Alteration of the QT/RR relationship in patients with idiopathic ventricular tachycardia

Drug-induced QT interval shortening: potential harbinger of proarrhythmia and regulatory perspectives

ATP-dependent potassium channel openers such as pinacidil and levcromakalim have long been known to shorten action potential duration and to be profibrillatory in non-clinical models, raising concerns on the clinical safety of drugs that shorten QT interval. Routine non-clinical evaluation of new drugs for their potential to affect cardiac repolarization has revealed that drugs may also shorten QT interval. The description of congenital short QT syndrome in 2000, together with the associated arrhythmias, suggests that drug-induced short QT interval may be proarrhythmic, and an uncanny parallel is evolving between our appreciation of the short and the long QT intervals. Epidemiological studies report an over-representation of short QT interval values in patients with idiopathic ventricular fibrillation. Therefore, as new compounds that shorten QT interval are progressed further into clinical development, questions will inevitably arise on their safety. Arising from the current risk-averse clinical and regulatory environment and concerns on proarrhythmic safety of drugs, together with our lack of a better understanding of the clinical significance of short QT interval, new drugs that substantially shorten QT interval will likely receive an unfavourable regulatory review unless these drugs fulfil an unmet clinical need. This review provides estimates of parameters of QT shortening that may be of potential clinical significance. Rufinamide, a recently approved anticonvulsant, illustrates the current regulatory approach to drugs that shorten QT interval. However, to further substantiate or confirm the safety of these drugs, their approval may well be conditional upon large-scale post-marketing studies with a focus on cardiac safety.

Electrocardiographic predictors of sudden cardiac death

Sudden cardiac death (SCD) is widespread and the most serious of the cardiac diseases, accounting for over half of cardiovascular mortality in adults in the United States, and nearly 1 in 3 of these patients does not report symptoms of cardiac disease before the sudden death. Quantifying the left ventricular ejection fraction is currently the best way to risk-stratify patients for SCD and identify those who are most likely to benefit from the insertion of an implantable cardiac defibrillator (ICD). The strategy of systemically placing ICDs in patients at risk of SCD is expensive and leads to substantial psychological hardship. However, noninvasive electrocardiographic indices of depolarization and repolarization may better identify patients who are at an increased risk of SCD. Therefore, developing an approach to identify electrocardiographic changes associated with the highest risk of arrhythmic death could markedly improve patient selection for ICD therapy. This report describes electrocardiographic parameters that may be useful in identifying patients at risk of SCD. The state of the science currently suggests that it is unlikely that a single electrocardiographic parameter will predict SCD, but rather a risk stratification algorithm based on a combination of electrocardiographic parameters may yield the best result.

[Prognostic value of Poincare plot as nonlinear parameter of chaos theory in patients with myocardial infarction]

INTRODUCTION

There are different proofs about association of autonomic nervous system dysfunction, especially nonlinear parameters, with higher mortality after myocardial infarction.

OBJECTIVE

The objective of the study was to determine predictive value of Poincaré plot as nonlinear parameter and other significant standard risk predictors: ejection fraction of the left ventricle, late potentials, ventricular arrhythmias, and QT interval.

METHOD

The study included 1081 patients with mean follow-up of 28 months (ranging from 0-80 months). End-point of the study was cardiovascular mortality. The following diagnostic methods were used during the second week: ECG with commercial software Schiller AT-10: short time spectral analysis of RR variability with analysis of Poincaré plot as nonlinear parameter and late potentials; 24-hour ambulatory ECG monitoring: QT interval, RR interval, QT/RR slope, ventricular arrhythmias (Lown > II); echocardiography examinations: systolic disorder (defined as EF < 40%).

RESULTS

There were 103 (9.52%) cardiovascular deaths during the follow-up. In univariate analysis, the following parameters were significantly correlated with mortality: mean RR interval < 800 ms, QT and RR interval space relationship as mean RR interval < 800 ms and QT interval > 350 ms, positive late potentials, systolic dysfunction, Poincaré plot as a point, ventricular arrhythmias (Lown > II). In multivariate analysis, the significant risk predictors were: Poincaré plot as a point and mean RR interval lower than 800 ms.

CONCLUSION

Mean RR interval lower than 800 ms and nonlinear and space presentation of RR interval as a point Poincaré plot were multivariate risk predictors.

Reproducibility of QT parameters derived from 24-hour ambulatory ECG recordings in healthy subjects

AIM

To estimate the reproducibility of QT parameters derived from 24-hour ambulatory ECG recordings.

METHOD

Ten healthy volunteers aged 25 to 41 years participated. In two 24-hour ambulatory ECG recordings obtained 1 day apart, the QT interval was measured manually at stable heart rates in approximately 16 periods during daytime and 6 periods during nighttime. The association between the QT and RR interval was described by linear regression for day and nighttime separately and the following QT parameters were calculated: the QT interval at heart rate 60 beats/min during daytime (QT(60)day), slope(day), slope(night), and the difference in QT(60) between day and nighttime (DeltaQT(60)). The QT parameters were assessed four times for each participant to discriminate method inaccuracy from day to day variation. The reproducibility was estimated as the coefficient of repeatability, the relative error, and the ratio between within-subject variability and between-subject variability.

RESULTS

The coefficient of repeatability, the relative error and the ratio, respectively, were 19 ms, 1.8% and 0.5 for QT(60)day, 0.076, 21% and 0.68 for slope(day), 0.116, 43% and 1.37 for slope(night), and 37 ms, 325% and 1.19 for DeltaQT(60) when estimating the overall day to day reproducibility. Inaccuracy of QT measurement accounted for approximately 40% of this variation, whereas the error caused by selecting segments was small.

CONCLUSION

QT(60)day has a high reproducibility and may with advantage replace the conventional QT interval measured on a resting ECG. To assess QT dynamics, the slope of the regression line during daytime is suitable and the short term reproducibility acceptable for clinical trials. Regarding slope(night) and DeltaQT(60), the variation is high and the parameters should be used with caution.

Assessment of the stability of the individual-based correction of QT interval for heart rate

BACKGROUND

Modeling the relationship between QT intervals and previous R-R values remains a challenge of modern quantitative electrocardiography. The technique based on an individual regression model computed from a set of QT-R-R measurements is presented as a promising alternative. However, a large set of QT-R-R measurements is not always available in clinical trials and there is no study that has investigated the minimum number of QT-R-R measurements needed to obtain a reliable individual QT-R-R model. In this study, we propose guidelines to ensure appropriate use of the regression technique for heart rate correction of QT intervals.

METHOD

Holter recordings from 205 healthy subjects were included in the study. QT-R-R relationships were modeled using both linear and parabolic regression techniques. Using a bootstrapping technique, we computed the stability of the individual correction models as a function of the number of measurements, the range of heart rate, and the variance of R-R values.

RESULTS

The results show that the stability of QT-R-R individual models was dependent on three factors: the number of measurements included in its design, the heart-rate range used to design the model, and the T-wave amplitude. Practically our results showed that a set of 400 QT-R-R measurements with R-R values ranging from 600 to 1000 ms ensure a stable and reliable individual correction model if the amplitude of the T wave is at least 0.3 mV. Reducing the range of heart rate or the number of measurements may significantly impact the correction model.

CONCLUSION

We demonstrated that a large number of QT-R-R measurements (approximately 400) is required to ensure reliable individual correction of QT intervals for heart rate.

Effect of age on diurnal changes of 24-hour QT interval variability

QT interval on the surface electrocardiogram (ECG) reflects the time for repolarization of myocardium and prolongation of QTc is strongly associated with sudden cardiac death. Studies using novel techniques on beat-to-beat QT interval variability (QTV) have shown that it is influenced by the autonomic nervous system and is a predictor of sudden cardiac death. In this study, we examined the awake and sleep changes in QTV in 39 normal adults (mean age, 35 years) and 10 children (mean age, 11 years) using 24-hour ECG records. We obtained eight 5-minute segments of ECG sampled at 1000 Hz from the 24-hour records. Our results show that there is a diurnal variation of QTvm, detrended QT interval variance corrected for mean QT, and QTvi, an index of QTvm divided by heart rate variability corrected for mean heart rate. There was a significant increase in mean QT during sleep, whereas there was a significant decrease in QTvm and QTvi. QTvi significantly increased during the early morning hours. There were significant but modest correlations between the average 24-hour and awake QTvi and age (p < 0.01). There were also decreased low-frequency and high-frequency powers of QT during sleep. Coherence between heart rate and QT interval fluctuations in the range of 0-0.5 Hz, and especially in the high-frequency range (0.15-0.5 Hz), was significantly lower in adults than in children (p < 0.0002). These findings demonstrate diurnal fluctuations in ventricular repolarization lability. We speculate that these effects may relate to changes in cardiac autonomic function and may contribute to the well-known diurnal variation in the incidence of ventricular arrhythmias.

QT interval dynamics predict mortality in high-risk patients after myocardial infarction

OBJECTIVE

The purpose of this study was to evaluate the prognostic properties of the QT/RR relationship in survivors of myocardial infarction with severe left ventricular dysfunction.

DESIGN

The QT/RR interval slope was assessed from 24-h electrocardiographic recordings obtained from 241 patients representative of survivors of an acute myocardial infarction with an ejection fraction < or =35%. During a median follow-up of 752 days, 63 patients died.

RESULTS

In a multivariate analysis QT/RR at night-time was an independent predictor of all-cause mortality. A linear all-cause mortality predictor score was calculated from age, ejection fraction, QRS duration and QT/RR night, allowing identification of a high-risk one-third of the patients with a relative risk of death at 4.0.

CONCLUSION

This study demonstrates independent prognostic information of night-time QT/RR dynamics in patients at high risk after myocardial infarction. A linear predictor score calculated from age, ejection fraction, QRS duration and QT/RR night identifies a high-risk one-third of the patients with a 1-year mortality of 36% vs 9% in the remaining patients.

Spatial features in body surface potential maps of patients with ventricular tachyarrhythmias with or without coronary artery disease

Body surface potential maps (BSPM) from patients with coronary artery disease or no structural heart disease were analyzed with respect to their spatial features and QT/QTc dispersion in order to determine whether BSPM allows identification of patients with ventricular fibrillation. QRST integral maps and QT/QTc dispersion were acquired from simultaneous recordings of 62 ECG leads during sinus rhythm in patients with idiopathic ventricular fibrillation (n=13), ventricular fibrillation and coronary artery disease (n=22), coronary artery disease without ventricular fibrillation (n=21) and healthy controls (n=18). The Karhunen-Loeve transformation was applied to reduce the dimensionality of the data matrix of the QRST map to eight coefficients. Linear discriminant analysis allowed discrimination between idiopathic ventricular fibrillation patients and controls with high sensitivity (85%) and specificity (89%). However, discrimination between coronary artery disease patients with or without ventricular fibrillation was poor (68% and 67%, respectively). QTc dispersion calculated from BSPM was longer in idiopathic ventricular fibrillation patients than in controls (99+/-30 ms vs 70+/-14 ms, P=0.009) in contrast to QTc dispersion taken from 12-lead ECG (53+/-21 ms vs. 47+/-12 ms, P=n.s.). No significant difference was noted for coronary artery disease patients with or without ventricular fibrillation. In conclusion, repolarization disturbances detected by BSPM allow identification of ventricular fibrillation patients without structural heart disease. However, our results do not suggest a major impact of QT/QTc dispersion or QRST integral mapping for identification of ventricular fibrillation patients with coronary artery disease.

Effect of verapamil on QT interval dynamicity

Using measurements of QT/RR slopes with a computerized Holter system, QT interval dynamicity was evaluated in 19 patients with normal structural heart before and 2 months after oral treatment with verapamil prescribed for paroxysmal atrioventricular nodal reentrant tachycardia. Verapamil significantly shortened QT at low heart rates, mainly in the diurnal period, and this characteristic may explain, in part, the previously reported protective effect of verapamil against torsades de pointes.

QT variability before and after episodes of nonsustained ventricular tachycardia in patients with hypertrophic cardiomyopathy

This study examined the changes in QT dynamics occurring during 5-minute intervals sampled immediately before and 1 hour after episodes of nonsustained ventricular tachycardia (VT) in patients with hypertrophic cardiomyopathy (HCM). Twenty-four hour Holter recordings were performed in 10 patients with HCM in the absence of antiarrhythmic medications and processed by the ELA Medical QT analysis software. All sinus complexes were averaged over 30-second segments and 2,880 templates were created. For each template, a mean corrected QTec (time interval between the onset of QRS and the end of the T wave) and QTac (time interval between the onset of the QRS and the peak of the T wave) were calculated, with their standard deviations (SDQTe and SDQTa) taken as indices of QT variability. The slopes of the regression line for the QTe and QTa against the corresponding RR also were calculated. Forty 5-minute segments were analyzed immediately before (sample A) and 1 hour after (sample B) 20 episodes of nonsustained VT. QTac was significantly longer in group A than in group B (321 +/- 20 vs 312 +/- 22, P < 0.0001) and SDQTa was significantly lower (2.8 +/- 1.2 vs 4.7 +/- 3.7, P < 0.03). There were no significant differences in QTec, SDQTe, QTe/RR and QTa/RR before and after the episodes. Our data indicate that in patients with HCM, the averaged QTac is significantly longer and the QTa variability significantly lower before episodes of nonsustained VT.

Effect of atenolol or metoprolol on waking hour dynamics of the QT interval in myocardial infarction

In subjects with a recent acute myocardial infarction, the hour immediately following awakening is associated with an abrupt exaggeration of heart rate-dependent changes and variability of the QT interval. Beta blockers were observed to blunt these waking hour changes.

Effect of exercise-induced ischemia on QT interval dispersion

An increased spatial dispersion of ventricular repolarization duration (QT dispersion) is associated with an increased vulnerability to arrhythmias. This study was designed to examine the effect of exercise on QT dispersion in ischemic heart disease (IHD). QT dispersion, corrected QT dispersion, and percentage change in uncorrected and corrected QT dispersion between rest and peak exercise were examined in 14 members of a control group, 17 patients with IHD, and 14 patients with IHD who were receiving beta-blockers (IHD-B). All subjects had undergone a standard Bruce protocol exercise test, and QT intervals were measured at rest and peak exercise with a digitizing tablet interfaced to a personal computer. QT dispersion at rest was markedly increased in the IHD group compared with that in the control and IHD-B groups, respectively (corrected QT dispersion in milliseconds), 74 +/- 7, 40 +/- 4, 49 +/- 5, p < 0.03). The corrected QT dispersion at peak exercise was greater in the IHD group compared with that in the control group (57 +/- 5 vs 26 +/- 3 msec, p < 0.03). The percentage change in QT dispersion with exercise was significantly higher in the IHD group (52% +/- 5%) compared with that in both the control group (28% +/- 4%, p < 0.002) and the IHD-B group (30% +/- 3%, p < 0.01). A larger mean QT dispersion at peak exercise and an increased percentage change in QT dispersion with exercise may help explain the increased susceptibility of the IHD group for arrhythmias. The cardioprotective action of beta-blockers may be explained by their blunting effect on exercise-related changes in QT dispersion.

Ventricular arrhythmias in apparently healthy subjects

While it is assumed that the normal heart does not predispose to serious arrhythmias, several conditions are now being recognized as being associated with short-lasting ventricular arrhythmias. It also becomes clear that idiopathic VT (or repetitive monomorphic VT) sometimes exists on the background of a compromised heart. Whether this dysfunction is due to the arrhythmia or vice versa is not evident. Finally, VF occurs in patients who, at a first glance, have no apparent heart disease, and it is then called idiopathic VF. These complex electrical abnormalities probably reflect disorders, which often are genetically determined. Recognition of these syndromes, often characterized by abnormal repolarization or a disturbed autonomic function is possible if appropriate techniques are used.

Ambulatory assessment of the QT interval in patients with hypertrophic cardiomyopathy: risk stratification and effect of low dose amiodarone

This study aims to assess the dynamics of the QT interval in patients with hypertrophic cardiomyopathy (HCM). Three consecutive QT intervals and the preceding RR intervals were measured on 24 hour ambulatory electrocardiograms at 30-minute intervals in ten high risk patients with HCM (sudden cardiac death [SCD] and/or documented ventricular fibrillation), aged 29 +/- 17 years, compared with ten age and sex matched low risk patients with HCM (no syncope, no adverse family history, and no ventricular tachycardia on Holter monitoring), and ten normal subjects. Another ten patients who were on amiodarone therapy (200-mg daily) were also studied. Patients with intraventricular conduction defects were excluded. There were 4,424 pairs of QT intervals and their preceding RR intervals were measured in this study. A nonsignificant prolongation in the QT interval and a significant prolongation in QTc values (Bazett's and Fridericia's formulas) were demonstrated in patients with HCM compared with normals. There were no significant differences in the QT and QTc between high and low risk patients. The slope of regression line for the QT against RR interval was significantly different between normals and HCM (0.1583 +/- 0.040 vs. 0.2017 +/- 0.043, P < 0.05), but not between high and low risk patients. Amiodarone significantly prolonged the QT and QTc without significantly altering the slope of the regression line (0.2017 +/- 0.043 vs 0.2099 +/- 0.037, NS). Our findings support the observations that there is a prolonged QT interval in patients with HCM and that there is no significant use dependent effect of amiodarone on ventricular repolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Is there an abnormal QT interval in sudden cardiac death survivors with a "normal" QTc?

Correcting the QT interval for heart rate may mask ventricular repolarization abnormalities and may lead to a misinterpretation of the physiologic and pathophysiologic findings. In this study the QT/R-R relationship was studied in eight sudden cardiac death (SCD) survivors without overt structural heart disease and compared with eight age- and sex-matched normal subjects. All patients were in a drug-free state. The QT intervals and their preceding R-R intervals were measured on electrocardiogram (ECG) strips from 24-hour Holter recordings at hourly intervals. The differences in mean heart rate, mean QT intervals, and QTc values between SCD survivors and normal subjects were not statistically significant. There was a significant correlation between the QT and R-R intervals in normal subjects (tau = 0.71 +/- 0.13, p < 0.05) and in SCD survivors (tau = 0.79 +/- 0.07, p < 0.05). However, the regression line of the QT interval against the R-R interval was significantly (p < 0.01) altered in SCD survivors compared with normal subjects. These observations suggest that there is abnormal ventricular repolarization (QT interval) despite an apparently normal QTc using Bazett's formula in these patients. Evaluation of the QT/R-R relationship by means of 24-hour ambulatory Holter ECG monitoring may provide a useful clinical tool for the assessment of ventricular repolarization abnormalities.