survived with a left ventricular ejection fraction (LVEF) ≤35% or >35%.

METHODS AND RESULTS

We selected 127 ECG and systolic blood pressure (SPB) recordings from outpatients with CHF all of whom had been followed up for 30 months. We calculated RR and SPB variability by power spectral analysis and QTend VI, QTpeak VI, Tpeak Tend VI. We then subdivided data patients into three groups SCD, LVEF ≤ 35%, and LVEF > 35%. The LVEF was higher in the SCD than in the LVEF ≤ 35% group, whereas no difference was found between the SCD and LVEF > 35% groups. QTend VI, QTpeak VI, and Tpeak Tend VI were higher in the SCD and LVEF ≤ 35% groups than in the LVEF > 35% group. Multivariate analysis detected a negative relationship between all repolarization variability indexes, low frequency obtained from RR intervals and LVEF.

CONCLUSIONS

Our data show that variability in the first (QTpeak VI) and second halves of the QT interval (Tpeak -Tend VI) significantly contributes to the QTend VI in patients with CHF. Further studies should investigate whether these indexes might help stratify the risk of SCD in patients with a moderately depressed LVEF.

Selected physiological effects of ultrafine particles in acute cardiovascular morbidity

Ultrafine particles (UFPs) have emerged as a potentially important environmental health concern as they are produced in large numbers by vehicle emissions and may contribute to previously reported associations between traffic pollution and acute cardiovascular morbidity. This review examines recent epidemiological evidence of UFP exposures and selected physiological outcomes that may be modified as part of the underlying causal pathway(s) linking particulate air pollution and acute cardiovascular morbidity. Outcomes examined included changes in heart rate variability (HRV) (autonomic function), ST-segment depression (myocardial ischemia), QT-interval (ventricular repolarization), and endothelial vasomotor function. Twenty-two studies were reviewed in total: 10 prospective panel studies and 12 randomized cross-over studies. Sixteen studies identified a significant relationship between UFPs and at least one of the above outcomes and current evidence generally supports the biological plausibility of a relationship between UFPs and acute cardiovascular morbidity. However, discrepancies were apparent in the direction of observed associations, particularly for HRV and ventricular repolarization. Reasons for these discrepancies may include differences in particle composition, time-point of clinical evaluation, and population susceptibilities. Nevertheless, evidence to date suggests that UFPs have a measureable impact on physiological measures known to be altered in cases of acute cardiovascular morbidity. Moving forward, expanded use of personal exposure measures is recommended for prospective panel studies to minimize exposure misclassification. In addition, effort should be made to include more women in studies of the acute cardiovascular effects of UFPs as findings to date generally reflect responses in men.

Microgravity effects on ventricular response to heart rate changes

The effect of simulated microgravity on ventricular repolarization (VR) has been evaluated on healthy volunteers by a 5-day Head Down (-6°) Bed Rest (HDBR) maneuver. QT to RR and QT(p) (measured until the peak of the T wave) to RR hystereses have been measured during a tilt table test, and differences between them have been studied to better understand possible changes in the final part of the repolarization. To characterize the hystereses, two indices have been computed: M(90), quantifying adaptation lag in beats, and α evaluating the slope of parabolic regression fitting. Significant differences between QT and QT(p) were found before, but not after HDBR. Specifically, before HDBR was considerable lower for QT(p) than for QT, while α was significantly higher. After HDBR, M(90) and a took essentially the same values for QT and QT(p). This fact evidenced the different effect of HDBR on QT to RR and QT(p) to RR adaptations, and suggest HDBR could lead to an impairment in ventricular repolarization dispersion.

DeltaT50--a new method to assess temporal ventricular repolarization variability

BACKGROUND

Increased beat-to-beat variability in cardiac repolarization time is a tentative risk marker of drug-induced torsades de pointes. We developed a new, automatic method based on the temporal variability of the T-wave down slope to assess this variability.

METHOD AND RESULTS

Leads V(1) to V(6) of resting electrocardiograms were recorded in 42 healthy subjects (18-68 years, 22 men). The temporal variability at 50% of the T-wave down slope, deltaT50 (1.5 ± 0.41 milliseconds; range, 0.86-2.66 milliseconds), was measured with an accuracy of 1 millisecond on at least 9 pairs of electrocardiogram complexes with a signal-to-noise ratio more than 10 and changes in the R-R interval less than 150 milliseconds. The correlation between repeated measurements of deltaT50 was high. DeltaT50 was measured without corrections for age, sex, heart rate, T-wave amplitude, signal-to-noise ratio, R-R variability, and QTcF because none of these factors explained more than 4% of the within-subject deltaT50 variability.

CONCLUSION

The beat-to-beat repolarization variability was measured with high fidelity with the deltaT50 method and was a robust measure in healthy volunteers.

Respiration effect on wavelet-based ECG T-wave end delineation strategies

The main purpose of this paper is to study the influence of the mechanical effect of respiration over the T-wave end delineation. We compared the performance of an automatic delineation system based on the wavelet transform (WT), considering single lead (SL), global delineation locations obtained from SL annotations (SLR), and multilead (ML) approaches. The linear relation between the variations on T-wave end locations obtained with each of the methods and the mechanical effect of respiration was quantified using spectral coherence and ARARX modeling both in simulated signals and in real data. We also explored the evolution of the vectorcardiographic spatial loop using the projection on the main direction of the WT in the region close to the T-wave end ( T(e)) and its relation with respiration. The dispersion of the additional T-wave end location error due to respiration was reduced by 15% using SLR with respect to SL, while ML allows for a reduction of around 40%. The percentage of that error correlated with respiration was in average 99% using SL while 82% and 72% using SLR and ML, respectively. Thus, results suggest that ML is the most adequate strategy for T-wave delineation, allowing the reduction of the instability of T-wave end location caused by respiration.

Measurement and regulation of cardiac ventricular repolarization: from the QT interval to repolarization morphology

Ventricular repolarization (VR) is a crucial step in cardiac electrical activity because it corresponds to a recovery period setting the stage for the next heart contraction. Small perturbations of the VR process can predispose an individual to lethal arrhythmias. In this review, I aim to provide an overview of the methods developed to analyse static and dynamic aspects of the VR process when recorded from a surface electrocardiogram (ECG). The first section describes the list of physiological and clinical factors that can affect the VR. Technical aspects important to consider when digitally processing ECGs are provided as well. Special attention is given to the analysis of the effect of heart rate on the VR and its regulation by the autonomic nervous system. The final section provides the rationale for extending the analysis of the VR from its duration to its morphology. Several modelling techniques and measurement methods will be presented and their role within the arena of cardiac safety will be discussed.

Multilead analysis of T-wave alternans in the ECG using principal component analysis

T-wave alternans (TWA) is a cardiac phenomenon associated with the mechanisms leading to sudden cardiac death. Several methods exist to automatically detect and estimate TWA in the ECG on a single-lead basis, and their main drawback is their poor sensitivity to low-amplitude TWA. In this paper, we propose a multilead analysis scheme to improve the detection and estimation of TWA. It combines principal component analysis with a single-lead method based on the generalized likelihood ratio test. The proposed scheme is evaluated and compared to a single-lead scheme by means of a simulation study, in which different types of simulated and physiological noise are considered under realistic conditions. Simulation results show that the multilead scheme can detect TWA with an SNR 30 dB lower and allows the estimation of TWA with an SNR 25 dB lower than the single-lead scheme. The two analysis schemes are also applied to stress test ECG records. Results show that the multilead scheme provides a higher detection power and that TWA detections obtained with this scheme are significantly different in healthy volunteers and ischemic patients, whereas they are not with the single-lead scheme.