Comparative usefulness of beat-to-beat QT dispersion and QT interval fluctuations for identifying patients with organic heart disease at risk for ventricular arrhythmias

QT interval variability in body surface ECG: measurement, physiological basis, and clinical value: position statement and consensus guidance endorsed by the European Heart Rhythm Association jointly with the ESC Working Group on Cardiac Cellular Electrophysiology

This consensus guideline discusses the electrocardiographic phenomenon of beat-to-beat QT interval variability (QTV) on surface electrocardiograms. The text covers measurement principles, physiological basis, and clinical value of QTV. Technical considerations include QT interval measurement and the relation between QTV and heart rate variability. Research frontiers of QTV include understanding of QTV physiology, systematic evaluation of the link between QTV and direct measures of neural activity, modelling of the QTV dependence on the variability of other physiological variables, distinction between QTV and general T wave shape variability, and assessing of the QTV utility for guiding therapy. Increased QTV appears to be a risk marker of arrhythmic and cardiovascular death. It remains to be established whether it can guide therapy alone or in combination with other risk factors. QT interval variability has a possible role in non-invasive assessment of tonic sympathetic activity.

QT Variability Index

The QT Variability Index (QTVI) is a non-invasive measure of repolarization lability that has been applied to a wide variety of subjects with cardiovascular disease. It is a ratio of normalized QT variability to normalized heart rate variability, and therefore includes an assessment of autonomic nervous system tone. The approach assesses beat-to-beat variability in the duration of the QT and U wave in conventional surface electrocardiographic recordings, as well as determines the heart rate variability (HRV) from the same recording. As opposed to T wave alternans, QTVI assesses variance in repolarization at all frequencies. Nineteen studies have published data on QTVI in healthy individuals, while 20 have evaluated its performance in cohorts with cardiovascular disease. Six studies have assessed the utility of QTVI in predicting VT/VF, cardiac arrest, or cardiovascular death. A prospective study utilizing QTVI to determine therapy allocation has not been performed, and therefore the final determination of the value of the metric awaits definitive exploration.

Sodium channel blockers enhance the temporal QT interval variability in the right precordial leads in Brugada syndrome

BACKGROUND

Temporal QT interval variability is associated with sudden cardiac death. The purpose of this study was to evaluate temporal QT interval variability in Brugada syndrome (BS).

METHODS

We measured QT and RR intervals in precordial leads (V(1)-V(6)) based on 12-beat resting ECG recordings from 16 BS patients (B group) with spontaneous ST elevation in right precordial leads (V(1)-V(2)) and from 10 patients with normal hearts (C group). We measured the response in B group before and after administration of pilsicainide (1 mg/kg). The standard deviation (QT-SD, RR-SD) of the time domain and total frequency power (QT-TP, RR-TP) were calculated for all precordial leads, and the latter was to analyze the frequency domain.

RESULTS

The right precordial leads in BS exhibited an additional and prominent ST elevation (coved-type) after pilsicainide administration. Both QT-SD and QT-TP values were significantly more increased in B, than in C (5.1 +/- 1.2 vs 3.6 +/- 0.2 and 23.4 +/- 2.9 vs 12.3 +/- 1.7 msec(2), P < 0.01, respectively) and after pilsicainide administration in B. (5.1 +/- 0.4 vs 3.9 +/- 0.3, 25.8 +/- 3.4 vs 16.3 +/- 2.6 msec(2), P < 0.01, respectively) However, QT-SD and QT-TP did not significantly change in any of other leads (V(3)-V(6)) and RR-SD and RR-TP were similar for both groups, as well as after intravenous pilsicainide administration in B.

CONCLUSIONS

The temporal QT interval variability was identified in BS. Moreover, sodium channel blocker induced temporal fluctuation in QT interval and it may possibly provide a substrate for ventricular arrhythmia in BS patients.

Impact of study design on proarrhythmia prediction in the SCREENIT rabbit isolated heart model

INTRODUCTION

Prediction of the propensity of a compound to induce Torsades de Pointes continues to be a formidable challenge to the pharmaceutical industry. Development of an in vitro model for assessment of proarrhythmic potential offers the advantage of higher throughput and reduced compound quantity requirements when compared to in vivo studies. A rabbit isolated heart model (SCREENIT) has been reported to identify compounds with proarrhythmic potential based on the observance of compound-induced triangulation and instability of the monophasic action potential (MAP), ectopic beats, and reverse-use dependence of prolongation of the MAP duration. Previous reports have indicated that this model qualitatively identifies proarrhythmic compounds and suggest the use of this model to assign safety margins for human clinical use. The intent of this series of studies was to evaluate the impact of study design on the proarrhythmic concentration predicted by this model.

METHODS

Nine compounds of varying proarrhythmic potential and a negative control were tested in a blinded fashion using a series of different experimental protocols: Compounds were tested at multiple concentration ranges and extended perfusion times were also evaluated.

RESULTS

In general when the dataset is viewed as a whole, the model did identify proarrhythmic compounds, however the concentration at which action potential prolongation, triangulation, instability, reverse-use dependence and ectopic beats occurred often varied based on the concentration range selected. Further analysis using extended compound perfusion times demonstrated that variability may be due in part to lack of adequate equilibration of compound with the cardiac tissue.

DISCUSSION

We report that the model correctly identified proarrhythmic agents in a qualitative manner, but that study design impacts the proarrhythmic concentration derived from the model.

Cardiac safety strategies

This meeting was organised by IIR Life Sciences. It was chaired by Brian Guth, (head of General Pharmacology at Boehringer Ingelheim Pharma) and brought together scientists and clinicians from the pharmaceutical industry, university and regulatory agencies. The meeting presented emerging trends in cardiac safety, including its regulatory context pertaining to ICH S7A, S7B and E14. ICH S7A and S7B highlight the importance of the hERG test and telemetric studies in non-rodents. ICH E14 describes the clinical 'thorough QT study' that is required by the FDA for any new drug. Marked physiological variability in QT interval over time can be observed, partly as a result of fluctuation in autonomic tone. Beat-to-beat QT variability and T-wave morphology should be considered as a part of an integrated estimate of proarrhythmic risk. A case study illustrated the predictivity of preclinical data for proarrhythmic risk in humans, showing the importance of evaluating QT effects in patients to establish a safety margin.

Increased myocardial repolarization lability and reduced cardiac baroreflex sensitivity in individuals with high-normal blood pressure

BACKGROUND

Recent guidelines for the management of arterial hypertension have proposed that, to prevent cardiovascular disease, lifestyle modifications are required even in the case of high-normal blood pressure (HNBP).

OBJECTIVE

To assess myocardial repolarization and spontaneous cardiac baroreflex sensitivity (BRS) in newly diagnosed and never-treated individuals.

DESIGN AND PARTICIPANTS

We studied healthy individuals with HNBP according to the 2003 European Society of Hypertension-ESC guidelines and, for comparison, patients with renovascular hypertension (RVH) and healthy individuals with normal blood pressure (NBP).

MAIN OUTCOME MEASURES

Electrocardiogram and beat-to-beat blood pressure were recorded and spontaneous cardiac baroreflex sensitivity and the temporal QT interval variability index (QTVI) were calculated.

RESULTS

Individuals with HNBP had increased QTVI values compared with those with NBP (-1.23 +/- 0.37 compared with -1.52 +/- 0.26; P < 0.05), whereas patients with RVH had additionally increased QTVI values that were greater than those in healthy individuals with NBP or HNBP (-0.81 +/- 0.75; P < 0.05 compared with both groups). BRS was reduced in both groups of individuals with increased blood pressures compared with NBP (8.2 +/- 4.1 ms/mmHg for individuals with HNBP, 6.1 +/- 4.3 ms/mmHg for patients with RVH and 10.8 +/- 3.5 ms/mmHg for NBP; P < 0.05 for both).

CONCLUSION

In otherwise healthy individuals, even a moderate blood pressure increase is associated with increased myocardial repolarization lability and reduced baroreflex sensitivity (BRS). Patients with RVH have an additionally increased QTVI, with values similar to those reported in congestive heart failure. Future studies are needed to establish the value of QTVI and BRS measurements among individuals with HNBP in predicting the risk of progression to hypertension and end-organ damage.

Clinical implication of T-wave morphology analysis as a new repolarization descriptor

BACKGROUND

T-wave morphology analysis (TMA) quantifies irregularities of ventricular repolarization based on singular value decomposition of the 12-lead electrocardiogram (ECG). Furthermore, TMA is useful for risk stratification of patients with myocardial infarction (MI), although gender differences in TMA and the relationship between TMA and heart diseases are unknown. The aim of this study was to evaluate the significance of TMA in healthy individuals and patients with heart diseases.

METHODS AND RESULTS

Patients with heart disease and either with or without an implanted cardioverter defibrillator (ICD, n = 33, 57+/-16 years; non-ICD, n = 50, 67+/-10 years) were studied. Normal control ECGs (n = 114) were selected from Marquette's database (NC, 33+/-13 years) and the TMA descriptors, including T-wave morphology dispersion (TMD) and percentage of the loop area (PL), were calculated. TMD was significantly lower in group NC males than in the group NC females (11+/-5.9 vs 22+/-16, p < 0.0001). PL was significantly higher in group NC than in the ICD and non-ICD groups (0.63+/-0.12 vs 0.53+/-0.15, p < 0.0001). TMD of group NC was significantly lower than that of the ICD and non-ICD groups (14+/-11 vs 47+/-27, p < 0.0001).

CONCLUSION

There are gender differences in TMD. Abnormal values for TMA could reflect abnormalities of ventricular repolarization.

Sex Difference in the Relationship Between Insulin Resistance and Corrected QT Interval in Non-Diabetic Subjects

BACKGROUND

Men with a prolonged corrected QT (QTc) interval have an increased risk of cardiovascular mortality and sudden death, even in healthy individuals. In addition, prolonged QTc is a predictor of mortality in diabetics. However, the relationship between insulin resistance and QTc is not clarified in non-diabetic healthy people. The present study was performed to observe the association between QTc and insulin resistance in Korean non-diabetic subjects.

METHODS AND RESULTS

In the current study there was a total of 874 subjects (520 men, 354 women, mean age: 45.9+/-11.0 years) who underwent a medical check-up at the health promotion center at Kangbuk Samsung Hospital from January 2002 to May 2002. Age, sex, height, body weight, blood pressure, blood cholesterol, triglyceride, high-density lipoprotein cholesterol (HDL-C), fasting insulin, and fasting glucose levels were measured. The QT intervals were corrected using Bazett's formula (QTc = QT/square root of RR). Homeostasis model assessments (HOMA) were performed to assess the correlation between insulin resistance indices and the QTc interval. The mean QTc interval was significantly longer in females (417+/-24 ms) than in males (402+/-23 ms) (p < 0.001). After adjusting the variables related to the QTc interval, the differences in QTc between men and women were statistically significant (p < 0.001). A significant positive correlation was found between QTc and age, glucose, and blood pressure in male subjects. Female subjects showed positive correlation between QTc and age, glucose, blood pressure, total cholesterol, low-density lipoprotein cholesterol, triglyceride, fasting insulin, and the HOMA index, and a negative correlation between QTc and HDL-C. Multiple regression analysis showed that in men, age (beta = 0.480, p < 0.001) and diastolic blood pressure (beta = 0.280, p < 0.001) were predictors of QTc. In women, age (beta = 0.321, p = 0.008), diastolic blood pressure (beta = 0.324, p = 0.006) and HOMA index (beta = 3.508, p = 0.033) were predictors of QTc.

CONCLUSIONS

The present study of Korean healthy subjects shows that QTc was more prolonged in females than in males. In normoglycemic female subjects, insulin resistance was an independent determinant of the prolongation of QTc.

Effect of Physical Training on Corrected QT Dispersion in Patients With Nonischemic Heart Failure

BACKGROUND

The effect of physical training (PT) on QTc dispersion and ventricular tachycardia (VT) remains unclear in patients with nonischemic heart failure.

METHODS AND RESULTS

Eight patients with nonischemic heart failure performed PT using a bicycle ergometer and their exercise tolerance increased (4.9+/-1.8 to 7.0+/-2.5 METs, p<0.05) and QTc dispersion decreased (71+/-22 to 48+/-24 ms, p<0.05). However, PT did not change the frequency of VT.

CONCLUSION

Physical training could improve QTc dispersion in patients with nonischemic heart failure, possibly by improving the autonomic nerve system.