Diurnal variation of the QT interval--influence of the autonomic nervous system

Circadian rhythms govern cardiac repolarization and arrhythmogenesis

Sudden cardiac death exhibits diurnal variation in both acquired and hereditary forms of heart disease ^{1, 2}, but the molecular basis is unknown. A common mechanism that underlies susceptibility to ventricular arrhythmias is abnormalities in the duration (e.g. short or long QT syndromes, heart failure) ^3-5 or pattern (e.g. Brugada syndrome) ⁶ of myocardial repolarization. Here we provide the first molecular evidence that links circadian rhythms to vulnerability in ventricular arrhythmias in mice. Specifically, we show that cardiac ion channel expression and QT interval duration (an index of myocardial repolarization) exhibit endogenous circadian rhythmicity under the control of a novel clock-dependent oscillator, Krüppel-like factor 15 (Klf15). Klf15 transcriptionally controls rhythmic expression of KChIP2, a critical subunit required for generating the transient outward potassium current (I_to). ⁷ Deficiency or excess of Klf15 causes loss of rhythmic QT variation, abnormal repolarization and enhanced susceptibility to ventricular arrhythmias. In sum, these findings identify circadian transcription of ion channels as a novel mechanism for cardiac arrhythmogenesis.

Estimation and modeling of QT-interval adaptation to heart rate changes

This paper introduces a new method for QT-interval estimation. It consists in a batch processing mode of the improved Woody's method. Performance of this methodology is evaluated using synthetic data. In parallel, a new model of QT-interval dynamics behavior related to heart rate changes is presented. Since two kinds of QT response have been pointed out, the main idea is to split the modeling process into two steps: 1) the modeling of the fast adaptation, which is inspired by the electrical behavior at the cellular level relative to the electrical restitution curve, and 2) the modeling of the slow adaptation, inspired by experimental works at the cellular level. Both approaches are based on a low-complexity autoregressive process whose parameters are estimated using an unbiased estimator. This new modeling of QT adaptation, combined with the presented QT-estimation process, is applied to several ECG recordings with various heart rate variability dynamics. Its potential is then illustrated on ECG recorded during rest, atrial fibrillation episodes, and exercise. Meaningful results in agreement with physiological knowledge at the cellular level are obtained.

Frequency domain assessment of the coupling strength between ventricular repolarization duration and heart period during graded head-up tilt

We test the hypothesis that the degree of correlation between ventricular repolarization duration (VRD) and heart period (HP) carries information on cardiac autonomic regulation. The degree of correlation was assessed in the frequency domain using squared coherence function during an experimental protocol known to gradually induce a shift of sympathovagal balance toward sympathetic predominance (ie, graded head-up tilt). We observed a progressive decrease of squared coherence with tilt table inclination, thus confirming the working hypothesis. The VRD-HP uncoupling occurs in the high-frequency band, centered on the respiratory rate, thus suggesting that vagal withdrawal is responsible for the VRD-HP uncoupling.

The ECG vertigo in diabetes and cardiac autonomic neuropathy

The importance of diabetes in the epidemiology of cardiovascular diseases cannot be overemphasized. About one third of acute myocardial infarction patients have diabetes, and its prevalence is steadily increasing. The decrease in cardiac mortality in people with diabetes is lagging behind that of the general population. Cardiovascular disease is a broad term which includes any condition causing pathological changes in blood vessels, cardiac muscle or valves, and cardiac rhythm. The ECG offers a quick, noninvasive clinical and research screen for the early detection of cardiovascular disease in diabetes. In this paper, the clinical and research value of the ECG is readdressed in diabetes and in the presence of cardiac autonomic neuropathy.

Changes in ventricular repolarization duration during typical daily emotion in patients with Long QT syndrome

OBJECTIVE

Intense emotions are known triggers of sudden cardiac death. However, the effect of typical daily emotion on repolarization has not been examined. We examined whether QT interval changes as a function of typical daily emotion in patients at risk for cardiac events in the context of emotion.

METHODS

We studied 161 patients (n = 114 females; mean age, 35 years) with the congenital form of the Long QT Syndrome during daily activities. Each day for 3 days, a 12-hour Holter recording was completed. Patients were paged ten times per day at random times and rated the intensity of 16 prespecified emotions during the preceding 5 minutes. Measurements of QT interval and interbeat intervals were synchronized with emotion ratings.

RESULTS

Low Arousal Positive Affect was associated with significant increases in QT interval corrected for heart rate (using Fridericia's QTc) (p < .001), whereas higher arousal Activated Positive Affect (p < .001) and Activated Negative Affect (p < .01) were associated with significant decreases in QTc. Changes in QTc as a function of daily emotion ranged from 5-ms increases to 11-ms decreases. High-frequency heart rate variability (vagal tone) was positively correlated with QTc (p < .001). The effects of each positive emotion variable on QTc were greater in LQT2 than LQT1 patients (p < .001).

CONCLUSION

Ventricular repolarization duration (QTc) changes dynamically as a function of daily emotion. These changes are relatively small and do not constitute a risk in themselves. In the context of other risk factors, however, they may contribute to ventricular arrhythmias in vulnerable populations.

Cardiovascular disease, chronopharmacotherapy, and the molecular clock

Cardiovascular functions such as heart rate and blood pressure show 24h variation. The incidence of cardiovascular diseases including acute myocardial infarction and arrhythmia also exhibits diurnal variation. The center of this circadian clock is located in the suprachiasmatic nucleus in the hypothalamus. However, recent findings revealed that each organ, including cardiovascular tissues, has its own internal clock, which has been termed a peripheral clock. The functional roles played by peripheral clocks have been reported recently. Since the peripheral clock is considered to play considerable roles in the processes of cardiac tissues, the identification of genes specifically regulated by this clock will provide insights into its role in the pathogenesis of cardiovascular disorders. In addition, the discovery of small compounds that modulate the peripheral clock will help to establish chronotherapeutic approaches. Understanding the biological relevance of the peripheral clock will provide novel approaches to the prevention and treatment of cardiovascular diseases.

Inotropic response of cardiac ventricular myocytes to beta-adrenergic stimulation with isoproterenol exhibits diurnal variation: involvement of nitric oxide

RATIONALE

Although >10% of cardiac gene expression displays diurnal variations, little is known of their impact on excitation-contraction coupling.

OBJECTIVE

To determine whether the time of day affects excitation-contraction coupling in rat ventricles.

METHODS AND RESULTS

Left ventricular myocytes were isolated from rat hearts at 2 opposing time points, corresponding to the animals resting or active periods. Basal contraction and [Ca(2+)](i) was significantly greater in myocytes isolated during the resting versus active periods (cell shortening 12.4+/-0.3 versus 11.0+/-0.2%; P<0.05 and systolic [Ca(2+)](i) 422+/-12 versus 341+/-9 nmol/L; P<0.01. This corresponded to a greater sarcoplasmic reticulum (SR) Ca(2+) load (672+/-20 versus 551+/-13 nmol/L P<0.001). The increase in systolic [Ca(2+)](i) in response to isoproterenol (>3 nmol/L) was also significantly greater in resting versus active period myocytes, reflecting a greater SR Ca(2+) load at this time. This diurnal variation in response of Ca(2+)-homeostasis to isoproterenol translated to a greater incidence of arrhythmic activity in resting period myocytes. Inhibition of neuronal NO synthase during stimulation with isoproterenol, further increased systolic [Ca(2+)](i) and the percentage of arrhythmic myocytes, but this effect was significantly greater in active period versus resting period myocytes. Quantitative RT-PCR analysis revealed a 2.65-fold increase in neuronal NO synthase mRNA levels in active over resting period myocytes (P<0.05).

CONCLUSIONS

The threshold for the development of arrhythmic activity in response to isoproterenol is higher during the active period of the rat. We suggest this reflects a reduction in SR Ca(2+) loading and a diurnal variation in neuronal NO synthase signaling.

RT variability unrelated to heart period and respiration progressively increases during graded head-up tilt

Open-loop linear parametric models were exploited to describe ventricular repolarization duration (VRD) variability during graded head-up tilt. Surface ECG and thoracic movements were recorded in 15 healthy humans (age: 24-54 yr, median: 28 yr; 6 women and 9 men). Tilt table inclinations ranged from 15 to 90 degrees and were varied in steps of 15 degrees . All subjects underwent recordings at every step in random order. Heart period was assessed as the time difference between two consecutive R-wave peaks (RR) and the respiratory signal (R) as the sampling of the thoracic movement signal at the R-wave peaks. VRD was measured automatically as the temporal difference between the R-wave peak and T-wave apex (RT(a)) or T-wave end (RT(e)). The best model decomposed RT variability as due to RR changes (RR-related RT variability) to direct respiratory-related inputs (R-related RT variability) and to unknown rhythmical sources unrelated to RR changes and R (RR-R-unrelated RT variability). Using this model, RT(e) variability was found to be less predictable than RT(a) variability and composed of a smaller fraction of RR-related RT variability and a larger fraction of RR-R-unrelated RT variability. Predictability progressively decreased with tilt table angles, suggesting increased complexity of RT regulation. RT variance progressively increased with tilt table inclination. This increase was characterized by a gradual rise of the amount of RR-R-unrelated RT variability, whereas the amount of RR-related RT variability remained unchanged. These results suggest that the amount of RT variability, complexity of RT dynamics, and amount of RR-R-unrelated RT variability increase with the magnitude of the sympathetic drive directly related to tilt table inclination. We propose the utilization of the amount of RR-R-unrelated RT variability instead of overall RT variability as an indirect measure of autonomic regulation directed to ventricles.

The effect of different stressors on the QT interval and the T wave

Prolonged QT interval is associated with the generation of life-threatening arrhythmias and sudden death. However, neither the relation between QT duration and heart rate, nor the association between mental stress and QT time has been clarified. Aim: The relationship between QT duration and smoking, cardiovascular reactivity, and mental stress as well as newer methods of QT correction were studied. Methods: In six laboratory experiments 166 volunteers were studied. Smoking, treadmill exercise, mental arithmetic and videogame were applied as stressors. Besides fixed formulae, study and subject-specific QT correction methods were also used. Results: 1. Bazett formula is not appropriate to compare QT durations. 2. Acute smoking has no effect on QT time. 3. QT changes are related to cardiovascular reactivity. 4. Mental stress may induce QT prolongation. 5. Bifid T waves often develop during mental and isometric stress. Conclusions: New methods for QT correction are more reliable than preformed formulae. QT prolongation and bifid T waves may be the links between mental stress and life threatening arrhythmias.

Cardiac repolarization interval in end-stage diabetic and nondiabetic renal disease

BACKGROUND AND HYPOTHESIS

QT interval length is influenced by autonomic nervous activity. In patients with diabetic autonomic neuropathy, both prolongation and shortening of ventricular repolarization has been reported. We studied diabetic and nondiabetic uremic patients to assess the effects of autonomic neuropathy on QT interval length.

METHODS

24-hour electrocardiogram recordings were performed in 12 diabetic and 11 nondiabetic renal transplantation patients, and in 12 control patients. Mean and corrected QT interval (QTc) during the 24-h period and intervals at predetermined heart rates at day and night periods were determined. The degree of autonomic neuropathy was assessed with cardiovascular autonomic function tests and measurement of heart rate variability.

RESULTS

In the diabetic group, severe autonomic neuropathy was present; in nondiabetic uremic patients, abnormalities were less severe. Mean QTc interval during 24 h was 444 +/- 24, 447 +/- 21, and 442 +/- 19 ms in the diabetic and nondiabetic uremic patients, and in the control groups, respectively, without any between-group difference. QT and QTc interval length did not differ among the groups when measured at heart rates of 70, 80, 90, or 100 beats/min.

CONCLUSIONS

In patients with autonomic failure caused by diabetes and/or uremia, QT interval length cannot be used as a diagnostic indicator of cardiac autonomic neuropathy.

Heart rate and catecholamine contribution to QT interval shortening on exercise

BACKGROUND

QT interval shortens with exercise. Some of this shortening is due to an increase in heart rate, and some is due to other effects of exercise, probably mostly neuroendocrine effects. Data from subjects with cardiac transplants have suggested that non-heart rate-related changes in QT interval on exercise are due to the effects of circulating catecholamines.

HYPOTHESIS

We sought to determine whether changes in plasma catecholamine levels with exercise are an important contributor to non-heart rate-related QT interval shortening.

METHODS

Subjects with DDD pacemakers were recruited. Subjects had QT intervals measured at rest, during a low fixed level exercise test designed to increase heart rate to about 110 beats/min, and, after resting, during pacing at a heart rate of 110 beats/min. Catecholamine levels were measured at each stage of the study.

RESULTS

QT interval at rest was 420 +/- 12 ms, during pacing 366 +/- 16 ms, and on exercise 325 +/- 14 ms. This then gave the proportion of QT interval shortening due to heart rate as 68.6 +/- 9.3% of total QT shortening, with the range between 35 and 95.6%. There was no proportionality between the degree of QT interval shortening on exercise that was not due to increases in heart rate and changes in plasma catecholamine levels.

CONCLUSION

Two-thirds of exercise-induced QT interval shortening are due to an increase in heart rate, and one-third to other effects. Changes in plasma catecholamine levels on exercise were not closely related to changes in the QT interval on exercise.

Prolonged QT interval and reduced heart rate variability in patients with uncomplicated essential hypertension

A prolonged QT interval is a risk factor for ischemic heart disease in hypertensive subjects. Heart rate variability (HRV) is both an index of autonomic function and an important prognostic factor in several diseases. The aim of the present study was to evaluate the relation between a prolonged QT interval and autonomic nervous system function in patients with untreated uncomplicated essential hypertension. Two hundred and fifteen untreated patients with essential hypertension underwent a Holter ECG equipped with software dedicated to HRV and QT analyses. Nine percent of the patients showed a corrected QT (QTc) >or=440 ms. The HRV indexes in the time domain (SDNN, SDNN index, RMSSD, and pNN50) were significantly reduced in the patients with a prolonged QTc compared to those with a normal QTc (SDNN 24 h: 126.4+/-29.9 vs. 143.9+/-35.4 ms, p=0.02; SDNN index [nighttime]: 85.9+/-32.4 vs. 115.5+/-36.7 ms, p=0.0006; RMSSD 24 h: 22.2+/-7.7 vs. 31.2+/-13.0 ms, p=0.0007; pNN50 24 h: 4.4+/-4.9 vs. 9.7+/-8.4%, p=0.0006). The linear correlation analysis between QTc length and HRV parameters showed a significant negative correlation with all the time-domain indexes. Such a correlation was maintained for RMSSD 24 h, pNN50 24 h and SDNN index (nighttime) after correction for gender and age. The present study shows that, even prior to the development of cardiac hypertensive disease, a prolongation of the QTc and a reduced HRV, both markers of cardiovascular risk, coexist in a proportion of patients with untreated essential hypertension. Further studies are warranted to evaluate whether the combination of such markers can identify hypertensive patients at risk for life-threatening arrhythmias and sudden death. (Hypertens Res 2008; 31: 2003-2010).

Low-complexity autoregressive modeling of the fast and slow QT adaptation to heart rate changes

The aim is to develop a new model of the QT interval dynamics behavior related to heart rate changes. Since two kinds of QT response have been pointed out, the main idea is to split the modeling process into two steps: 1) the modeling of the "fast" adaptation, which is inspired by the electrical behavior at the cellular level relative to the electrical restitution curve, 2) the modeling of the "slow" adaptation, inspired by experiments works at the cellular level. Both are modeled as low-complexity autoregressive process whose parameters are computed using an unbiased estimator. The relevance of this approach is illustrated on several ECG recordings where the variations of the heart rate are various (rest, atrial fibrillation episodes, exercise). Significant results are obtained in agreement with the physiological knowledge at the cellular level.

Baseline values and sotalol-induced changes of ventricular repolarization duration, heterogeneity, and instability in patients with a history of drug-induced torsades de pointes

The authors investigated whether computerized parameters quantifying ventricular repolarization delay, heterogeneity, and instability characterize individuals who developed drug-induced Torsades de Pointes. Assessing an individual's propensity to Torsades de Pointes when exposed to a QT-prolonging drug is challenging because baseline QT prolongation has limited predictive value. Five-minute digital 12-lead electrocardiograms were acquired at baseline and after a sotalol challenge in 16 patients who had a history of Torsades de Pointes in the context of a QT-prolonging drug and 17 patients who did not have such history. Computerized measurements of QTc, T peak to T end intervals (TpTe), TpTe/QTc, and QT variability were implemented, and novel quantifiers of ventricular repolarization heterogeneity from the early (ERD) and late (LRD) part of the T wave were investigated. Compared with electrocardiograms of patients without a history of Torsades de Pointes, the baseline electrocardiograms of patients with a history of Torsades de Pointes had a longer QTc and an increased repolarization heterogeneity of the early part of the T wave (ERD30%: 44 +/- 13 vs 35 +/- 8 ms, P = .02). On sotalol, the electrocardiograms from individuals with Torsades de Pointes revealed a delay of the terminal part of the T wave that was not present in patients without Torsades de Pointes (TpTe: 27 +/- 40 vs -2 +/- 21 ms, P = .02; LRD70%: 20 +/- 29 vs 2 +/- 4 ms, P = .04). Results suggest that the electrocardiogram abnormalities characterizing patients with a history of Torsades de Pointes are (1) an increased repolarization heterogeneity at baseline and (2) a sotalol-induced prolongation of the terminal part of the T wave.

Mental stress may induce QT-interval prolongation and T-wave notching

BACKGROUND

The effects of active and passive mental stress (PMS) on the QT interval were studied by using an intraindividual regression method of QT-interval correction for heart rate.

METHODS

Thirty healthy males (age 21.2 +/- 1.8 years) performed a mental arithmetic for 1 minute, which was considered as active mental stress (AMS) because of the performance requirement. A 1-minute unpleasant video clip was used for PMS. Two baseline and two (an early and a late) ECGs were prepared in both mental stress periods. The individual QT-RR relationship was assessed by linear regression analysis of 7-15 (11.0 +/- 1.9) controlled QT-RR data pairs, also obtained from ECGs gained during a successive set of 9 isometric stretching exercises.

RESULTS

Heart rate has increased significantly at both measurements in response to AMS (P < 0.0001), but not in response to passive stress. QTc significantly prolonged early in AMS (P = 0.0004), then normalized by the end of the period. During PMS, no significant QTc changes were observed. The evolution of bifid T waves was noted in 14 subjects: 8 presented bifid T waves during both AMS and exercise, and 6 during only exercise.

CONCLUSIONS

AMS and PMS elicit different cardiovascular reactions. Our results indicate that changes in the autonomic tone, probably abrupt sympathetic predominance, may cause QTc prolongation and bifid T waves. This suggests that besides stress quality and intensity, the dynamics of stress application and perception also influence repolarization.

Circadian rhythms in cardiac arrhythmias and opportunities for their chronotherapy

It is now well established that nearly all functions of the body, including those that influence the pharmacokinetics and pharmacodynamics of medications, exhibit significant 24-hour variation. The electrical properties of the heart as well as cardiac arrhythmias also vary as circadian rhythms, even though the suboptimal methods initially used for their investigation slowed their identification and thorough characterization. The application of continuous Holter monitoring of the electrical properties of the heart has revealed 24-hour variation in the occurrence of ventricular premature beats with the peak in events, in diurnally active persons, between 6 a.m. and noon. After the introduction of implantable cardioverter-defibrillators, ventricular tachycardia or fibrillation were also found to peak in the same period of the day. Even defibrillator energy requirements show circadian variation, thus supporting the need for a temporal awareness in the therapeutic approach to arrhythmias. Imbalanced autonomic tone, circulating levels of catecholamines, increased heart rate and blood pressure, all established determinants of cardiac arrhythmias, show circadian variations and underlie the genesis of the circadian pattern of cardiac arrhythmias. Arrhythmogenesis appears to be suppressed during nighttime sleep, and this can influence the evaluation of the efficacy of antiarrhythmic medications in relation to their administration time. Unfortunately, very few studies have been undertaken to assess the proper timing (chronotherapy) of antiarrhythmic medications as means to maximize efficacy and possibly reduce side effects. Further research in this field is warranted and could bring new insight and clinical advantage.

Cardiotoxicity of antimalarial drugs

There are consistent differences in cardiovascular state between acute illness in malaria and recovery that prolong the electrocardiographic QT interval and have been misinterpreted as resulting from antimalarial cardiotoxicity. Of the different classes of antimalarial drugs, only the quinolines, and structurally related antimalarial drugs, have clinically significant cardiovascular effects. Drugs in this class can exacerbate malaria-associated orthostatic hypotension and several have been shown to delay ventricular depolarisation slightly (class 1c effect), resulting in widening of the QRS complex, but only quinidine and halofantrine have clinically significant effects on ventricular repolarisation (class 3 effect). Both drugs cause potentially dangerous QT prolongation, and halofantrine has been associated with sudden death. The parenteral quinoline formulations (chloroquine, quinine, and quinidine) are predictably hypotensive when injected rapidly, and cardiovascular collapse can occur with self-poisoning. Transiently hypotensive plasma concentrations of chloroquine can occur when doses of 5 mg base/kg or more are given by intramuscular or subcutaneous injection. At currently recommended doses, other antimalarial drugs do not have clinically significant cardiac effects. More information on amodiaquine, primaquine, and the newer structurally related compounds is needed.

Circadian rhythms in myocardial metabolism and contractile function: influence of workload and oleate

Multiple extracardiac stimuli, such as workload and circulating nutrients (e.g., fatty acids), known to influence myocardial metabolism and contractile function exhibit marked circadian rhythms. The aim of the present study was to investigate whether the rat heart exhibits circadian rhythms in its responsiveness to changes in workload and/or fatty acid (oleate) availability. Thus, hearts were isolated from male Wistar rats (housed during a 12:12-h light-dark cycle: lights on at 9 AM) at 9 AM, 3 PM, 9 PM, and 3 AM and perfused in the working mode ex vivo with 5 mM glucose plus either 0.4 or 0.8 mM oleate. Following 20-min perfusion at normal workload (i.e., 100 cm H(2)O afterload), hearts were challenged with increased workload (140 cm H(2)O afterload plus 1 microM epinephrine). In the presence of 0.4 mM oleate, myocardial metabolism exhibited a marked circadian rhythm, with decreased rates of glucose oxidation, increased rates of lactate release, decreased glycogenolysis capacity, and increased channeling of oleate into nonoxidative pathways during the light phase. Rat hearts also exhibited a modest circadian rhythm in responsiveness to the workload challenge when perfused in the presence of 0.4 mM oleate, with increased myocardial oxygen consumption at the dark-to-light phase transition. However, rat hearts perfused in the presence of 0.8 mM oleate exhibited a markedly blunted contractile function response to the workload challenge during the light phase. In conclusion, these studies expose marked circadian rhythmicities in myocardial oxidative and nonoxidative metabolism as well as responsiveness of the rat heart to changes in workload and fatty acid availability.

Accuracy of Uncorrected versus Corrected QT Interval for Prediction of Torsade de Pointes Associated with Intravenous Haloperidol

STUDY OBJECTIVE

To determine if the uncorrected QT interval (QT(u)) more accurately predicts drug-induced torsade de pointes than QT intervals corrected using the Bazett's (QT(B)), Fridericia (QT(Frid)), or Framingham (QT(Fram)) methods.

DESIGN

Retrospective analysis of a previously reported case-control study of risk factors for haloperidol-induced torsade de pointes.

SETTING

Large tertiary care teaching hospital.

PATIENTS

Forty-six critically ill patients who received intravenous haloperidol for sedation; seven developed torsade de pointes.

MEASUREMENTS AND MAIN RESULTS

The QT intervals were measured manually by one investigator from electrocardiograms performed before and during haloperidol therapy. Logistic regression analysis for prediction of torsade de pointes was performed, incorporating QT(u), QT(B), QT(Frid), QT(Fram), and RR intervals measured during treatment. Receiver operating characteristics (ROC) curves were constructed. Primary outcome measures were proportion of explained variation (maximum-rescaled R2); area under the ROC curves for QT(u), QT(B), QT(Frid), QT(Fram), and RR interval; and sensitivity and specificity for prediction of haloperidol-induced torsade de pointes. The QT(u) was associated with the highest R2 compared with QT(Fram), QT(Frid), QT(B), and RR interval (0.77, 0.73, 0.68, 0.53, and 0.30, respectively). No significant differences in areas under the ROC curves were found between any of the QT-interval methods. Areas under the ROC curves for QT(u) and QT(Fram) trended toward being greater than that associated with the RR interval. All QT-interval methods were highly sensitive (100% for each), whereas the RR interval was less sensitive (86%); QT(u) and QT(Fram) were most specific (82%) compared with the QT(Frid) (72%), QT(B) (64%), and RR interval (36%).

CONCLUSION

Compared with QT(B) and QT(Frid), the QT(u) and QT(Fram) best predicted haloperidol-induced torsade de pointes in critically ill patients; the QT(Fram) offered no advantage over the QT(u).

Evaluation of a 12-Lead Digital Holter System for 24-Hour QT Interval Assessment

BACKGROUND

Drug induced QT prolongation may precipitate life threatening cardiac arrhythmias. Evaluation of the QT prolonging effect of new pharmaceutical agents in a 'thorough QT/QTc study' is being mandated by FDA. The purpose of this study was to evaluate an automated 12-lead digital Holter system for a thorough QT/QTc study.

METHODS

Five healthy volunteers underwent 24-hour digital Holter monitoring. Each recording underwent a fully automated QT analysis (AQA) followed by an onscreen complete manual over read (MOR). Each recording was analyzed twice at least 2 weeks apart. The effect of data sampling (5-min segment/hour), the system sensitivity to detect 5-ms increase in QT, and the ability to assess circadian variation were evaluated.

RESULTS

The AQA resulted in identical QT for the first and second analyses, but with obvious errors in QT measurements. Compared to the complete onscreen MOR, the mean QT was longer with AQA (416 +/- 41 vs. 387 +/- 30 ms, p < 0.001), correlation; r = 0.3. The reproducibility of AQA with complete MOR was very good (QT: 387 +/- 30 vs. 387 +/- 30 ms, coefficient of variation: 0.2%, r = 0.986. The 5-min mean QT intervals correlated well with the hourly mean QT intervals (r = 0.994, p < 0.001, coefficient of variation = 1 ms) and both showed a similar circadian variation. The system was sensitive to detect a 5-ms change in QT intervals (5 +/- 2 ms, coefficient of variation = 0.6%, r = 0.998, p < 0.001).

CONCLUSIONS

The AQA is not an acceptable method, while the automatic analysis with complete MOR is a highly sensitive and reproducible method. Data sampling by analyzing 5-min segments per hour is sensitive and reproducible.

Diurnal variation in QT dispersion in patients with chronic heart failure

QT dispersion is defined as the difference in QT interval among the different leads of the standard 12-lead electrocardiogram and reflects inhomogeneity of myocardial repolarization. Dispersion of repolarization is an important electrophysiologic feature that is considered fundamental for the initiation of ventricular fibrillation. However, no data exist regarding the diurnal variation of QT dispersion measured from simultaneous 12-lead recording in chronic heart failure patients. The aim of this study was to identify diurnal variation in QT dispersion in patients with chronic heart failure. QT dispersion was measured in the 12-lead standard electrocardiogram in 11 patients with chronic heart failure. QT dispersion in these patients was increased in the afternoon compared to the morning. It is concluded that QT dispersion has a clear diurnal variation in patients with chronic heart failure. These findings have potentially significant implications for therapy and prevention of sudden cardiac death in patients with chronic heart failure.

A novel predictive pharmacokinetic/pharmacodynamic model of repolarization prolongation derived from the effects of terfenadine, cisapride and E-4031 in the conscious chronic av node—ablated, His bundle-paced dog

INTRODUCTION

Terfenadine, cisapride, and E-4031, three drugs that prolong ventricular repolarization, were selected to evaluate the sensitivity of the conscious chronic atrioventricular node--ablated, His bundle-paced Dog for defining drug induced cardiac repolarization prolongation. A novel predictive pharmacokinetic/pharmacodynamic model of repolarization prolongation was generated from these data.

METHODS

Three male beagle dogs underwent radiofrequency AV nodal ablation, and placement of a His bundle-pacing lead and programmable pacemaker under anesthesia. Each dog was restrained in a sling for a series of increasing dose infusions of each drug while maintained at a constant heart rate of 80 beats/min. RT interval, a surrogate for QT interval in His bundle-paced dogs, was recorded throughout the experiment.

RESULTS

E-4031 induced a statistically significant RT prolongation at the highest three doses. Cisapride resulted in a dose-dependent increase in RT interval, which was statistically significant at the two highest doses. Terfenadine induced a dose-dependent RT interval prolongation with a statistically significant change occurring only at the highest dose. The relationship between drug concentration and RT interval change was described by a sigmoid E(max) model with an effect site. Maximum RT change (E(max)), free drug concentration at half of the maximum effect (EC(50)), and free drug concentration associated with a 10 ms RT prolongation (EC(10 ms)) were estimated. A linear correlation between EC(10 ms) and HERG IC(50) values was identified.

DISCUSSION

The conscious dog with His bundle-pacing detects delayed cardiac repolarization related to I(Kr) inhibition, and detects repolarization change induced by drugs with activity at multiple ion channels. A clinically relevant sensitivity and a linear correlation with in vitro HERG data make the conscious His bundle-paced dog a valuable tool for detecting repolarization effect of new chemical entities.

Comparison of the four formulas of adjusting QT interval for the heart rate in the middle-aged healthy Turkish men

OBJECTIVE

The aim of this study was to evaluate the QT intervals at different rest heart rates in healthy middle-aged Turkish men and to compare the known four QT adjusting methods for heart rate.

METHODS AND RESULTS

The QT intervals were measured in electrocardiograms of 210 healthy men (mean age = 35-60 years). A curve relating QT intervals and heart rates from 45 to 135 beats/min was constructed for study population. Based on the formula of Bazett, Fridericia, and Framingham, adjusted QT intervals in these range of heart rates were separately estimated. An adjusting nomogram for different heart rates was created using a reference value, which was the measured QT interval at heart rate of 60 beats/min (QT(No) = QT + correcting number). These four QT correction methods were compared with each other. The reference value of QT interval at heart rate of 60 beats/min was 382 ms. The relationship between QT and RR interval was linear (r = 0.66, P < 0.001). Nomogram method corrected QT interval most accurately for all the heart rates compared with other three adjusting methods. At heart rates of 60-100 beats/min, the equation of linear regression was QT = 237 + 0.158 x RR (P < 0.001). Bazett's formula gave the poorest results at all the heart rates. The formulas of Fridericia and Framingham were superior to Bazett's formula; however, they overestimated QT interval at heart rate of 60-110 beats/min (P < 0.01). At lower rates (<60 beats/min), all methods except nomogram method, underestimated QT interval (P = 0.03).

CONCLUSION

Among four QT correction formulas, the nomogram method provides the most accurately adjusted values of QT interval for all the heart rates in healthy men. Bazett's formula fails to adjust the QT interval for all the heart rates.

Individual QT-R-R relationship: average stability over time does not rule out an individual residual variability: implication for the assessment of drug effect on the QT interval

BACKGROUND

Universal QT correction formulae have been shown to under or overcorrect the QT interval duration. Individual QT-R-R modeling has been proposed as a preferable solution for heart rate correction of QT intervals. However, the QT-R-R relationship stability over time needs to be evaluated.

METHODS

The present report is part of randomized, double-dummy, and placebo-controlled 4-way crossover phase 1 study (48 healthy volunteers). Each randomized period included a run-in placebo day followed the day after by drug administration, with moxifloxacin as a positive control for QT interval measurement. Digital Holter ECG data were analyzed using the "bin" approach. For each period, individual QT-R-R relationship were calculated using two different models (linear and parabolic log-log models).

RESULTS

The mean intrasubject variability for the alpha coefficient of the linear modeling (SDintra = 0.011 +/- 0.005) reached 28.6 +/- 10.2%. When the parabolic model was considered, the SDintra was 0.026 +/- 0.009 for the alpha coefficient. The QT-R-R relationship variability was in part related to long-term RR changes (R2 = 30%, P < 0.05). However, no significant time effect (ANOVA) was evidenced for QT-R-R coefficients. Moxifloxacin significantly increased the alpha coefficient of the QT-R-R relationship from 0.07 +/- 0.018 to 0.085 +/- 0.019, P < 0.05 (linear model).

CONCLUSIONS

The individual QT-R-R relationship shows a residual variability in part related to long-term autonomic changes. In addition, the QT-R-R relationship might be modulated by the drug tested. As a consequence, pretherapy QT-R-R relationship obtained in a given patient cannot be used as a fingerprint throughout a drug trial.

QT dynamicity: a prognostic factor for sudden cardiac death in chronic heart failure

INTRODUCTION

The aim of this study was to determine whether impaired adaptation of the QT interval to changes in heart rate predicts sudden death in patients with chronic heart failure (CHF).

METHODS

We prospectively included 175 CHF patients in sinus rhythm. QT dynamicity was evaluated by analyzing 24-h Holter recordings. The linear regression slope of QT interval measured to the apex and to the end of T wave plotted against RR intervals was calculated using a dedicated Holter algorithm.

RESULTS

Mean follow-up was 29.9+/-17.9 months. There were 48 deaths, of which 21 were sudden. The actuarial 3-year mortality rates were 38.4% for overall mortality and 14.1% for sudden death. Of all the parameters, an increased QTe/RR slope (>0.28) was the strongest independent predictor of sudden death (relative risk 3.47, 95% confidence interval 1.43-8.40, p=0.006).

CONCLUSION

Increased 24-h QTe dynamicity is independently predictive of sudden death among patients with heart failure. This simple parameter may help to stratify risk and select patients who may benefit from antiarrhythmic prophylaxis.

Day-night pattern of sudden death in obstructive sleep apnea

BACKGROUND

The risk of sudden death from cardiac causes in the general population peaks from 6 a.m. to noon and has a nadir from midnight to 6 a.m. Obstructive sleep apnea is highly prevalent and associated with neurohormonal and electrophysiological abnormalities that may increase the risk of sudden death from cardiac causes, especially during sleep.

METHODS

We reviewed polysomnograms and the death certificates of 112 Minnesota residents who had undergone polysomnography and had died suddenly from cardiac causes between July 1987 and July 2003. For four intervals of the day, we compared the rates of sudden death from cardiac causes among people with obstructive sleep apnea and the following: the rates among people without obstructive sleep apnea, the rates in the general population, and the expectations according to chance. For each interval, we assessed the median apnea-hypopnea index and the relative risk of sudden death from cardiac causes. We similarly analyzed sudden death from cardiac causes during three time intervals that correlate with usual sleep-wake cycles.

RESULTS

From midnight to 6 a.m., sudden death from cardiac causes occurred in 46 percent of people with obstructive sleep apnea, as compared with 21 percent of people without obstructive sleep apnea (P=0.01), 16 percent of the general population (P<0.001), and the 25 percent expected by chance (P<0.001). People with sudden death from cardiac causes from midnight to 6 a.m. had a significantly higher apnea-hypopnea index than those with sudden death from cardiac causes during other intervals, and the apnea-hypopnea index correlated directly with the relative risk of sudden death from cardiac causes from midnight to 6 a.m. For people with obstructive sleep apnea, the relative risk of sudden death from cardiac causes from midnight to 6 a.m. was 2.57 (95 percent confidence interval, 1.87 to 3.52). The analysis of usual sleep-wake cycles showed similar results.

CONCLUSIONS

People with obstructive sleep apnea have a peak in sudden death from cardiac causes during the sleeping hours, which contrasts strikingly with the nadir of sudden death from cardiac causes during this period in people without obstructive sleep apnea and in the general population.

Abnormal ventricular repolarization in hypertensive patients: role of sympatho-vagal imbalance and left ventricular hypertrophy

BACKGROUND

An increased risk for life-threatening arrhythmias and sudden death has been observed in hypertensive patients, associated with either left ventricular hypertrophy (LVH) or prolonged QT interval. To investigate the influence of autonomic imbalance and LVH on QT interval in hypertensive patients, we compared two different models of LVH: hypertension and endurance physical training.

METHODS

Forty-seven untreated subjects affected by essential hypertension and 35 endurance runners, with a similar degree of LVH, were enrolled into the study. All subjects underwent 24-h ambulatory ECG recording and morning blood sampling for catecholamines. Heart rate variability was evaluated by spectral analysis and a computerized algorithm was used to measure the QT interval; QTc was then computed by the Bazett's formula. Left ventricular mass index (LVMI) was assessed by echocardiogram.

RESULTS

No difference in LVMI was found between hypertensive patients and athletes. Athletes showed lower heart rate (64 +/- 1 vs. 75 +/- 1 bpm, p<0.001, mean +/- S.E.M.) and shorter QTc (401 +/- 3 vs. 434 +/- 4 ms, p<0.001) than hypertensive patients throughout the 24-h period. Athletes showed a higher vagal drive compared to hypertensive patients as suggested by bradycardia and higher values of vagal indices, which negatively correlated with QTc. Plasma norepinephrine was significantly lower in athletes than in hypertensive patients (p<0.05) and positively correlated with QTc.

CONCLUSION

Despite similar degrees of LVH, hypertensive patients show QTc lengthening, as compared to athletes. Heart rate variability and plasma norepinephrine levels suggest sympathetic predominance in hypertensive patients, which could contribute to abnormal ventricular repolarization, thus identifying patients with an increased arrhythmic risk.

QT PRODACT: Inter-facility Variability in Electrocardiographic and Hemodynamic Parameters in Conscious Dogs and Monkeys

In safety pharmacology studies, the effect of a test compound on the electrocardiogram is routinely examined by using conscious dogs. However, the results may be widely variable. The monkey, on the other hand, has scarcely been used for such studies; and as yet, there have not been reported studies on monkeys conducted at several facilities with a standard protocol. In the present study, we examined inter-facility variabilities in electrocardiographic and hemodynamic parameters as described below. We analyzed the data from 8 facilities (9 test groups) on dogs and 5 facilities (7 test groups) on monkeys. This data was obtained from the studies employing the following standard protocol: dl-Sotalol or a vehicle (0.5 w/v% methylcellulose solution) was given to animals; and the PR interval, QRS duration, QT interval, heart rate, and mean blood pressure were determined time-sequentially before and after administration of the vehicle or dl-sotalol in each test group. dl-Sotalol produced a prolongation of the maximum mean QTcF interval in conscious dogs and QTcB interval in conscious monkeys by more than 10% in every test group. No difference in the corrected QT interval among the test groups was observed in dogs, but a difference was observed in monkeys.

Characterization of QT Interval Adaptation to RR Interval Changes and Its Use as a Risk-Stratifier of Arrhythmic Mortality in Amiodarone-Treated Survivors of Acute Myocardial Infarction

A new method is proposed to evaluate the dynamics of QT interval adaptation in response to heart rate (HR) changes. The method considers weighted averages of RR intervals (RR) preceding each cardiac beat to express RR interval history accounting for the influence on repolarization duration. A global optimization algorithm is used to determine the weight distribution leading to the lowest regression residual when curve fitting the [QT, RR1 data using a patient-specific regression model. From the optimum weight distribution, a memory lag L90 is estimated, expressing the delay in the QT adaptation to HR changes. On average, RR intervals of the past 150 beats (approximately 2.5 min) are required to model the QT response accurately. From a clinical point of view, the interval of the initial tens of seconds to one minute seems to be most important in the majority of cases. A measure of the optimum regression residual (r(opt)) has been calculated, discriminating between post-myocardial infarction patients at high and low risk of arrhythmic death while on treatment with amiodarone. A similar discrimination has been achieved with a variable expressing the character of QT lag behind the RR interval dynamics.

Abnormalities of the repolarization characteristics of patients with heart failure progress with symptom severity

BACKGROUND

Congestive heart failure is a common condition with high mortality. Many of these deaths are sudden and unexpected. Ventricular action potential, surface repolarization (QT interval), and dispersion of repolarization are prolonged in the failing heart, contributing to arrhythmogenesis and sudden death. We studied the relationship between QT and heart rate (RR interval) from ambulatory recordings using a novel method in patients with ischemic heart disease and varying degrees of left-ventricular impairment (IHD) and compared them to healthy subjects (HS). We compare the degree of abnormality with the functional impairment and ejection fraction.

METHODS

Using a previously described automated method for continuous estimation of the QT/RR characteristic that incorporates a correction formula for compensation of QT adaptation lag (VERDA, Del Mar Reynolds Medical Ltd., Hertford, UK), we compared recordings from 41 IHD patients with age-matched HS.

RESULTS

IHD Patients have prolonged 24-hour mean QTo (461 ms vs 426 ms, P < 0.01), and abnormal rate dependence relative to controls (24-hour mean slope: 0.20 vs 0.14, P < 0.001; J: 0.38 vs 0.28, P < 0.001). There is increased temporal variation in J with respect to HS. These abnormalities of repolarization increase with worsening NYHA class, but do not correlate with ejection fraction.

CONCLUSIONS

The use of a universal correction formula to compare dynamic QT data in IHD patients is inappropriate. The observed progressive abnormalities may be responsible for the high incidence of sudden death through promotion of arrhythmias.

Beat-to-beat QT dynamics in healthy subjects

BACKGROUND

Measures of QT dynamics express repolarization abnormalities that carry prognostic information, but the reproducibility of beat-to-beat QT dynamics has never been established. The QT interval is prolonged at night, but how the circadian rhythm and heart rate influence the dynamic QT measurements is still unsettled. The aims of the present study were: (1) to describe the reproducibility of beat-to-beat QT dynamics with respect to intrasubject, between-subject, and between-observer variability and (2) to describe the normal range, circadian variation, and heart rate dependence of QT dynamics.

METHODS

Ambulatory Holter recordings were performed three times on 20 healthy volunteers and were analyzed by two experienced cardiologists. Slope and intercept of the QT/RR regression, the variability of QT and R-R intervals expressed as the standard deviation, and the relation between QT and RR variability expressed as a variability ratio were measured among other QT dynamics.

RESULTS

The reproducibility of all QT dynamics was good. All QT dynamics showed circadian variation when calculated on an hourly basis. The day/night variation in slope could be explained by the differences in heart rate, whereas the day/night variation in intercept was heart rate independent.

CONCLUSION

The present study shows that reliable automatic QT measurements could be performed, encouraging further evaluation of the clinical value of QT dynamics in risk stratification of cardiac patients.

Diagnostic performance of various QTc interval formulas in a large family with long QT syndrome type 3: Bazett's formula not so bad after all

BACKGROUND

Recently, we identified a novel mutation of SCN5A (1795insD) in a large family with LQTS3. The aim of this study was to assess whether the various proposed corrections of the QT interval to heart rate help to improve the identification of carriers of the mutant gene.

METHODS

The study group consisted of 101 adult family members: 57 carriers and 44 noncarriers (mean age 44.6 +/- 14.6 and 40.3 +/- 12.8 years, respectively). In all individuals a 12-lead ECG, exercise ECG, and 24-hour Holter ECG were obtained.

RESULTS

Correction for heart rate significantly improved the diagnostic performance of the QT interval. Diagnostic performance of the Bazett formula was similar to that of the newer formulas (Fridericia, Hodges, Framingham, and a logarithmic formula). At a cut-off value of 440 ms, the Bazett corrected QT interval was associated with a sensitivity and specificity of 90% and 91%, respectively. Using the 24-hour Holter ECG, a prolonged QTc at heart rates less than 60 beats/min was almost pathognomonic for genetic mutation (sensitivity and specificity both 99%), whereas the QTc calculated at the lowest heart rate using Bazett's formula provided full discrimination.

CONCLUSION

In the present family, the resting ECG gave a good indication about the presence or absence of genetic mutation but a 24-hour Holter recording was mandatory to ascertain the diagnosis. In the diagnosis of this form of LQTS3, Bazett's formula was at least as good as other proposed corrections of the QT interval to heart rate.

Dopamine receptor agonists differ in their actions on cardiac ion channels

Four dopamine receptor agonists used for the treatment of Parkinson's disease (apomorphine, pergolide, ropinirole and sumanirole) were evaluated for the ability to block human ether-a-go-go related gene (hERG) K(+) channels and to modify the duration of canine Purkinje fiber action potentials. Apomorphine, pergolide and ropinirole blocked the hERG-mediated currents with IC(50) values of 2.4, 0.12 and 1.2 microM, respectively. When evaluated in an action potential duration assay, pergolide significantly shortened action potential duration at 90% repolarization (APD(90)) whereas apomorphine and ropinirole significantly prolonged repolarization. Sumanirole only partially blocked hERG K(+) channels at the highest tested concentration (10 microM) and did not modify action potential duration over the tested concentration range (0.65-65 microM). Taken together, these data provide evidence that dopamine receptor agonists developed for the treatment of Parkinson's disease differentially influence hERG K(+) channel function and cardiac action potential duration.

Acute effects of cigarette smoking on the QT interval in healthy smokers

This study assesses the effect of a first cigarette smoked in the morning on the QT interval duration in healthy habitual smokers after not smoking overnight. This study demonstrates that although after cigarette smoking the QT(Bc) values increase, the QT(Fc) and QT(Lc) values remained unchanged, and an increased sympathetic response was evident. The discrepancies between the results seen by different QT correction formulas emphasize the importance of selecting an accurate method for heart rate correction when evaluating and interpreting alterations in QT values.

Alteration of QT rate dependence reflects cardiac autonomic imbalance in patients with obstructive sleep apnea syndrome

QT rate dependence is one of the major properties of ventricular repolarization with its circadian and autonomic modulations. The authors postulated that dynamic alterations in QT interval adaptation could help characterize patients with cardiac autonomic alterations, like those with obstructive sleep apnea syndrome (OSAS). To assess ventricular repolarization features in patients with OSAS, QT parameters and their dynamicity along RR intervals were compared from 24-hour ECG data of patients with and without this syndrome, assessing cardiac autonomic nervous system equilibrium by means of time-domain and frequency-domain analyses of heart rate variability (HRV). The study group consisted of 74 consecutive patients referred to the Sleep Laboratory for clinically suspected OSAS. The syndrome was confirmed in 30 (40.5%) patients according to standard polysomnographic criteria. QT length related to heart rate (HR) was found significantly shorter for HR < 70 beats/min in patients with OSAS(-1.32 +/- 0.35)compared with patients without OSAS(-1.99 +/- 0.40; P < 0.01). This flattened relationship was correlated with the severity of the sleep related disorder. Using multiple linear regression analysis, the apnea/hypopnea index and nocturnal normalized high frequencies (HFnu) were the most significant predictors of the QT/RR slope(R = 0.61; P < 0.0001). OSAS is significantly associated with a flattened relationship between QT duration and RR interval at low HRs. The alteration of cardiac parasympathetic tone occurring in severe OSAS patients may explain this altered rate dependent adaptation of myocardial repolarization.

Circadian variation of cardiac K+ channel gene expression

BACKGROUND

Many cardiac arrhythmias have their own characteristic circadian variations. Because the expression of many genes, including clock genes, is regulated variably during a day, circadian variations of ion channel gene expression, if any, could contribute to the fluctuating alterations of cardiac electrophysiological characteristics and subsequent arrhythmogenesis.

METHODS AND RESULTS

To examine whether cardiac K+ channel gene expression shows a circadian rhythm, we analyzed the mRNA levels of 8 Kv and 6 Kir channels in rat hearts every 3 hours throughout 1 day. Among these channels, Kv1.5 and Kv4.2 genes showed significant circadian variations in their transcripts: approximately 2-fold increase of Kv1.5 mRNA from trough at Zeitgeber time (ZT) 6 to peak at ZT18 and a completely reverse pattern in Kv4.2 mRNA ( approximately 2-fold increase from trough at ZT18 to peak at ZT6). Actually, along with the variations in the immunoreactive proteins, the density of the transient outward and steady-state currents in isolated myocytes and the responses of atrial and ventricular refractoriness to 4-aminopyridine in isolated-perfused hearts showed differences between ZT6 and ZT18, a circadian pattern comparable to that of Kv1.5 and Kv4.2 gene expression. Reversal of light stimulation almost inverted these circadian rhythms, although pharmacological autonomic blockade only partially attenuated the rhythm of Kv1.5 but not of Kv4.2 transcripts.

CONCLUSIONS

Among all the cardiac K+ channels, Kv1.5 and 4.2 channels are unique in showing characteristic circadian patterns in their gene expression, with Kv1.5 increase during the dark period partially dependent on beta-adrenergic activities and Kv4.2 increase during the light period independent of the autonomic nervous function.

Ventricular repolarization is prolonged in nondipper hypertensive patients: role of left ventricular hypertrophy and autonomic dysfunction

OBJECTIVE

The aim of this study was to investigate the influence of circadian behavior of blood pressure, left ventricular hypertrophy, and autonomic function on QTc interval duration in untreated hypertensive patients.

DESIGN

Hypertensive patients underwent simultaneous blood pressure and ECG 24-h ambulatory monitoring. Patients were classified into two groups on the basis of a lack of nocturnal fall in blood pressure, as dippers and nondippers. A group of normotensive healthy subjects was studied as controls.

METHODS

QT and QTc intervals were automatically computed and spectral analysis was applied to RR interval time series from the same electrocardiogram (ECG) recordings. Left ventricular mass index (LVMI) was computed by echocardiogram.

RESULTS

No difference among the three groups was found concerning mean values and circadian pattern of heart rate; by contrast, QTc was significantly longer in nondippers compared to dippers or to normotensive subjects, particularly at night-time, whereas all groups exhibited similar circadian variations in heart rate. Compared to dippers, nondippers showed significantly higher LVMI, which positively correlated with QTc, and parasympathetic withdrawal, which negatively correlated with QTc.

CONCLUSIONS

Nondippers show a prolonged ventricular repolarization throughout the 24-h period, absent either in dippers or normotensives. The association of left ventricular hypertrophy and vagal deactivation may lead to prolongation of QTc, potentially facilitating ventricular arrhythmias in nondipper hypertensive patients.

Modulation of QT interval during autonomic nervous system blockade in humans

BACKGROUND

It is thought that the autonomic nervous system modulates QT interval, but traditional autonomic blockade combining propranolol and atropine has produced conflicting results. We used the alternative approach of interrupting neurotransmission at the level of autonomic ganglia to determine its effect on the QT interval.

METHODS AND RESULTS

We infused trimethaphan at increasing doses (0.5 to 10 mg/min IV) while monitoring heart rate, heart rate variability spectra, QT interval, and blood pressure in 10 normal volunteers, 9 patients with multiple system atrophy (MSA), and 8 patients with pure autonomic failure (PAF). The QT interval was corrected for heart rate using Bazett's formula (QTc). Patients with PAF had very low heart rate variability and a prolonged QTc at baseline (465+/-8 ms) compared with patients with MSA (448+/-6 ms) and normal subjects (432+/-6 ms). In normal subjects, trimethaphan dose-dependently prolonged QTc (to 469+/-7 ms), decreased RR interval (995+/-45 to 670+/-35 ms), and abolished heart rate variability. In MSA patients, trimethaphan also prolonged QTc (to 463+/-7 ms) and reduced heart rate variability but did not significantly change RR interval (from 813+/-38 to 801+/-39).

CONCLUSIONS

Autonomic blockade prolongs QT interval in normal subjects to a similar duration as in PAF patients. Furthermore, blocking residual autonomic tone in PAF patients is associated with a further increase in QT interval length. Patients with MSA have greater residual sympathetic tone and greater prolongation of the QT interval during ganglionic blockade than PAF patients.

Rate-correction technique for QT interval in long-term telemetry ECG recording in beagle dogs

The establishment of a new rate-correction method for the QT interval is presented for long-term telemetry ECG recording in free-moving beagle dogs. First, in order to define the QT-RR relation to derive the correction formula, the diurnal variations of the QT and RR intervals and the influencing factors were analyzed, and the QT-RR regression coefficient beta was estimated under various conditions: steady and non-steady states of animal, light and dark periods, and over 24 h. In the results, the diurnal rhythm of the QT interval was synchronized with the RR interval reflecting the physical and emotional states of the animal. The coefficient beta had considerable variation during the day: beta; 0.276 +/- 0.052 (maximum to minimum: 0.495 to 0.153). Thus, it was considered that the ideal rate-correction technique for telemetry ECG requires the selection of a flexible coefficient beta adjusted to the condition of the measurement. Therefore, rate-correction utilizing analysis of covariance estimating the coefficient beta for each dog, was compared with previously proposed formulas which fix the rate-correction coefficient, based on the capacity to dissociate the effects of heart rate on the QT interval. This was then tested by the levels of discrimination apparent in the QT prolongation caused by a class III antiarrhythmic drug, which ranked the formulas on the levels of correction achieved as follows: covariance adjustment > Matsunaga > Van de Water > Bazett. Thus, the rate-correction method utilizing analysis of covariance is proven adequate for data from telemetry ECG recordings.

Sex differences in repolarization homogeneity and its circadian pattern

The reason for sex differences in arrhythmic risk remains unclear. Heterogeneity of ventricular repolarization is directly linked to arrhythmogenesis; thus we investigated repolarization homogeneity and its circadian pattern in men and women. During 24-h Holter recordings in 60 healthy subjects (27 males), a 12-lead electrocardiogram (ECG) was obtained every 30 s. RR and QT intervals, and, after singular-value decomposition, two characteristics of repolarization homogeneity were calculated in each ECG. Corrected QT (QTc) values were obtained using an individually optimized heart rate (HR) correction formula. All values were averaged over 10-min time bands from 0000 to 2400. There were substantial sex differences in both global repolarization homogeneity (measured by the total cosine of the angle between QRS and T wave vectors) and regional homogeneity of repolarization (quantified independently by the relative T wave residuum). Whereas women throughout the 24 h followed more closely the pattern of inverse sequence between depolarization and repolarization, they also showed much higher localized repolarization heterogeneity than men. In both women and men, repolarization irregularity was greatest during morning hours. A sex difference was also observed for HR and QTc interval; however, the circadian patterns of the repolarization homogeneity descriptors were different from those of HR and QTc intervals.

Influence of cardiac autonomic neuropathy on heart rate dependence of ventricular repolarization in diabetic patients

OBJECTIVE

Prolongation of the QT interval and increased QT dispersion are associated with a poor cardiac prognosis. The goal of this study was to assess the long-term influence of the autonomic nervous system on the heart rate dependence of ventricular repolarization in patients with diabetic autonomic neuropathy (DAN).

RESEARCH DESIGN AND METHODS

We studied 27 subjects (mean age 51.8 years) divided into three age- and sex-matched groups: nine control subjects, nine diabetic subjects with DAN (mostly at a mild stage; DAN+), and nine diabetic subjects without DAN (DAN-). DAN was assessed on heart rate variations during standard maneuvers (Valsalva, deep-breathing, and lying-to-standing maneuvers). No subject had coronary artery disease or left ventricular dysfunction or hypertrophy, and no subject was taking any drugs known to prolong the QT interval. All subjects underwent electrocardiogram and 24-h Holter recordings for heart rate variations (time and frequency domain) and QT analysis (selective beat averaging QT/RR relation, nocturnal QT lengthening).

RESULTS

Rate-corrected QT intervals (Bazett formula) did not differ significantly between the three groups. The diurnal and nocturnal levels of low frequency/high frequency, an index of sympathovagal balance, were significantly reduced in DAN+ subjects. Using the selective beat-averaging technique, a day-night modulation of the QT/RR relation was evidenced in control and DAN- subjects. This long-term modulation was significantly different in DAN+ subjects, with a reversed day-night pattern and an increased nocturnal QT rate dependence.

CONCLUSIONS

In diabetic patients with mild parasympathetic denervation, QT heart rate dependence was found to be impaired, as determined by noninvasive assessment using Holter data. Analysis of ventricular repolarization could represent a sensitive index of the progression of neuropathy. The potential prognostic impact of a reversed day-night pattern with steep nocturnal QT/RR relation still remains to be defined.

Does heart rate identify sudden death survivors? Assessment of heart rate, QT interval, and heart rate variability

The objective was to test whether the circadian variability of several electrocardiographic variables distinguishes sudden cardiac death survivors from heart disease patients without a history of cardiac arrest and from normal subjects. Heart rate, heart rate variability, and QT interval have been reported to identify survivors of sudden cardiac death. Computer-assisted continuous QT measurement and heart rate variability analysis were performed on 24-hour Holter records for three groups: (1) 14 sudden death survivors; (2) 14 control patients with diagnosis and therapy matched to survivors; and (3) 14 healthy subjects. There were no significant differences in 24-hour mean RR and QT intervals between groups. However, heart rate was significantly different between the three groups at night but not during the day because the expected nighttime decline was markedly blunted in survivors and somewhat blunted in control patients. The QT interval and frequency domain heart rate variability measures followed a similar circadian pattern. The mean QTc was significantly longer in control patients. The QTc had a wide range in all groups, but less in sudden death survivors. Of ten common time and frequency domain heart rate variability indices, only SDANN and SDNN were significantly lower in sudden death survivors. Reduced circadian variation of heart rate, with marked blunting of the nighttime heart rate decline, identifies sudden cardiac death survivors as well as does SDANN and SDNN, and, in contrast to heart rate variability measures, can easily be obtained from a Holter report without complex calculations.