Diurnal pattern of QTc interval: how long is prolonged? Possible relation to circadian triggers of cardiovascular events

Circadian and gender effects on repolarization in healthy adults: a study using harmonic regression analysis

BACKGROUND

Sudden cardiac death and myocardial infarction have a circadian variation with a peak incidence in the early morning hours. Increased dispersion of repolarization facilitates the development of conduction delay necessary to induce sustained arrhythmia. Both QT-dispersion and T-wave peak to T-wave end (TpTe) have been proposed as markers of dispersion of myocardial repolarization.

METHODS

Forty healthy adults (20 women), age 35-67 years old, with normal EKGs, echocardiograms, stress tests, and tilt-table tests were analyzed during a 27-hour hospital stay. EKGs were done at eight different time points. QT-intervals, QT-dispersion, and TpTe were measured at each time point. Harmonic regression was used to model circadian periodicity, P < 0.05 was considered significant.

RESULTS

The composite QT-interval was longer in women than in men (416 + or - 17 msec vs 411 + or - 20 msec, respectively, P = 0.006). The QT-dispersion among all leads was greater in men than women (37 + or - 13 msec vs 30 + or - 11 msec, respectively, P < 0.0001); a similar difference was found in the precordial leads. Harmonic regression showed that QT-dispersion had a significant circadian variation, primarily in men. In men, the maximum QT-dispersion occurred at 6 AM (45 + or - 15 msec). TpTe also had a significant circadian variation that was not affected by gender in the majority of leads.

CONCLUSIONS

A circadian variation exists in the dispersion of myocardial repolarization, as measured by both TpTe and QT-dispersion. Men and women have a different circadian variation pattern. Further studies regarding the mechanisms and clinical implications are needed.

Assessment of QT interval and QT dispersion during electroconvulsive therapy using computerized measurements

BACKGROUND

Electroconvulsive therapy (ECT) used in the treatment of severe psychiatric disorders induces stimulation of the autonomic nervous system with initial parasympathetic outflow immediately followed by a sympathetic response. These responses induce an initial bradycardia, arrhythmias, and hypertension. QT dispersion (QTD), defined as maximal QT interval minus minimal QT interval on 12 leads of the surface electrocardiogram, reflects regional heterogeneity of ventricular repolarization. The effects of electrical stimulus due to ECT on QT interval and QTD are of considerable interest.

OBJECTIVE

: This study was designed to investigate the effects of electrical stimulation caused by ECT on RR interval, QT interval, the rate-corrected QT (QTc) interval, QTD, and the rate-corrected QTD (QTcD) under general anesthesia using computerized measurements.

METHODS

Thirty psychiatric patients scheduled for ECT were studied under propofol anesthesia. A 12-lead electrocardiogram was monitored to measure parameters. Muscle paralysis was achieved by administering succinylcholine 1 mg/kg intravenously, and the efficacy of ECT was determined by the tourniquet technique.

RESULTS

The RR interval and QT interval decreased significantly immediately after electrical stimulus, and returned to the baseline level 1 minute after electrical stimulus. In 25 out of 30 patients, the baseline value of QTc interval was higher than the normal limits, and the QTc interval decreased significantly for 2 minutes after electrical stimulus. In 27 out of 30 patients, the baseline values of QTD and QTcD were higher than the normal limits, and the QTD and QTcD increased significantly from immediately after electrical stimulus to 5 minutes after electrical stimulus.

CONCLUSIONS

The QTc interval, QTD, and QTcD, which were associated with increased risks of ventricular arrhythmias, increased significantly before anesthetic induction in patients with major depression. Electrical stimulus during ECT induced further increases of the QTD and QTcD.

Safety assessment of prucalopride in elderly patients with constipation: a double-blind, placebo-controlled study

Chronic constipation is common among nursing home residents. The aim of this study was to evaluate safety, tolerability and pharmacokinetics of the selective 5HT(4) receptor agonist prucalopride in elderly, chronically constipated patients in nursing homes. A multicentre, phase II, randomized, double-blind dose-escalation study in 89 elderly constipated nursing home residents treated with placebo, 0.5, 1 or 2 mg prucalopride once daily for 28 days was analysed. Adverse events, vital signs, ECG, Holter monitor and pharmacokinetics were assessed (Clinicaltrials.gov identifier: NCT00627692). Patients' mean age was 83 years; 88% had a history of cardiovascular diseases. Most frequent adverse events, at least possibly related to prucalopride, were diarrhoea and abdominal pain. Relative to placebo, there were no differences in vital signs, ECG corrected QT interval, ECG morphology parameters, or incidence of supraventricular or ventricular arrhythmias on Holter monitoring. Plasma prucalopride concentrations increased proportionally with administered dose. Prucalopride up to 2 mg once daily for 4 weeks was safe and well-tolerated by constipated elderly patients, with no differences vs placebo in ECG or a range of Holter-monitoring parameters.

Update on the evaluation of a new drug for effects on cardiac repolarization in humans: issues in early drug development

Following reports of death from cardiac arrhythmias with drugs like terfenadine and cisapride, the International Conference for Harmonization formulated a guidance (E14) document. This specifies that all new drugs must undergo a 'thorough QT/QTc' (TQT) study to detect drug-induced QT prolongation, a surrogate marker of ventricular tachycardia, especially torsades de pointes (TdPs). With better understanding of data from several completed TQT studies, regulatory requirements have undergone some changes since the E14 guidance was implemented in October 2005. This article reviews the implications of the E14 guidance and the changes in its interpretation including choice of baseline QT, demonstration of assay sensitivity, statistical analysis of the effect of new drug and positive control, and PK-PD modelling. Some issues like use of automated QT measurements remain unresolved. Pharmaceutical companies too are modifying Phase 1 studies to detect QTc liability early in order to save time and resources. After the E14 guidance, development of several drugs that prolong QTc by >5 ms is being abandoned by sponsors. However, all drugs that prolong the QT interval do not increase risk of TdP. Researchers in regulatory agencies, academia and industry are working to find better biomarkers of drug-induced TdP which could prevent many useful drugs from being prematurely abandoned. Drug-induced TdP is a rare occurrence. With fewer drugs that prolong QT interval reaching the licensing stage, knowing which of these drugs are torsadogenic is proving to be elusive. Thus, paradoxically, the effectiveness of the E14 guidance itself has made prospective validation of new biomarkers difficult.

The International Society for Bipolar Disorders (ISBD) consensus guidelines for the safety monitoring of bipolar disorder treatments

OBJECTIVES

Safety monitoring is an important aspect of bipolar disorder treatment, as mood-stabilising medications have potentially serious side effects, some of which may also aggravate existing medical comorbidities. This paper sets out the International Society for Bipolar Disorders (ISBD) guidelines for the safety monitoring of widely used agents in the treatment of bipolar disorder. These guidelines aim to provide recommendations that take into consideration the balance between safety and cost-effectiveness, to highlight iatrogenic and preventive clinical issues, and to facilitate the broad implementation of therapeutic safety monitoring as a standard component of treatment for bipolar disorder.

METHODS

These guidelines were developed by an ISBD workgroup, headed by the senior author (MB), through an iterative process of serial consensus-based revisions. After this, feedback from a multidisciplinary group of health professionals on the applicability of these guidelines was sought to develop the final recommendations.

RESULTS

General safety monitoring recommendations for all bipolar disorder patients receiving treatment and specific monitoring recommendations for individual agents are outlined.

CONCLUSIONS

These guidelines are derived from evolving and often indirect data, with minimal empirical cost-effectiveness data available to provide guidance. These guidelines will therefore need to be modified to adapt to different clinical settings and health resources. Clinical acumen and vigilance remain critical ingredients for safe treatment practice.

Unintended consequences of reducing QT-alert overload in a computerized physician order entry system

PURPOSE

After complaints of too many low-specificity drug-drug interaction (DDI) alerts on QT prolongation, the rules for QT alerting in the Dutch national drug database were restricted in 2007 to obviously QT-prolonging drugs. The aim of this virtual study was to investigate whether this adjustment would improve the identification of patients at risk of developing Torsades de Pointes (TdP) due to QT-prolonging drug combinations in a computerized physician order entry system (CPOE) and whether these new rules should be implemented.

METHODS

During a half-year study period, inpatients with overridden DDI alerts regarding QT prolongation and with an electrocardiogram recorded before and within 1 month of the alert override were included if they did not have a ventricular pacemaker and did not use the low-risk combination cotrimoxazole and tacrolimus. QT-interval prolongation and the risk of developing TdP were calculated for all patients and related to the number of patients for whom a QT-alert would be generated in the new situation with the restricted database.

RESULTS

Forty-nine patients (13%) met the inclusion criteria. In this study population, knowledge base-adjustment would reduce the number of alerts by 53%. However, the positive predictive value of QT alerts would not change (31% before and 30% after) and only 47% of the patients at risk of developing TdP would be identified in CPOEs using the adjusted knowledge base.

CONCLUSION

The new rules for QT alerting would result in a poorer identification of patients at risk of developing TdP than the old rules. This is caused by the many non-drug-related risk factors for QT prolongation not being incorporated in CPOE alert generation. The partial contribution of all risk factors should be studied and used to create clinical rules for QT alerting with an acceptable positive predictive value.

Assessing proarrhythmic potential of drugs when optimal studies are infeasible

Assessing the potential for a new drug to cause life-threatening arrhythmias is now an integral component of premarketing safety assessment. International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use Guideline (ICH) E14 recommends the "Thorough QT Study" (TQT) to assess clinical QT risk. Such a study calls for careful evaluation of drug effects on the electrocardiographic QT interval at multiples of therapeutic exposure and with a positive control to confirm assay sensitivity. Yet for some drugs and diseases, elements of the TQT Study may be impractical or unethical. In these instances, alternative approaches to QT risk assessment must be considered. This article presents points to consider for evaluation of QT risk when alternative approaches are needed.

Prognostic value of ventricular repolarization prolongation in resistant hypertension: a prospective cohort study

OBJECTIVE

The prognostic value of prolonged ventricular repolarization in patients with resistant hypertension is unknown. The aim of this prospective study was to investigate the usefulness of electrocardiographic QT-interval parameters as predictors of cardiovascular morbidity and mortality.

METHODS

At baseline, 538 resistant hypertensive patients had five QT-interval components measured in standard 12-lead ECGs: maximum QRS, QTpeak, QTend, JT and Tpeak-to-end-interval durations. Primary endpoints were a composite of fatal and nonfatal cardiovascular events, all-cause and cardiovascular mortalities. Multiple Cox regression assessed the associations between QT-interval parameters and subsequent endpoints.

RESULTS

After a median follow-up of 4.8 years, 69 (12.8%) patients died, 46 from cardiovascular causes, and 107 (19.9%) fatal or nonfatal cardiovascular events occurred. After adjustment for several traditional risk factors, including 24-h ambulatory systolic blood pressure, an increment of 1 SD (35 ms) in QTcend-interval was associated with hazard ratios of 1.38 (1.15-1.67), 1.51 (1.16-1.98) and 1.30 (1.03-1.64), respectively, for the composite endpoint, cardiovascular mortality and all-cause mortality. Further adjustment for left ventricular hypertrophy attenuated the relative risks, but they remained significant for cardiovascular mortality (1.45, 1.07-1.97) and for the composite endpoint (1.35, 1.11-1.66). After full adjustment, a prolonged QTcend-interval (> or =460 ms) conferred a 1.7-fold (1.1-2.6) higher risk of having a future fatal or nonfatal cardiovascular event. No other QT-interval component added further prognostic information to QTcend-interval duration.

CONCLUSIONS

Prolonged ventricular repolarization is a risk marker for cardiovascular morbidity and mortality in patients with resistant hypertension, over and beyond traditional cardiovascular risk factors, including ambulatory blood pressure and left ventricular hypertrophy.

QT interval prolongation in future SIDS victims: a polysomnographic study

OBJECTIVE

Previous data have suggested that a prolonged QTc interval during the first days of life can be associated with some cases of sudden infant death syndrome (SIDS). Analysis of heart rate variability during sleep in future SIDS victims has shown findings compatible with an imbalance in autonomic tone. We hypothesized that some future SIDS infants could have longer QTc intervals during sleep, compared with healthy control infants, and that this difference would correlate with the autonomic imbalance already found in these infants.

METHODS

QTc intervals and a heart rate autoregressive power spectral analysis were calculated during the same periods in the polysomnographic sleep recordings of 18 infants who eventually died of SIDS and of 18 control infants. The control infants were matched for sex, gestational age, postnatal age, birth weight, and sleep position. The median postnatal age was 8 weeks.

RESULTS

Compared with control infants, future SIDS victims were characterized by having longer QTc intervals during total sleep (P = 0.019), rapid eye movement sleep (P = 0.045) and non-rapid eye movement sleep (P = 0.029). When the night was divided into 3 equal parts, this difference was always present but was most marked during the last part of the night. There was, respectively, a negative and a positive correlation between parasympathetic activity and sympathovagal balance and median and maximum QTc interval values.

CONCLUSION

Compared with QTc intervals in matched control infants, QTc intervals were increased in future SIDS victims. Such a prolongation could be related to the autonomic dysfunction already reported in these patients.

Changes of the corrected QT interval in healthy boys and girls over day and night

AIMS

The study was designed to detect changes in corrected QT intervals over day and night in both sexes in healthy children.

METHODS AND RESULTS

The corrected QT interval was calculated from 24 h ECGs obtained from 282 healthy children aged 6 months to 18 years. The QTc interval as measured by the 24 h recording differs to the standard ECG measurement which is in average of 40-50 ms shorter. The QTc interval changes little over a 24 h period and is remarkably constant despite significant heart rate changes in healthy children.

CONCLUSION

The routine ECG-even if the calculated values differ markedly from those obtained over 24 h-seems to be a good screening method for the measurement of corrected QT intervals, because the corrected QT interval is kept constant over the whole day in healthy children.

[Standard-ECG]

The ECG is still the basis of technical investigations in cardiology. New descriptions and interpretations have been developed for left ventricular hypertrophy, Q-wave, arrhythmogenic diseases such as long and short QT syndrome, Brugada syndrome, syndrome of early repolarization, and arrhythmogenic right ventricular disease.

[Long term electrocardiography (Holter monitoring)]

During the past almost 50 years Holter monitoring has become an established non-invasive diagnostic tool in clinical electrophysiology. It allows ECG recording independent of stationary monitoring facilities during daily life and, therefore, contains much information. In the beginning the main interest was directed towards quantitative and qualitative assessment of arrhythmias, their circadian behaviour, and the circadian behaviour of the heart rate. With advances in technology the analysis spectrum of Holter monitoring expanded, and it was used also for detection of silent myocardial ischaemia. New digital recorders and computers with large capacities made it possible to measure every single heart beat very accurately, which was a prerequisite for heart rate variability and QT-interval analysis, which provided new knowledge about the autonomic modulation of the heart rate and the circadian dynamicity of the QT interval, respectively. Beyond arrhythmia analysis Holter monitoring was increasingly used to assess prognosis in different cardiac conditions. It can also be valuable in assessing transient symptoms possibly related to arrhythmias or device dysfunction, which will not necessarily be revealed by simple device control.

Prognostic significance of inverse spatial QRS-T angle circadian pattern in myocardial infarction survivors

BACKGROUND

We investigated the predictive value of the spatial QRS-T angle (QRSTA) circadian variation in myocardial infarction (MI) patients.

METHODS

Analyzing 24-hour recordings (SEER MC, GE Marquette) from 151 MI patients (age 63 +/- 12.7), the QRSTA was computed in derived XYZ leads. QRS-T angle values were compared between daytime and night time. The end point was cardiac death or life-threatening ventricular arrhythmia in 1 year.

RESULTS

Overall, QRSTA was slightly higher during the day vs. the night (91 degrees vs. 87 degrees, P = .005). However, 33.8% of the patients showed an inverse diurnal QRSTA variation (higher values at night), which was correlated to the outcome (P = .001, odds ratio 6.7). In multivariate analysis, after entering all factors exhibiting univariate trend towards significance, inverse QRSTA circadian pattern remained significant (P = .036).

CONCLUSION

Inverse QRSTA circadian pattern was found to be associated with adverse outcome (22.4%) in MI patients, whereas a normal pattern was associated (96%) with a favorable outcome.

Accurately measured and properly heart-rate corrected QTc intervals show little daytime variability

BACKGROUND

Circadian QTc changes have been reported, with conflicting results. Spontaneous QTc variability is important for pharmaceutical cardiac safety studies.

OBJECTIVE

The purpose of this study was to investigate QTc variability in accurately measured and heart-rate corrected daytime data of healthy subjects.

METHODS

Continuous 12-lead ECGs were recorded in 53 healthy volunteers. For each recording, approximately 320 ECG samples of 10 seconds were obtained throughout the daytime period, all preceded by stable heart rates. In each ECG sample, QT interval was measured on superimposed 12 leads by two independent cardiologists and reconciled. Four RR-interval expressions were used: (a) average of the first three RR of the ECG sample, (b) RR average of the 10-second sample, (c) average of RR intervals in a 2-minute history, and (d) RR intervals of an independently established individual QT/RR hysteresis profile. For all RR-interval expressions, QT intervals were corrected using the Fridericia formula and individually optimized curvature correction. QTc variability was measured by intraindividual QTc standard deviations.

RESULTS

With Fridericia correction and the RR expressions (a) to (d), QTc variability obtained was (a) 9.45 +/- 1.70 ms, (b) 7.80 +/- 1.48 ms, (c) 6.37 +/- 1.64 ms, and (d) 5.81 +/- 1.75 ms. With individualized correction, QTc variability was (a) 8.16 +/- 1.71 ms, (b) 6.71 +/- 1.41 ms, (c) 5.22 +/- 1.13 ms, and (d) 4.56 +/- 1.18 ms. All differences (b) vs (a), (c) vs (b), and (d) vs (c) were highly statistically significant (P <10(-12) in all cases).

CONCLUSION

Previously reported large QTc variability largely results from methodologic imprecision. Little QTc variability is present in daytime recordings of healthy subjects. Consequently, QT-related pharmaceutical cardiac safety studies can be made smaller without decreasing their power.

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.

Effects of atorvastatin on ventricular late potentials and repolarization dispersion in patients with hypercholesterolemia

Emerging evidence suggests that statins have a favorable impact on the reduction of arrhythmia events and sudden cardiac death in patients with structural heart disease. We aimed to investigate the possibly and directly favorable effects of statins on ventricular late potentials, QT dispersion, and transmural dispersion of repolarization attained by analyzing clinical electrocardiography (ECG) risk stratification parameters in patients with hypercholesterolemia without structural heart disease. In total, 82 patients (45 females; mean age, 62 +/- 10 years) with hypercholesterolemia were enrolled in this prospective study to examine the effects of statin therapy (atorvastatin 10 mg/day for 3 months) on ECG risk stratification parameters. Surface 12-lead ECG and signal-average ECG (SAECG) were recorded before and after statin treatment. The SAECG parameters, QT dispersion, Bazett-corrected QT (QTc) dispersion, T wave peak-to-end interval (Tpe), and percentage of Tpe/QT interval were calculated and compared before and after statin therapy. Twelve-lead ambulatory 24-hour ECGs were recorded in 12 patients. The results demonstrated that after statin therapy for 3 months, serum levels of total cholesterol and low-density lipoprotein cholesterol were significantly reduced (both p values < 0.001). However, neither significant changes of each SAECG parameter nor the frequency of late potentials were demonstrated after atorvastatin therapy. In addition, no significant changes in QT dispersion, QTc dispersion, Tpe, or Tpe/QT were found. However, 24-hour ambulatory ECG revealed a flattening effect of circadian variation of QTc dispersion after atorvastatin therapy. In conclusion, the favorable antiarrhythmia effect of atorvastatin (10 mg/day) therapy cannot be directly reflected by analyzing these noninvasive ECG risk stratification parameters in low-risk patients with hypercholesterolemia.

Current concepts in the mechanisms and management of drug-induced QT prolongation and torsade de pointes

Drug-induced long QT syndrome is characterized by a prolonged corrected QT interval (QTc) and increased risk of a polymorphic ventricular tachycardia known as torsade de pointes (TdP). We review mechanisms, predispositions, culprit agents, and management of this potentially fatal phenomenon. Virtually all drugs that prolong QTc block the rapid component of the delayed rectifier current (I(kr)). Some drugs prolong QTc in a dose-dependent manner, others do so at any dose. Most patients that develop drug-induced TdP have underlying risk factors. Female sex is the most common. Implicated drugs include class 1A and III antiarrhythmics, macrolide antibiotics, pentamidine, antimalarials, antipsychotics, arsenic trioxide, and methadone. Treatment for TdP includes immediate defibrillation for hemodynamic instability and intravenous magnesium sulfate. Potassium levels should be maintained in the high normal range, and all QT prolonging agents must be promptly discontinued.

[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.

Value of electrocardiographic and ankle-brachial index abnormalities for prediction of coronary atherosclerosis in asymptomatic subjects with type 2 diabetes mellitus

Type 2 diabetes mellitus (DM) is associated with increased cardiovascular risk, in part due to accelerated subclinical atherosclerosis. Electrocardiographic (ECG) and ankle-brachial index (ABI) abnormalities are used to screen for cardiovascular risk in the clinic. However, their capacity to identify patients with type 2 DM with nonobstructive subclinical atherosclerosis is unknown. Associations of ECG and ABI abnormalities with coronary artery calcium (CAC), a measure of coronary atherosclerosis, were examined using multivariable ordinal regression modeling in 589 asymptomatic patients with type 2 DM. Sensitivity, specificity, and positive and negative predictive values were determined. CAC was prevalent (44% CAC>00; 32% CAC>5th percentile score) despite normal electrocardiograms (64%) and ABIs (97%) in most subjects. Neither ECG nor ABI changes predicted CAC after adjusting for age, gender, and race. ECG abnormalities were neither sensitive nor specific for detection of CAC>100, >400, or>75th percentile (sensitivities 0.43, 0.45, and 0.34; specificities 0.69, 0.66, and 0.63, respectively). ABI abnormalities were not sensitive (0.03, 0.04, and 0.03) but had high specificity (0.98, 0.98, and 0.98). In subjects with normal electrocardiograms and ABIs, extensive CAC was remarkably prevalent (CAC>00 in 24%). In conclusion, ECG and ABI abnormalities failed to detect patients with subclinical coronary atherosclerosis and therefore may be of limited value in identifying many asymptomatic patients with type 2 DM at increased risk of cardiovascular disease.

[Early risk predictors of sudden cardiac death after myocardial infarction: results of follow up of 881 patients]

INTRODUCTION

It has been shown that depolarization disorders, autonomic dysfunction, and systolic dysfunction of the left ventricle are associated with sudden cardiac death after myocardial infarction.

OBJECTIVE

The objective of study was to examine the prognostic value of the most important predictors in the first week after myocardial infarction.

METHOD

Study included 881 patients who were followed up from 1 to 60 months. During the first week after myocardial infarction, following examination were performed: ECG with standard leads and X, Y, Z orthogonal leads, vectorcardiogram, QT interval, late potentials, short-time spectral analysis of RR variability, nonlinear (Poincaré plot) analysis and echocardiogram.

RESULTS

In univariate analysis, the following parameters measured on the first day were important predictors of sudden cardiac death: lower LF/HF ratio (<1.5) (p = 0.000), T wave inversion in X lead (p = 0.000), high P wave in D2 lead (p=0.030), and diminished systolic function (p = 0.000). In multivariate analysis, the following parameters were significant risk predictors: T wave inversion in X lead, lower LF/HF ratio, positive late potentials and the left ventricle systolic dysfunction.

CONCLUSION

The parameters of the left ventricle systolic disfunction with sympathicovagal imbalance and electric instability are the key risk predictors in the first few days after myocardial infarction.

Ventricular repolarization dynamics during different sleep stages in the subacute phase of myocardial infarction

BACKGROUND

Ventricular arrhythmias after myocardial infarction (MI) are often nocturnal. However, the arrhythmogenic effects of sleep after MI are unknown. We examined the effects of sleep stages on QT dynamicity and tested the hypothesis of a differential effect of sleep stage on the QT/RR relationship after recent MI, versus in healthy controls (HC).

METHODS

Polysomnography and electrocardiograms were simultaneously recorded in 21 men in the subacute phase of a first uncomplicated MI, and in 10 age-matched, male HC. QT dynamicity (QT/RR slope) and parameters of QT interval were measured during wakefulness, stages 1-4 of nonrapid eye movement (non-REM) sleep, and REM sleep.

RESULTS

Mean QT and RR intervals increased through all sleep stages in both MI survivors and HC. The Bazett-corrected QT interval remained stable from wakefulness throughout all sleep stages. QT/RR slopes remained stable from wakefulness to stage 3 in both groups. However, unlike in MI survivors, the QT/RR slopes decreased and remained significantly lower during deep sleep and REM sleep in HC.

CONCLUSION

An abnormal QT/RR relationship in deep sleep and REM sleep was observed after a recent MI, reflecting an insufficient shortening of ventricular repolarization with increasing heart rates, which might have important implications in the nocturnal distribution of ventricular arrhythmias after MI.

Gender differences in arrhythmias

Electrocardiographic and electrophysiologic differences between men and women have long been noted. Women have a higher intrinsic heart rate than men, along with a longer corrected QT interval and a shorter sinus nodal recovery time. The incidence of and risk factors for a variety of arrhythmias differ between men and women. Atrioventricular nodal reentry tachycardia has a 2:1 female-to-male predominance, while accessory pathways are twice as frequent in men. Although atrial fibrillation is more prevalent in men of all age groups, the absolute numbers of men and women with atrial fibrillation are equal, and the associated morbidity and mortality experienced by women with atrial fibrillation appear to be worse. Women have a lower incidence of sudden cardiac death, and female survivors of sudden cardiac death have a lower frequency of spontaneous or inducible ventricular tachycardia. On the other hand, drug-induced torsade de pointes and symptomatic long QT syndrome have a female predominance. Therefore, greater caution should be used when prescribing QT-prolonging drugs in women. The incidence of arrhythmias is increased during pregnancy, and management of pregnant patients poses a significant challenge. The mechanisms of these gender differences are unclear but may be related to hormonal effects and the shorter QT interval in adult males. Pharmacologic and nonpharmacologic therapies are usually equally efficacious, but the risks of pharmacologic therapy are different in men and women. Atrial fibrillation may be more difficult to treat in women.

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.

Antimicrobial-associated QT interval prolongation: pointes of interest

Until recently, cardiac toxicity manifesting in the form of arrhythmias related to QT interval prolongation was uncommonly appreciated within the antimicrobial class of drugs, but it was well described among antiarrhythmic agents. Antimicrobials that are associated with QT prolongation include the macrolides/ketolides, certain fluoroquinolones and antimalarials, pentamidine, and the azole antifungals. Although, in most cases, mild delays in ventricular repolarization caused by these drugs are clinically unnoticeable, they may serve to amplify the risk for torsades de pointes (TdP) when prescribed in the setting of other risk factors. Conditions or variables that influence proarrhythmic risk include sex, age, electrolyte derangements, structural heart disease, pharmacokinetic/pharmacodynamic interactions, and genetic predisposition. It is important that clinicians be knowledgeable about drugs with QT liability, as well as the risk factors that increase the probability of TdP. Additionally, because TdP remains a difficult-to-measure adverse event, we must rely upon multiple data sources to determine the risk versus the benefit for newly approved drugs.

The prognostic accuracy of different QT interval measures

BACKGROUND

The QT intervals accuracy for predicting arrhythmic death varies between studies, possibly due to differences in the selection of the lead used for measurement of the QT interval. The purpose of this study was to analyze the prognostic accuracy of all known ways to select the lead.

METHODS AND RESULTS

Three institutions that used different methods for measuring QT intervals provided their QT databases. They included more than 3500 twelve-lead surface ECGs. The data represented low- and high-risk patients of the normal population (survivors vs dead from cardiovascular causes), acute myocardial infarction (survivors versus death from all causes) and remote myocardial infarction (with vs without a history of ventricular arrhythmia). The prognostic accuracy was defined as the area under the Receiver Operator Curve (ROC-area). The most accurate standard leads were I and aVL and the least accurate was AVR. The most accurate precordial lead was V4. The prognostic accuracy of the longest QT interval was higher than for any standard lead. The prognostic accuracy of the mean of the three longest QT intervals was equal to or slightly lower than for the longest QT interval.

CONCLUSIONS

The highest prognostic accuracy is obtained with the longest QT interval. The accuracies of the lead selection methods are so different that it can explain a substantial part of the differences between otherwise similar studies in the literature. We recommend the use of the mean value of the three longest QT intervals.

Risk factors for prolonged QTc among US adults: Third National Health and Nutrition Examination Survey

BACKGROUND

QT interval prolongation can lead to torsades de pointes, a potentially fatal arrhythmia. Although research exists on the relationship between QT prolongation and clinical outcome, few studies have described risk factors for prolonged QT interval in the general population.

METHODS

The Third National Health and Nutrition Examination Survey (NHANES III) collected electrocardiogram interval data on 8561 subjects over 40 years of age and projected results to the US population. QT was corrected for heart rate using Fridericia's formula. Logistic regression analyses were performed to identify factors independently associated with prolonged QTc interval, defined as being in the upper 5% of the population QTc interval distribution. Analyses were conducted separately for women and men as a result of differences in the QT distribution between the sexes and also because of potential effect modification. Analytical variables included age, race/ethnicity, electrolyte measurements, body mass index, the recent use of QT-prolonging drugs and past medical histories of stroke, thyroid disease, hypertension, diabetes and myocardial infarction.

RESULTS

Age, female sex, hypocalcemia (men), hypokalemia (women), and a history of thyroid disease and myocardial infarction (men) were associated with a prolonged QTc interval. In addition, taking QT-prolonging medications in the past month was associated with more than a twofold increase in the odds of prolonged QTc interval in both men and women.

CONCLUSIONS

Healthcare practitioners should be aware that a prolonged QTc interval is a potential indicator of cardiovascular risk, and should exercise caution in prescribing potentially QT-prolonging medications to certain patients.

Prolonged QTc interval and risk of sudden cardiac death in a population of older adults

OBJECTIVES

This study sought to investigate whether prolongation of the heart rate-corrected QT (QTc) interval is a risk factor for sudden cardiac death in the general population.

BACKGROUND

In developed countries, sudden cardiac death is a major cause of cardiovascular mortality. Prolongation of the QTc interval has been associated with ventricular arrhythmias, but in most population-based studies no consistent association was found between QTc prolongation and total or cardiovascular mortality. Only very few of these studies specifically addressed sudden cardiac death.

METHODS

This study was conducted as part of the Rotterdam Study, a prospective population-based cohort study that comprises 3,105 men and 4,878 women aged 55 years and older. The QTc interval on the electrocardiogram was determined during the baseline visit (1990 to 1993) and the first follow-up examination (1993 to 1995). The association between a prolonged QTc interval and sudden cardiac death was estimated using Cox proportional hazards analysis.

RESULTS

During an average follow-up period of 6.7 years (standard deviation, 2.3 years) 125 patients died of sudden cardiac death. An abnormally prolonged QTc interval (>450 ms in men, >470 ms in women) was associated with a three-fold increased risk of sudden cardiac death (hazard ratio, 2.5; 95% confidence interval, 1.3 to 4.7), after adjustment for age, gender, body mass index, hypertension, cholesterol/high-density lipoprotein ratio, diabetes mellitus, myocardial infarction, heart failure, and heart rate. In patients with an age below the median of 68 years, the corresponding relative risk was 8.0 (95% confidence interval 2.1 to 31.3).

CONCLUSIONS

Abnormal QTc prolongation on the electrocardiogram should be viewed as an independent risk factor for sudden cardiac death.

Diurnal variation of ventricular repolarization in a large family with LQT3-Brugada syndrome characterized by nocturnal sudden death

BACKGROUND

In patients with long-QT syndrome type 3 (LQT3), symptoms occur particularly at rest or during sleep. As to the underlying mechanism, excessive prolongation of the QT interval at slow heart rates probably plays a role.

OBJECTIVES

The purpose of the present study was to investigate QT interval prolongation unrelated to heart rate comparing nighttime and daytime in a family with features of both LQT3 and Brugada syndrome.

METHODS

The study group consisted of 38 carriers of the mutant gene (SCN5A, 1795insD) and 30 noncarrier family members, who served as controls. Holter monitoring was performed with beat-to-beat QT interval measurement. In addition, in a subset of subjects, an exercise test and a pacing test (carriers only) with measurement of the RT interval were performed.

RESULTS

In carriers, the slope between heart rate and QT interval was significantly steeper during nighttime (0:00 a.m. to 6:00 a.m.) than during daytime (8:00 a.m. to 22:00 p.m.) (regression coefficient -6.18 and -2.80, respectively), (p=0.03),no such effect being observed in the noncarriers. Further, the RT interval was markedly shorter during recovery than during exercise in carriers but not in noncarriers. In contrast, during AAI pacing in the carriers, RT interval shortening along with increasing heart rate was followed by a comparable prolongation of the RT interval along with subsequent decreasing heart rate.

CONCLUSIONS

In this large LQT3-Brugada syndrome family, carriers of the mutant gene (SCN5A, 1795insD) are characterized by diurnal variation of ventricular repolarization by exhibiting QT interval prolongation, which is more pronounced during nighttime compared with daytime, even when taking into account differences in heart rate. The autonomic nervous system appears to play a role in mediating this effect. This observation may be of relevance for explaining the high incidence of nocturnal sudden death in this family, but this remains to be proven. In addition, whether our findings also apply to other families with LQT3 is uncertain.

QTc interval and survival in 75-year-old men and women from the general population

AIMS

The study concerns the relationship of the corrected QT (QTc) interval to 6.4 years of survival and to measures of cardiac function, such as echocardiographic variables and plasma levels of brain natriuretic peptide (BNP), in 75-year-old people.

METHODS AND RESULTS

QTc was measured in a 12-lead electrocardiogram (ECG) in 210 men and 223 women, comprising a randomly selected sample from the general population (70% participation rate). The Sicard 440/740 computer-analysis program, with Hodges' formula for heart rate-based QT correction, was used. The optimal cut-off point for predicting survival according to the receiver operating characteristic curve was found between 429 and 430 ms. Individuals with a QTc interval of > or =430 ms (n = 115) had decreased survival when compared with those with shorter QTc interval (n = 318); the relative risk was 2.4 (95% confidence interval 1.5-3.7). The predictive ability of QTc reflects an association between QTc and the following variables: BNP, left ventricular mass, and left ventricular ejection fraction (but not diastolic filling patterns). Both Hodges' and Bazett's formulae for heart rate correction of the QT interval were useful for predicting survival. The median QTc was 415 ms using Hodges' formula and 430 ms with Bazett's formula. The QRS component of QTc predicted survival better than the rest of the QTc interval and was approximately as useful as the QTc interval itself.

CONCLUSION

The computer-derived QTc obtained from the ordinary 12-lead ECG identifies high-risk individuals among elderly people from the general population.

Absence of QTc-Interval-Prolonging or Hemodynamic Effects of a Single Dose of Bitter-Orange Extract in Healthy Subjects

STUDY OBJECTIVE

To evaluate the hemodynamic and electrocardiographic effects of a single dose of commercially available bitter-orange dried-fruit extract, which is increasingly being used in dietary supplements.

DESIGN

Randomized, double-blind, placebo-controlled, crossover study.

SETTING

University of Connecticut, Storrs Campus.

SUBJECTS

Eighteen healthy volunteers aged 18 years or older.

INTERVENTION

Subjects were given either placebo or bitter-orange dried-fruit extract (450 mg standardized to 27 mg of m- or p-synephrine) in phase 1. The opposite treatment was given during phase 2 after a washout period of at least 7 days.

MEASUREMENTS AND MAIN RESULTS

The rate-corrected QT (QTc) interval and blood pressure were measured before dosing and at 1, 3, 5, and 8 hours after dosing. Mean+/-SD values of the maximum postdose values were compared between groups. Subjects receiving bitter-orange extract versus those receiving placebo had similar postdose QTc intervals (402+/-29 vs 403+/-24 msec, p=0.653), systolic blood pressure (114+/-10 vs 115+/-8 mm Hg, p=0.686) and diastolic blood pressure (68+/-9 vs 68+/-8, p=0.879).

CONCLUSION

Bitter-orange dried-fruit extract standardized to m- or p-synephrine 27 mg did not significantly alter the QTc interval or blood pressure after a single dose was administered. Future studies are necessary to ensure the safety of this herbal product with multiple doses.

Pharmacokinetic-pharmacodynamic modeling in the data analysis and interpretation of drug-induced QT/QTc prolongation

In this review, factors affecting the QT interval and the methods that are currently in use in the analysis of drug effects on the QT interval duration are overviewed with the emphasis on (population) pharmacokinetic-pharmacodynamic (PK-PD) modeling. Among which the heart rate (HR) and the circadian rhythm are most important since they may interfere with the drug effect and need to be taken into account in the data analysis. The HR effect or the RR interval (the distance between 2 consecutive R peaks) effect is commonly eliminated before any further analysis, and many formulae have been suggested to correct QT intervals for changes in RR intervals. The most often used are Bazett and Fridericia formulae introduced in 1920. They are both based on the power function and differ in the exponent parameter. However, both assume the same exponent for different individuals. More recent findings do not confirm this assumption, and individualized correction is necessary to avoid under- or overcorrection that may lead to artificial observations of drug-induced QT interval prolongation. Despite the fact that circadian rhythm in QT and QTc intervals is a well-documented phenomenon, it is usually overlooked when drug effects are evaluated. This may result in a false-positive outcome of the analysis as the QTc peak due to the circadian rhythm may coincide with the peak of the drug plasma concentration. In view of these effects interfering with a potential drug effect on the QTc interval and having in mind low precision of QT interval measurements, a preferable way to evaluate the drug effect is to apply a population PK-PD modeling. In the literature, however, there are only a few publications in which population PK-PD modeling is applied to QT interval prolongation data, and they all refer to antiarrhythmic agents. In this review, after the most important sources of variability are outlined, a comprehensive population PK-PD model is presented that incorporates an individualized QT interval correction, a circadian rhythm in the individually corrected QT intervals, and a drug effect. The model application is illustrated using real data obtained with 2 compounds differing in their QT interval prolongation potential. The usefulness of combining data of several studies is stressed. Finally, the standard approach based on the raw observations and formal statistics, as described in the Preliminary Concept paper of the International Conference on Harmonization, is briefly compared with the method based on population PK-PD modeling, and the advantages of the latter are outlined.

Circadian periodicity of heart rate variability in hospitalized angor patients

The relationship between unstable angor (angina) and circadian periodicity of heart rate variability (HRV) was explored in a group of patients hospitalized in a coronary care unit (CCU). Patients were classified as normal (whose symptoms had non-cardiovascular origin, n=8), moderate angor (n=13) and severe angor (n=11). A fourth group of ambulatory healthy volunteers (n=12) was included. Individual 24 h Holter records were analyzed, mean RR and standard deviation of RR (SDNN) being obtained from 1 h-length windows. For frequency domain analysis, 5 min-length windows were employed. The spectral components analyzed were total power (spectral power between 0.01 and 0.5 Hz), low frequency power (LF: power between 0.04 and 0.15 Hz), and high frequency power (HF: power between 0.15 and 0.4 Hz). In addition, LF to HF areas ratio (L/H) was computed. Mesor, amplitude and acrophase for every 24 h rhythm were calculated by cosinor analysis. As compared to ambulatory controls, admission to the CCU diminished amplitude and phase-delayed the circadian oscillation of most HRV parameters, except for SDNN. Moderate angor patients showed decreased amplitude of RR and L/H and augmented amplitude of SDNN when compared to normal hospitalized subjects. A phase delay of about 1.5 h for RR intervals and a phase advance of 3.5-6 h for LFA and SDNN were found in the moderate angor group when compared to normal. Amplitude of 24 h variation of total power decreased in severely angor patients and the circadian oscillation of HF (an indicator of vagal control on the heart) became free running. A phase delay of 2.5 h in SDNN acrophase was found in severely affected patients when compared to moderate. The results indicate that severity of unstable angor correlates with desynchronization of parasympathetic control of heart rate.

Electrocardiographic effects of an Ephedra-Free, multicomponent weight-loss supplement in healthy volunteers

STUDY OBJECTIVE

Metabolife 356, an ephedra-containing weight-loss product, substantially increases the corrected QT (QTc) interval. Metabolife Ephedra Free, a similar supplement, contains caffeine and extracts of green tea, garcinia cambogia, and yerba mate. Its electrocardiographic (ECG) effects are not known. Therefore, we sought to determine the effect of this supplement on the QTc interval.

DESIGN

Randomized, double-blind, placebo-controlled, crossover study.

SETTING

University of Connecticut, Storrs Campus.

SUBJECTS

Twenty healthy volunteers. Intervention. A single capsule containing half the normal recommended dose of Metabolife Ephedra Free or matching placebo was administered in crossover fashion, with a 7-day washout period between treatments.

MEASUREMENTS AND MAIN RESULTS

Baseline and three postdose ECG measurements were obtained, and QTc intervals were measured over a 5-hour study period. No significant differences in the QTc interval or other ECG variables were observed between the Metabolife Ephedra Free and placebo groups.

CONCLUSION

At half the recommended single dose, Metabolife Ephedra Free does not affect the QTc interval or other ECG variables over 5 hours. Dose-response studies and studies of longer duration should be conducted.

Antipsychotic drugs and QT prolongation

Antipsychotic drugs (AD) are effective and frequently prescribed to more females than males. AD may cause serious cardiovascular side-effects, including prolonged QT interval, eventually leading to torsades de pointes (TdP) and sudden death. Epidemiologic data and case-control studies indicate an increased rate of sudden death in psychiatric patients taking AD. This review summarizes current knowledge about the QT prolonging effects of AD and gives practical suggestions. Amisulpride, clozapine, flupenthixol, fluphenazine, haloperidol, melperone, olanzapine, perphenazine, pimozide, quetiapine, risperidone, sulpiride, thioridazine and ziprasidone cause a QT prolongation ranging from 4 ms for risperidone to 30 ms for thioridazine. Our knowledge about the QT-prolonging effects of many AD is still limited. Females are under-represented in most studies. Many studies were conducted or supported by pharmaceutical companies. To avoid prodysrhythmia caused by QT prolongation, other factors influencing QT interval have to be considered, such as other drugs affecting the same pathway, hypokalemia, hypomagnesemia, bradycardia, increased age, female sex, congestive heart failure and polymorphisms of genes coding ion channels or enzymes involved in drug metabolism. Because the response of a patient to AD is individual, an electrocardiogram recording the QT interval has to be performed at baseline, after AD introduction and after occurrence of any factor that might influence the QT interval.

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.

Effects of shift work on QTc interval and blood pressure in relation to heart rate variability

OBJECTIVES

There is evidence that shift work contributes to excess cardiovascular mortality. The purpose of this study was to determine the effects of shift work on heart rate-corrected QT (QTc) and blood pressure in relation to heart rate variability (CV(RR)).

METHODS

The study population consisted of 153 male shiftworkers and 87 male day workers who were employed at a copper-smelting plant. The QTc interval, total power spectral density (t-PSD) of 100 RR intervals, PSDs with frequencies of 0.01 Hz-0.15 Hz and 0.15 Hz-0.40 Hz (PSD(LF) and PSD(HF)), CV(RR), low frequency (LF) component variability and high frequency (HF) component variability (CCV(LF) and CCV(HF)) and %LF (PSD(LF)/(PSD(LF)+PSD(HF)) .100) were measured (LF and HF components are thought to reflect the sympathetic and parasympathetic activities, respectively).

RESULTS

The QTc interval was significantly longer in the shiftworkers than in the day workers, although there was no significant difference in systolic or diastolic blood pressure between the two groups. Also, the CCV(LF) and log(PSD(LF)) were significantly depressed in the shiftworkers. In the day workers, the QTc interval was significantly related to the CV(RR), log(t-PSD), CCV(HF), log(PSD(HF)),%LF, and log(LF/HF ratio) (partial correlation coefficient r=-0.305, -0.377, -0.312, -0.355, 0.297, and 0.277, respectively). In the shiftworkers only two relations of the QTc interval to non-specific CV(RR) and log(t-PSD) were significant.

CONCLUSION

The clear association between long QTc interval and reduced parasympathetic activity observed in the day workers did not exist in the shiftworkers. Rather, the shiftworkers had a selective reduction in sympathetic drive, which may have compensated for prolonged QTc interval due to shift work. On the other hand, the impact of shift work on hypertension remains unclear, although reduced sympathetic drive in the shiftworkers may explain the absence of differences in blood pressure between shiftworkers and day workers to some extent.

EFFECT OF AUTONOMIC NERVOUS FUNCTION ON QT INTERVAL IN DOGS

The effects of drugs on the QT interval should be evaluated precisely in the early stages of drug development because QT prolongation can trigger the so-called torsades de pointes, a life-threatening polymorphic ventricular tachycardia. It has been reported that the QT interval is affected by autonomic nervous tone besides the heart rate. In this study, we investigated the direct effect of autonomic nervous tone on the QT interval using the parameters of heart rate variability in dogs, when the RR interval was constant (400 or 700 msec). Our results showed that the QT interval at the high HF (high vagal tone) or low LF/HF ratio (low sympathetic tone) was longer than that at the low HF (low vagal tone) or high LF/HF ratio (high sympathetic tone), when the RR intervals were constant, and that the effect of vagal tone on the QT interval might be somewhat stronger than that of the sympathetic tone. The present observations would support the idea that sympathetic as well as parasympathetic tone regulates QT interval and that QT interval may be controlled physiologically by myocardial autonomic nerves via and not via a sinus node. Therefore, a more precise correction formula of QT interval could be established using autonomic parameters other than RR interval (heart rate), while the QT interval is widely known to be dependent on the RR interval or heart rate.

QT prolongation with antimicrobial agents: understanding the significance

Cardiac toxicity has been relatively uncommon within the antimicrobial class of drugs, but well described for antiarrhythmic agents and certain antihistamines. Macrolides, pentamidine and certain antimalarials were traditionally known to cause QT-interval prolongation, and now azole antifungals, fluoroquinolones and ketolides can be added to the list. Over time, advances in preclinical testing methods for QT-interval prolongation and a better understanding of its sequelae, most notably torsades de pointes (TdP), have occurred. This, combined with the fact that five drugs have been removed from the market over the last several years, in part because of QT-interval prolongation-related toxicity, has elevated the urgency surrounding early detection and characterisation methods for evaluating non-antiarrhythmic drug classes. With technological advances and accumulating literature regarding QT prolongation, it is currently difficult or overwhelming for the practising clinician to interpret these data for purposes of formulary review or for individual patient treatment decisions. Certain patients are susceptible to the effects of QT-prolonging drugs. For example, co-variates such as gender, age, electrolyte derangements, structural heart disease, end organ impairment and, perhaps most important, genetic predisposition, underlie most if not all cases of TdP. Between and within classes of drugs there are important differences that contribute to delayed repolarisation (e.g. intrinsic potency to inhibit certain cardiac ion currents or channels, and pharmacokinetics). To this end, a risk stratification scheme may be useful to rank and compare the potential for cardiotoxicity of each drug. It appears that in most published cases of antimicrobial-associated TdP, multiple risk factors are present. Macrolides in general are associated with a greater potential than other antimicrobials for causing TdP from both a pharmacodynamic and pharmacokinetic perspective. The azole antifungal agents also can be viewed as drugs that must be weighed carefully before use since they also have both pharmacodynamic and pharmacokinetic characteristics that may trigger TdP. The fluoroquinolones appear less likely to be associated with TdP from a pharmacokinetic perspective since they do not rely on cytochrome P450 (CYP) metabolism nor do they inhibit CYP enzyme isoforms, with the exception of grepafloxacin and ciprofloxacin. Nonetheless, patient selection must be carefully made for all of these drugs. For clinicians, certain responsibilities are assumed when prescribing antimicrobial therapy: (i) appropriate use to minimise resistance; and (ii) appropriate patient and drug selection to minimise adverse event potential. Incorporating information learned regarding QT interval-related adverse effects into the drug selection process may serve to minimise collateral iatrogenic toxicity.

Acquired QT interval prolongation and HERG: implications for drug discovery and development

Putative interactions between the Human Ether-a-go-go Related Gene (HERG), QT interval prolongation and Torsades de Pointes (TdP) are now integral components of any discussion on drug safety. HERG encodes for the inwardly rectifying potassium channel (I(Kr)), which is essential to the maintenance of normal cardiac function. HERG channel mutations are responsible for one form of familial long QT syndrome, a potentially deadly inherited cardiac disorder associated with TdP. Moreover, drug-induced (acquired) QT interval prolongation has been associated with an increase in the incidence of sudden unexplained deaths, with HERG inhibition implicated as the underlying cause. Subsequently, a number of non-cardiovascular drugs which induce QT interval prolongation and/or TdP have been withdrawn. However, a definitive link between HERG, QT interval prolongation and arrhythmogenesis has not been established. Nevertheless, this area is subject to ever increasing regulatory scrutiny. Here we review the relationship between HERG, long QT syndrome and TdP, together with a summary of the associated regulatory issues, and developments in pre-clinical screening.

Safety profile of oral and intravenous moxifloxacin: Cumulative data from clinical trials and postmarketing studies

BACKGROUND

The established safety profile of the fluoroquinolones has been disrupted in the past decade by the detection of low-frequency but potentially serious adverse events that have led to the license suspension, voluntary withdrawal, or restricted use of specific members of the class. Moxifloxacin is a broad-spectrum, advanced-generation fluoroquinolone that has potent activity against respiratory tract infections in adults in both oral and IV formulations.

OBJECTIVE

The goal of this article was to provide an overview of the cumulative safety data on both oral and IV moxifloxacin, including data from the most recent clinical trials and postmarketing studies.

METHODS

Data from clinical trials of moxifloxacin were captured from an electronic database maintained by the manufacturer. Safety data for oral moxifloxacin were obtained from 30 Phase II/III comparator studies (n = 7,368 moxifloxacin, n = 5,687 comparators), 1 Phase IV study (n = 18,374), and 4 postmarketing observational studies (n = 27,756). Safety data for IV moxifloxacin were obtained from 2 Phase III comparator studies (n = 550 maxifloxacin, n = 579 comparators). In addition, pharmacokinetic data were reviewed.

RESULTS

In Phase II/III comparator studies, gastrointestinal complaints were the most common adverse drug reactions (ADRs) associated with both formulations of moxifloxacin, with nausea occurring in 7.1% and 3.1% of patients receiving oral and IV moxifloxacin, respectively, and diarrhea occurring in 5.2% and 6.2% of patients. Discontinuation rates due to ADRs with oral and IV moxifloxacin were 2.7% and 6.0%, and mortality rates were 0.3% and 4.0%. Similar rates of withdrawal and mortality were observed in the comparator groups. There was no evidence that moxifloxacin caused disturbances in glucose metabolism in patients with or without diabetes mellitus, and there was no evidence of an increased risk for cardiovascular adverse events. Pharmacokinetic analyses indicated that dose adjustment of moxifloxacin does not appear to be necessary in elderly patients, those with renal dysfunction, or those with mild to moderate hepatic impairment. The pharmacokinetics of moxifloxacin have not been studied in patients with severe hepatic insufficiency. Moxifloxacin does not interact with a number of commonly prescribed drugs, although its absorption is decreased by concomitant administration of iron and cationic antacids.

CONCLUSIONS

Based on evidence from >7000 patients in clinical trials and >46,000 patients in postmarketing studies, moxifloxacin is generally well tolerated. Its lack of significant drug interactions in target groups makes it an option in diabetic patients or the elderly, as well as in those with renal impairment.