Long QT: good, bad or indifferent?

Role of the rabbit whole-heart model for electrophysiologic safety pharmacology of non-cardiovascular drugs

Plenty of non-cardiovascular drugs alter cardiac electrophysiology and may ultimately lead to life-threatening arrhythmias. In clinical practice, measuring the QT interval as a marker for the repolarization period is the most common tool to assess the electrophysiologic safety of drugs. However, the sole measurement of the QT interval may be insufficient to determine the proarrhythmic risk of non-cardiovascular agents. Several other markers are considered in pre-clinical safety testing to determine potential harm on cardiac electrophysiology. Besides measuring typical electrophysiologic parameters such as repolarization duration, whole-heart models allow the determination of potential predictors for proarrhythmia. Spatial and temporal heterogeneity as well as changes of shape of the action potential can be easily assessed. In addition, provocation manoeuvers (either by electrolyte imbalances or programmed pacing protocols) may induce sustained arrhythmias and thereby determine ventricular vulnerability to arrhythmias. Compared with the human heart, the rabbit heart possesses a similar distribution of ion currents that govern cardiac repolarization, resulting in a rectangular action potential configuration in both species. In addition, similar biophysical properties of rabbit and human cardiac ion channels lead to a comparable pharmacologic response in human and rabbit hearts. Of note, arrhythmia patterns resemble in both species due to the similar effective size of human and rabbit hearts. Thus, the rabbit heart is particularly suitable for testing the electrophysiologic safety of drugs. Several experimental setups have been developed for studying cardiac electrophysiology in rabbits, ranging from single cell to tissue preparations, whole-heart setups, and in vivo models.

Does Sex Modify an Association of Electrophysiological Substrate with Sudden Cardiac Death? The Atherosclerosis Risk in Communities (ARIC) Study

Background

Sex is a well-recognized risk factor for sudden cardiac death (SCD). We hypothesized that sex modifies the association of electrophysiological (EP) substrate with SCD.

Objective

The purpose of this study was to determine whether there are sex differences in electrocardiographic (ECG) measures and whether sex modifies the association of ECG measures of EP substrate with SCD.

Methods

Participants from the Atherosclerosis Risk in Communities study with analyzable ECGs (n = 14,725; age 54.2 ± 5.8 years; 55% female; 74% white) were included. EP substrate was characterized by heart rate, QRS, QTc, Cornell voltage, spatial ventricular gradient (SVG), and sum absolute QRST integral (SAI QRST) ECG metrics. Two competing outcomes were adjudicated: SCD and non-SCD. Interaction of ECG metrics with sex was studied in Cox proportional hazards and Fine-Gray competing risk models. Model 1 was adjusted for prevalent cardiovascular disease (CVD) and risk factors. Time-updated model 2 was additionally adjusted for incident nonfatal CVD. Relative hazard ratio (RHR) and relative subhazard ratio with 95% confidence interval (CI) for SCD and non-SCD risk for women relative to men were calculated. Model 1 was adjusted for prevalent CVD and risk factors. Time-updated model 2 was additionally adjusted for incident nonfatal CVD.

Results

Over median follow-up of 24.4 years, there were 530 SCDs (incidence 1.72; 95% CI 1.58–1.88 per 1000 person-years). Women compared to men experienced a greater risk of SCD associated with Cornell voltage (RHR 1.18; 95% CI 1.06–1.32; P = .003), SAI QRST (RHR 1.16; 95% CI 1.04–1.30; P = .007), and SVG magnitude (RHR 1.24; 95% CI 1.05–1.45; P = .009), independently from incident CVD.

Conclusion

In women, the global EP substrate is associated with up to 24% greater risk of SCD than in men, suggesting differences in underlying mechanisms and the need for sex-specific SCD risk stratification.

Electrographic indices in migraine patients: A systematic review and meta-analysis

BACKGROUND AND AIM

Migraine patients can exhibit autonomic dysregulation, in turn leading to cardiac conduction and repolarization abnormalities. This systematic review and meta-analysis evaluated the electrocardiographic changes in migraineurs.

METHOD

PubMed and Embase databases were searched for human studies using the search terms 'migraine' and 'electrocardiogram' until 15th December 2018, identifying 108 and 131 studies.

RESULTS

Thirteen studies involving 667 migraineurs and 208 normal subjects included (mean age=30.7, total male percentage=19.8%) were included. A longer mean QTc interval (standard mean difference=7.89, 95% confidence interval=[3.29, 12.49], p=0.0008) and higher frequency of QTc prolongation (risk ratio [RR]=6.23, [2.86-13.58], p<0.00001), but no difference in PR-interval (SMD=4.33, [-3.90-12.56], p=0.30) were observed during migraine attacks compared to pain-free periods. P-wave dispersion was higher in migraine patients compared to controls (mean difference=3.62, [1.03-6.21], p=0.006). RR-interval were statistically indistinguishable between migraine patients and controls (SMD=0.08, [-0.65-0.81], p=0.83), or between migraineurs with and without aura (SMD=-0.03, [-0.44-0.38], p=0.89). Deep breathing ratio was significantly lower in migraineurs compared to controls (SMD=-0.27, 95% CI=[-0.46, -0.08], p=0.006) but similar between migraineurs with and without aura (SMD=-0.04, [-0.27-0.19], p=0.74). No significant difference in Valsalva ratio is found between migraineurs and controls (SMD=0.10, [-0.32-0.53], p=0.63) or between migraineurs with and without aura (SMD=-0.17, [-0.40-0.06], p=0.14). Root mean square of successive differences (RMSSD) (SMD=-0.07, [-1.10-0.95], p=0.89) and standard deviation of NN intervals (SDNN) (SMD=-0.10, [-0.61-0.41], p=0.71) did not significantly differ between migraine patients and controls.

CONCLUSION

Electrocardiographic alterations are observed in migraine patients compared to controls, especially during migraine attacks.

Inducible and reversible phenotypes in a novel mouse model of Friedreich's Ataxia

Friedreich's ataxia (FRDA), the most common inherited ataxia, is caused by recessive mutations that reduce the levels of frataxin (FXN), a mitochondrial iron binding protein. We developed an inducible mouse model of Fxn deficiency that enabled us to control the onset and progression of disease phenotypes by the modulation of Fxn levels. Systemic knockdown of Fxn in adult mice led to multiple phenotypes paralleling those observed in human patients across multiple organ systems. By reversing knockdown after clinical features appear, we were able to determine to what extent observed phenotypes represent reversible cellular dysfunction. Remarkably, upon restoration of near wild-type FXN levels, we observed significant recovery of function, associated pathology and transcriptomic dysregulation even after substantial motor dysfunction and pathology were observed. This model will be of broad utility in therapeutic development and in refining our understanding of the relative contribution of reversible cellular dysfunction at different stages in disease.

Electrocardiographic Impact of Myocardial Diffuse Fibrosis and Scar: MESA (Multi-Ethnic Study of Atherosclerosis)

Purpose To examine the associations of myocardial diffuse fibrosis and scar with surface electrocardiographic (ECG) parameters in individuals free of prior coronary heart disease in four different ethnicities. Materials and Methods This prospective cross-sectional study was approved by the institutional review boards, and all participants gave informed consent. A total of 1669 participants in the Multi-Ethnic Study of Atherosclerosis, or MESA, who were free of prior myocardial infarction underwent both ECG and cardiac magnetic resonance imaging. In individuals without a late gadolinium enhancement-defined myocardial scar (n = 1131), T1 mapping was used to assess left ventricular (LV) interstitial diffuse fibrosis. The associations of LV diffuse fibrosis or myocardial scar with ECG parameters (QRS voltage, QRS duration, and corrected QT interval [QTc]) were evaluated by using multivariable regression analyses adjusted for demographic data, risk factors for scar, LV end-diastolic volume, and LV mass. Results The mean age of the 1669 participants was 67.4 years ± 8.7 (standard deviation); 49.8% were women. Lower postcontrast T1 time at 12 minutes was significantly associated with lower QRS Sokolow-Lyon voltage (β = 15.1 µV/10 msec, P = .004), lower QRS Cornell voltage (β = 9.2 µV/10 msec, P = .031), and shorter QRS duration (β = 0.16 msec/10 msec, P = .049). Greater extracellular volume (ECV) fraction was also significantly associated with lower QRS Sokolow-Lyon voltage (β = -35.2 µV/1% ECV increase, P < .001) and Cornell voltage (β = -23.7 µV/1% ECV increase, P < .001), independent of LV structural indexes. In contrast, the presence of LV scar (n = 106) was associated with longer QTc (β = 4.3 msec, P = .031). Conclusion In older adults without prior coronary heart disease, underlying greater LV diffuse fibrosis is associated with lower QRS voltage and shorter QRS duration at surface ECG, whereas clinically unrecognized myocardial scar is associated with a longer QT interval. ^© RSNA, 2016 Online supplemental material is available for this article.

Analytic Reviews: Electrophysiologic Testing in the Intensive Care Unit

The role of electrophysiologic concepts and procedures in managing patients with potentially life-threatening ar rhythmias in the intensive care unit is discussed. These patients may be survivors of sudden cardiac arrest or myocardial infarction or may be admitted for syncope or sustained or nonsustained ventricular tachycardia. The value of electrophysiologic testing is discussed in terms of the distinction between wide QRS complex tachycardias that are supraventricular or ventricular in origin and those in which preexcitation syndromes may be important. Drug-induced ventricular arrhythmias are discussed, with specific emphasis on torsades de pointes. Finally, the use of His bundle recordings in pa tients with atrioventricular conduction disturbances is discussed. The methodology of electrophysiologic test ing, including stimulation protocols and interpretation of results, is described.

The value of neurologic and cardiologic assessment in breath holding spells

Objective: To evaluate the value of neurologic and cardiologic assessment and also the frequency of iron deficiency anemia in children with Breath Holding Spells (BHS).

Methods: The hospital charts of patients diagnosed with BHS between 2011 and 2013 were reviewed retrospectively.

Results: A total of 165 children (90 boys, 75 girls) with BHS comprised the study group. A matched group of 200 children with febrile convulsions served as controls. Among the first-degree relatives, 13.3% had BHS, 1.8% had febrile convulsions and 12.1% had epilepsy. The spells were cyanotic in 140 (84.8%) children and pallid or mixed in the remainder. BNS type was simple in 46.7% of patients and complicated in the remainder. Eighteen patients had abnormalities in electroencephalography, however only one patient was diagnosed with epilepsy. Sixty nine (47.9%) patients were found to have iron deficiency anemia.

Conclusion: Referral of children with clinically definite BHS to pediatric neurology or pediatric cardiology clinics and performance of echocardiography and EEG investigations for exclusion of heart disease or epilepsy appear unnecessary. However, performance of an electrocardiogram to search for prolonged QT syndrome should be considered although no patient in our series had any cardiologic abnormalities.

Use of QT-prolonging medications in US emergency departments, 1995-2009

PURPOSE

Emergency department (ED) patients receive medications that place them at risk for adverse events, including drug-induced prolongation of the QT interval, which can lead to Torsade de Pointes and sudden cardiac death. We report the frequency of prescription and co-prescription of QT-prolonging medications in US EDs and factors associated with high-risk prescribing practices.

METHODS

We analyzed the ED component of the National Hospital Ambulatory Medical Care Survey for 1995 through 2009. Yearly rates of visits involving the prescription of QT-prolonging medications were determined. Multivariate regression analyses identified factors associated with the prescription of two or more QT-prolonging medications.

RESULTS

Approximately 16.5 million visits annually (15.0%) involved prescription of a QT-prolonging drug, with 1.7 million (1.6%) involving multiple prescriptions. Visits associated with QT-prolonging drugs more than doubled over the study period (10.4% to 22.2%). Diphenhydramine, azithromycin, and ondansetron were most frequently implicated (46.1% of cases). The most commonly prescribed combination was diphenhydramine and famotidine, both QT-prolonging medications available over-the-counter. Female gender and older age were associated with co-prescription of QT-prolonging medications. The rate of EKG screening among visits associated with QT-prolonging drug combinations was low (20.9%), but more common than among visits without a QT-prolonging drug (OR 1.3; 95% CI 1.2-1.5).

CONCLUSION

Use of QT-prolonging medications is increasing in EDs nationally. A small number of agents account for a large proportion of these visits and may represent an area for targeted screening or monitoring interventions in the ED.

Corrected QT interval: a prognostic marker in patients with non-ST-segment elevation acute coronary syndrome?

Over many decades, the corrected QT (QTc) has become an established clinical tool for the prediction of sudden cardiac death and life-threatening ventricular arrhythmias and for monitoring adverse effects of pharmacological agents capable of triggering serious ventricular arrhythmias mainly associated with QTc prolongation. Recent evidence also suggests that QTc prolongation is a predictor of poor clinical outcome in patients with coronary artery disease, particularly in the setting of the acute coronary syndrome. Indeed, in the past few years, studies assessing the predictive role of QTc measurements have provided important information in this regard and suggest a potential role of the QTc in patient risk stratification. The incorporation of biomarkers of myocardial damage (ie cardiac troponins), clinical risk scores, and other biochemical and angiographic markers in the past two decades has considerably improved the risk stratification of patients presenting with acute coronary syndrome, but further refinement of our prognostic armamentarium is still required. This article reviews the information available regarding the potential role of the QTc as a marker of increased risk in patients with acute presentations of coronary artery disease.

Prolonged QTc as a predictor of mortality in acute ischemic stroke

OBJECTIVE

We sought to examine the relationship of the QTc interval with mortality and functional outcome after acute ischemic stroke, and determine whether a threshold cutoff is present beyond which risk of death increases.

METHODS

The QTc interval was measured for all patients presenting to the emergency department. The outcomes were mortality at 90 days and functional outcome at hospital discharge. The cutoffs were determined plotting martingale residuals.

RESULTS

Patients with a prolonged QTc interval were more likely to die within 90 days compared with patients without a prolonged interval (relative risk [RR] 2.5; 95% confidence interval [CI] 1.5-4.1; P < .001). The estimated survival at 90 days was 70.5% and 87.1%, respectively. This association retained statistical significance after adjusting for age and National Institutes of Health Stroke Scale score (RR 1.7; 95% CI 1.0-2.9; P = .043). Patients with a prolonged QTc interval were also more likely to have poor functional status compared with patients without a prolonged interval (odds ratio 1.8; 95% CI 1.2-3.0; P = .006). This association was not statistically significant after adjusting for age and National Institutes of Health Stroke Scale score (odds ratio 1.2; 95% CI 0.7-2.4). The identified threshold cutoffs for increased risk of death at 90 days were 440 milliseconds for women and 438 milliseconds for men.

CONCLUSION

There appears to be an increased risk of early death in patients with acute ischemic stroke and a prolonged QTc interval at the time of emergency department presentation. Prognosis appears to be worse with QTc intervals longer than 440 milliseconds in women and longer than 438 milliseconds in men.

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.

A method for constructing and estimating the RR-memory of the QT-interval and its inclusion in a multivariate biomarker for torsades de pointes risk

The problem of drug-induced QT-interval prolongation has become a major economic and health issue in the pharmaceutical industry. Little has been done to link analytically the QT-interval prolongation and the risk of torsades de pointes. This paper introduces a method for analyzing the dynamic characteristics of a 24-hour electrocardiograph and offers an algorithm to measure the long-term memory of the RR-interval history in a single statistic. The memory statistic seems to improve the discrimination between healthy normal subjects and arrhythmia cases using only beat-to-beat information from the QT and RR intervals producing an impulse response function, which is completely independent of heart rate.

Alfuzosin delays cardiac repolarization by a novel mechanism

The United States Food and Drug Administration (FDA) uses alfuzosin as an example of a drug having QT risk in humans that was not detected in nonclinical studies. FDA approval required a thorough clinical QT study (TCQS) that was weakly positive at high doses. The FDA has used the clinical/nonclinical discordance as a basis for mandatory TCQS, and this requirement has serious consequences for drug development. For this reason, we re-examined whether nonclinical signals of QT risk for alfuzosin were truly absent. Alfuzosin significantly prolonged action potential duration (APD)(60) in rabbit Purkinje fibers (p < 0.05) and QT in isolated rabbit hearts (p < 0.05) at the clinically relevant concentration of 300 nM. In man, the QT interval corrected with Fridericia's formula increased 7.7 ms, which exceeds the 5.0-ms threshold for a positive TCQS. Effects on hK(v)11.1, hK(v)4.3, and hK(v)7.1/hKCNE1 potassium currents and calcium current were not involved. At 300 nM, approximately 30x C(max), alfuzosin significantly increased whole-cell peak sodium (hNa(v)1.5) current (p < 0.05), increased the probability of late hNa(v)1.5 single-channel openings, and significantly shortened the slow time constant for recovery from inactivation. Alfuzosin also increased hNa(v)1.5 burst duration and number of openings per burst between 2- and 3-fold. Alfuzosin is a rare example of a non-antiarrhythmic drug that delays cardiac repolarization not by blocking hK(v)11.1 potassium current, but by increasing sodium current. Nonclinical studies clearly show that alfuzosin increases plateau potential and prolongs APD and QT, consistent with QT prolongation in man. The results challenge the FDA grounds for the absolute primacy of TCQS based on the claim of a false-negative, nonclinical study on alfuzosin.

ILSI–HESI cardiovascular safety subcommittee dataset: An analysis of the statistical properties of QT interval and rate-corrected QT interval (QTc)

INTRODUCTION

The Health and Environmental Sciences Institute of the International Life Sciences Institute (ILSI/HESI) Cardiovascular Safety Subcommittee outlined a set of in vivo telemetry studies to determine how well this preclinical model identified compounds known to cause torsades de pointes (TdP) and prolong QT interval in humans. In the original analysis of these data, QT, QTcB (Bazett model), QTcF (Fridericia model), and QTcQ (animal-specific model) were evaluated. We further evaluate the statistical properties of these measurements, using a method that can properly account for the sources of variability in the dataset.

METHODS

The ILSI/HESI telemetry studies were conducted as a double Latin square design where eight dogs each received a vehicle control and three dose levels of a compound on four separate dosing days. We statistically analyzed the QT/QTc intervals using a repeated measures analysis of covariance and evaluate the powers for QT, QTcF and QTcQ based on simulations.

RESULTS

The analyses for QTcF and QTcB intervals show that all six compounds which were known to cause TdP in humans were identified as positive and all six compounds known to be free of TdP events in their clinical use had no statistically significant treatment-related effects, while the analyses for QTcQ identified all positive compounds except pimozide. The power analysis shows that the method can detect a 7% increment of QT, a 5% increment of QTcF, and a 4% increment of QTcQ, with greater than 80% of power when n=8.

DISCUSSION

We describe a repeated measures procedure to perform statistical analysis of covariance on Latin square designs and show that it can be used to detect meaningful changes in the analysis of QT/QTc intervals.

Computer-based analysis of dynamic QT changes: toward high precision and individual rate correction

BACKGROUND

New strategies are needed to improve the results of automatic measurement of the various parts of the ECG signal and their dynamic changes.

METHODS

The EClysis software processes digitally-recorded ECGs from up to 12 leads at 500 Hz, using strictly defined algorithms to detect the PQRSTU points and to measure ECG intervals and amplitudes. Calculations are made on the averaged curve of each sampling period (beat group) or as means +/- SD for beat groups, after being analyzed at the individual beat level in each lead. Resulting data sets can be exported for further statistical analyses. Using QT and R-R measured on beat level, an individual correction for the R-R dependence can be performed.

RESULTS

EClysis assigns PQRSTU points and intervals in a sensitive and highly reproducible manner, with coefficients of variation in ECG intervals corresponding to ca. 2 ms in the simulated ECG. In the normal ECG, the CVs are 2% for QRS, 0.8% for QT, and almost 6% for PQ intervals. EClysis highlights the increase in QT intervals and the decrease of T-wave amplitudes during almokalant infusion versus placebo. Using the observed linear or exponential relationships to adjust QT for R-R dependence in healthy subjects, one can eliminate this dependence almost completely by individualized correction.

CONCLUSIONS

The EClysis system provides a precise and reproducible method to analyze ECGs.

Novel indices of ventricular repolarization to screen post myocardial infarction patients

We propose novel indices of ventricular repolarization intervals, the JTp/JT, Tpe/JTp and Tpe/JT ratios. These indices have been compared with the duration of the ventricular repolarization intervals and other ratios in 17 normal subjects and 17 patients with old myocardial infarction. In the intervals and other ratios, the best separation between groups is obtained with the Tpe/QTp and Tpe/QT ratios with 94% sensitivity and 82% specificity, the proposed ratios increased sensitivity to 100% and specificity to 94%. These indices should be further tested to determine their usefulness in discriminating between OMI patients with and without susceptibility to ventricular arrhythmias.

Effect of ciprofloxacin and levofloxacin on the QT interval: is this a significant "clinical" event?

BACKGROUND

The widespread use of the fluoroquinolones has raised the question of the cardiac safety of these medications. This widespread use of this class of antibiotics has displayed their safety profile, which is actually more favorable than many other drug classes. The cardiac toxicity issue at the center of this discussion is the prolongation of the QT interval leading to torsade de pointes. Ciprofloxacin and levofloxacin, two of the more commonly used fluoroquinolones, are considered less likely than other fluoroquinolones to prolong the QT interval. The authors set out to evaluate the effect on the QT interval of patients after administration of ciprofloxacin and levofloxacin.

METHODS

A prospective evaluation of 38 consecutive patients evaluated by the infectious disease service and receiving either ciprofloxacin or levofloxacin was undertaken. Twelve-lead electrocardiograms were obtained at baseline and at least 48 hours after the first dose of the antibiotic was administered. Both the longest QT interval and the mean QT interval were evaluated. To account for variations in heart rate, the corrected QT interval was calculated by using Bazett's formula (QTc = QT(square root of) R-R). Statistical analysis was undertaken to assess for the presence of a change after the administration of the antibiotic.

RESULTS

Thirty-eight patients (mean age, 65 +/- 19 years), 23 women and 15 men, were studied. There was a small but significant increase in the longest QTc intervals over baseline in patients receiving levofloxacin; there was no significant change in the mean QTc interval. However, one patient who received levofloxacin was, statistically, an outlier and, on retrospective analysis, had demonstrated severe electrolyte disturbances at the time of the study. When this patient was excluded, the increase in the longest QTc interval was not significant. Patients receiving ciprofloxacin did not demonstrate any significant change in the longest QTc interval or mean QTc interval.

CONCLUSIONS

Neither levofloxacin nor ciprofloxacin significantly prolonged the mean QTc interval over baseline. When electrolyte deficiencies in one of the patients evaluated were taken into account, this also held true for the longest QTc interval. There is, therefore, evidence that taking ciprofloxacin or levofloxacin, assuming that there are not any concurrent risk factors, will not cause a significant prolongation in the QT interval.

QTc interval in patients with changing edematous states: implications on interpreting repeat QTc interval measurements in patients with anasarca of varying etiology and those undergoing hemodialysis

Associations have been described among weight, amplitude of QRS complexes, and QRS duration (QRSd) in patients with anasarca (AN), and changes in the amplitude of the QRS complexes, QRSd, and QTc after hemodialysis (HD) and in patients with heart failure with associated peripheral edema congestive heart failure. The objective of this study was to evaluate the hypothesis that changes in QTc in patients with AN and after HD are at least partially apparent, due to changing edematous states, and not totally due to altered electrophysiology. QTc was measured in patients with AN on admission, at peak weight (N = 28), and at their subsequent lowest weight (N = 12), in 28 control patients without change in weight during hospitalization, and in one patient before and after 26 HD sessions. In the patients with AN, the QTc was 451 +/- 36 ms on admission and dropped to 423 +/- 46 ms at peak weight (P = 0.005). QTc was 421 +/- 44 ms at peak weight and raised to 434 +/- 30 at subsequent lowest weight (P = 0.32). In the controls, QTc on admission and at discharge were 435 +/- 34 and 428 +/- 23 ms, correspondingly (P = 0.18). QTc increased from 472 +/- 18 ms before to 489 +/- 36 ms after HD (P = 0.017). Alterations in QTc in AN, or HD suggest that the changes in the QTc may be partially only apparent, and due to the electrocardiogram machine-based measurement of the attenuated/augmented QRST complexes resulting from fluid shifts.

Assessment of ventricular repolarization alterations in subjects with early repolarization

BACKGROUND

Although the electrocardiographic (ECG) features of early repolarization (ER) have been studied extensively, no systematic quantification of ventricular repolarization in subjects with ER has been conducted so far.

METHODS

The objective of the present study was to evaluate ECG and spatial vectorcardiographic (VCG) descriptors of ventricular repolarization in ER subjects and to associate them with the respective indices of ventricular depolarization. A digital 12-lead surface ECG was obtained from 108 young, healthy men with ER and 108 age-matched healthy controls. The maximum Q-onset-T-end interval (QT maximum), the maximum Q-onset-T-peak interval (QTp maximum), the respective QT dispersion values (QT maximum-QT minimum), the rate-corrected QTC maximum and QTpC maximum, the QRS duration, and the VCG markers spatial T amplitude, spatial QRS amplitude and spatial QRS-T angle, were evaluated in ER subjects and controls.

RESULTS

QT maximum (P = 0.05) and QTp maximum (P = 0.003) were higher in ER subjects than in controls, while QTC maximum (P < 0.0001) and QTpC maximum (P = 0.002) were lower in ER subjects than in controls. The QRS duration (P = 0.013), as well as the spatial T amplitude, the spatial QRS amplitude, and the spatial QRS-T angle were higher in ER subjects than in controls (P < 0.0001). The spatial T amplitude was not associated with the indices of ventricular depolarization neither in ER subjects, nor in controls.

CONCLUSIONS

Ventricular repolarization, as well as depolarization, is altered in young, healthy males with ER compared to age-matched healthy controls. Ventricular depolarization and repolarization indices in ER subjects are not associated to each other.

Torsade de pointes: the clinical considerations

Torsade de pointes is a form of polymorphic ventricular tachycardia occurring in a setting of prolonged QT interval on surface electrocardiogram. Congenital causes of prolonged QT interval occur in individuals with genetic mutations in genes that control expression of potassium and sodium channels and acquired causes are numerous, predominantly drugs causing prolonged QT interval by blockade of potassium channels. Among the drugs, antiarrhythmic agents most notably quinidine, sotalol, dofetilide and ibutilide have the potential to induce the fatal torsade de pointes. Many non-antiarrhythmic drugs can also cause torsade de pointes. Although it is important to distinguish between the congenital and the acquired forms of long QT syndrome as the later can often be reversed by correction of the underlying disorder or discontinuation of the offending drug, both forms are not mutually exclusive. Clinical considerations and management of torsade de pointes are described.

Are QT measurements on body surface ECG indicative of ventricular refractory patterns?

OBJECTIVE

Increased dispersion (DISP) of refractoriness (ERP) facilitates the induction of malignant ventricular arrhythmias. Accordingly, QT DISP on surface ECG, supposedly reflecting ERP DISP, has been proposed as a noninvasive marker for risk stratification. However, a comparative analysis of local ERPs and QT measurements is not available so far.

METHODS AND RESULTS

In 19 healthy dogs, standard 12 lead surface ECGs were recorded to measure QT and RR intervals. Based on these measurements, corrected QT intervals (QTc, Bazett formula) and DISP (maximum difference) of both QT and QTc intervals (QT-DISP and QTc-DISP, respectively) were calculated. Subsequently, 60 custom-made needle electrodes (12 mm long, 4 bipolar electrodes per needle, interelectrode distance 2.5 mm) were inserted into the left (LV) and right ventricle (RV). At each bipole of 14 randomly selected needle electrodes (8 LV, 6 RV) local ERPs were determined (extrastimulus technique, basic cycle length 1000 ms). Interventricular DISP of ERP (LV-RV-DISP) was defined as the difference between the longest and shortest ERP within both ventricles. Respective values were calculated for each ventricle (LV-DISP; RV-DISP). Scatter plots and correlation analysis did not reveal a significant correlation between QT, QTc, QT-DISP, QTc-DISP and any of the ERP measurements or calculations. Although not statistically significant, the closest correlation was found between QTc and mean ERP and between QTc-DISP and LV-RV-DISP.

CONCLUSION

QT measurements on surface ECG are poorly correlated with local ERPs. If anything, QT- or QTc-DISP might provide a rough estimate of interventricular, that is, global DISP of ERP. Local or even intraventricular DISP of ERP is definitely not reflected by these QT measurements.

QRS duration and QT interval predict mortality in hypertensive patients with left ventricular hypertrophy: the Losartan Intervention for Endpoint Reduction in Hypertension Study

Left ventricular hypertrophy is a risk factor for cardiovascular mortality, including sudden cardiac death. Experimentally, left ventricular hypertrophy delays ventricular conduction and prolongs action potential duration. Electrocardiographic QRS duration and QT interval measures reflect these changes, but whether these measures can further stratify risk in patients with electrocardiographic left ventricular hypertrophy is unknown. We measured the QRS duration and QT intervals from the baseline 12-lead electrocardiograms in the Losartan Intervention For Endpoint Reduction in Hypertension (LIFE) study, which included hypertensive patients with electrocardiographic evidence of left ventricular hypertrophy randomized to either losartan-based or atenolol-based treatment to lower blood pressure. In the present study, we related study baseline electrocardiographic measures to cardiovascular and all-cause mortality. There were 5429 patients (male 45.8%; mean age 66+/-7 years) included in the present analyses. After a mean follow-up of 4.9+/-0.8 years, there were 417 deaths from all causes, including 214 cardiovascular deaths. In separate univariate Cox regression analyses, QRS duration and several QT measures were significant predictors of cardiovascular mortality and all-cause mortality. However, in multivariate Cox analyses including all electrocardiographic measures and adjusting for other risk factors as well as treatment strategy, only QRS duration and maximum rate-adjusted QT(apex) interval remained as significant independent predictors of cardiovascular (P=0.022 and P=0.037, respectively) and all-cause mortality (P=0.038 and P=0.002, respectively). In conclusion, in a hypertensive risk population identified by electrocardiographic left ventricular hypertrophy, increased QRS duration and maximum QT(apex) interval can further stratify mortality risk even in the setting of effective blood pressure-lowering treatment.

Relation of QT interval and QT dispersion to regression of echocardiographic and electrocardiographic left ventricular hypertrophy in hypertensive patients: the Losartan Intervention For Endpoint Reduction (LIFE) study

BACKGROUND

In hypertensive patients, both echocardiographic and electrocardiographic left ventricular hypertrophy (LVH) increase the risk of sudden death, possibly in part because of LVH-induced proarrhythmic repolarization changes. Experimentally, regression of LVH normalizes ventricular electrophysiology.

METHODS

To assess the relation of regression of LVH to changes in electrocardiographic measures of ventricular repolarization, we studied 317 hypertensive (61.2% men, mean age 65 +/- 7 years) participants in the Losartan Intervention For Endpoint Reduction (LIFE) study with electrocardiographic evidence of LVH, at study baseline, and after 1 year of blood pressure-lowering treatment with losartan or atenolol and hydrochlorothiatzide as the first adjunct therapy if needed to reach target blood pressure of 140/90 mm Hg. As indexes of LVH, we used echocardiographically determined LV mass as well as the Sokolow-Lyon and Cornell voltages from the electrocardiogram. QT interval duration and QT dispersion from the 12-lead electrocardiogram were used as ventricular repolarization measures.

RESULTS

By using tertiles of LV mass change and adjusting for the difference in treatment (losartan or atenolol), shortening of the rate-adjusted QT intervals as well as reduction in QT(apex) dispersion were observed in the tertile showing the greatest decrease in LV mass but not in the tertile without substantial changes in LV mass despite a significant reduction in blood pressure. Similar results were obtained with the use of Sokolow-Lyon and Cornell voltage change tertiles.

CONCLUSIONS

In hypertensive patients with electrocardiographic evidence of LVH, regression of echocardiographically determined LV mass and electrocardiographic indexes of LVH may partially reverse the LVH-induced proarrhythmic repolarization changes. This may have a beneficial impact on the increased incidence of sudden death in these patients.

Time and frequency analysis of beat-to-beat R-T interval variability in patients with ischaemic left ventricular dysfunction providing evidence for non-neural control of ventricular repolarisation

BACKGROUND

Determinants of temporal lability in ventricular repolarisation are not fully recognised. We aimed to analyse the sources of RT variability by comparing normal subjects and patients after myocardial infarction (MI) with either depressed or preserved left ventricular (LV) function.

METHODS

One hundred and nine patients (27 women, 82 men, aged 51 +/- 9 years) were divided into three groups: 24 patients (pts) with an uncomplicated angiographically proven coronary heart disease (CHD-group), 59 post-MI pts with preserved LV function (LVEF > 40%, PMI-N-group) and 26 post-MI pts with depressed LV function (LVEF < 40%, PMI-L-group). An ECG signal of low-noise 512 heartbeats was recorded using a computer-assisted amplifier (16 bit, 2 kHz). The onset and offset of the R-wave and T-wave were determined automatically. The magnitude of R-R and R-T variability was measured as the standard deviation of all intervals (SD-RR and SD-RT, ms, respectively). Their relationship was quantified by the correlation coefficient r(RT/RR). Power spectral density of RR or RT variability was estimated with the FFT (Welch's averaged periodogram, Hanning window) and frequency relation was quantified using a squared coherence spectrum (SCS). For all spectral and cross-spectral measurements two frequency ranges were considered: high (0.15-0.50 Hz, HF) and low (0.04-0.15 Hz, LF). Spectral power and SCS of RR and RT variability for both ranges (HF(RR), LF(RR), HF(RT), LF(RT), SCS(HF), SCS(LF)), and the ratios LF/HF(RR) and LF/HF(RT) were drawn for comparisons. The central frequency of HF(RR) was considered as the frequency of respiration (f(resp), Hz).

RESULTS

In the PMI-L group the SD-RT was significantly greater compared to the remaining groups and accounted for almost 10% of the SDRR. Also, the coefficient r(RT/RR) was weakest in this group. The spectral indices of RR variability were similar in all groups, while the greatest value of the HFRT was observed in the PMI-L group. The SCS(LF) was insignificant in this group, contrary to the CHD and PMI-N groups. Additionally, there were significant negative relationships between f(resp) and spectral indices of RT variability in PMI-patients with depressed LV function.

CONCLUSION

A greater beat-to-beat variation in RT interval duration along with increased power of its HF component indicates an important role of respiration in ventricular repolarisation control, while reduced time- and frequency RT-RR relationships seem to relate to an impaired process of ventricular duration adaptation.

Prevalence of increased QT interval duration and dispersion in type 2 diabetic patients and its relationship with coronary heart disease: a population-based cohort

OBJECTIVE

To evaluate the prevalence of prolonged QT interval and dispersion in a population-based cohort of type 2 diabetic patients and their relationship with clinical and metabolic variables.

DESIGN

Cross-sectional population-based cohort.

SETTING

Diabetes clinics and general practitioners in Casale Monferrato (Northern Italy).

SUBJECTS

A total of 1357 patients with known type 2 diabetes (70% of the cohort).

MAIN OUTCOMES MEASURES

Albumin excretion rate and coronary heart disease (CHD); a standard supine 12-lead electrocardiogram (ECG) was recorded and coded according to the Minnesota code criteria. QT interval corrected for heart rate (QTc) > 0.44 s and QTc dispersion > 0.080 s were considered abnormally prolonged.

RESULTS

Prevalence of increased QTc duration and QTc dispersion were 25.8% (95% CI 23.5-28.3) and 33.1% (95% CI 30.6-35.7), with no sex differences. No metabolic differences were found, apart from fibrinogen and creatinine levels, which were higher in patients with increased QTc dispersion. Patients with CHD had higher mean adjusted values of QTc and QTc dispersion, whereas no association was found with albumin excretion rate (AER) and diabetes treatment. QTc duration and QTc dispersion were significantly correlated (0.17, P < 0.001). In multiple regression analysis, only CHD was independently associated with QTc, after adjustment for age and sex (beta=0.010, P < 0.001, R2=2.5%); as regards QTc dispersion, a similar association with CHD was found (beta=0.20, P < 0.001, R2=4.8%).

CONCLUSIONS

This population-based study shows a considerably high prevalence of increased QTc and QTc dispersion in type 2 diabetic patients and their association with CHD. These findings have both epidemiological and clinical relevance, as they might be implicated in the excess mortality risk of type 2 diabetic patients.

The effects of cigarette smoking on the heterogeneity of ventricular repolarization

BACKGROUND

Although the circulatory effects of cigarette smoking have been studied extensively, its impact on ventricular repolarization has not been adequately evaluated.

METHODS

The goal of our study was to determine whether cigarette smoking influences the spatial and temporal heterogeneity of ventricular repolarization in a population of young, healthy, male subjects. A digital 12-lead surface electrocardiogram was obtained from 1394 men recruited from the Hellenic Air Force and classified as smokers and nonsmokers. The maximum, minimum, and median QT intervals, QT dispersion (QT maximum - QT minimum), the rate-corrected maximum and median QT intervals, the slopes of the QT maximum/RR and QT median/RR regression equations, and the vectorcardiographic markers spatial T amplitude and spatial QRS-T angle were evaluated in the 2 groups.

RESULTS

Heart rate was significantly higher (P <.001) in smokers (n =691) compared with nonsmokers (n = 703). QT maximum, QT minimum, and QT median were significantly lower (P <.001), whereas the rate-corrected QT maximum (P =.04) and QT median (P =.06) were marginally higher in smokers than in nonsmokers. The spatial T amplitude was lower (P =.002), whereas the spatial QRS-T angle was higher (P =.01) in smokers compared with nonsmokers. Neither QT dispersion nor the slopes of the QT/RR and the spatial descriptors/RR regression equations differed between smokers and nonsmokers.

CONCLUSIONS

Ventricular repolarization is altered in young male cigarette smokers. The differences in the heterogeneity of ventricular repolarization between smokers and nonsmokers are mainly due to heart rate differences between the 2 study groups.

Relation of QT interval and QT dispersion to echocardiographic left ventricular hypertrophy and geometric pattern in hypertensive patients. The LIFE study. The Losartan Intervention For Endpoint Reduction

OBJECTIVE

In hypertensive patients, left ventricular hypertrophy (LVH) predicts increased mortality, in part due to an increased incidence of sudden death. Repolarization-related arrhythmogenesis may be an important mechanism of sudden death in hypertensive patients with LVH. Increased QT interval and QT dispersion are electrocardiographic (ECG) measures of ventricular repolarization, and also risk markers for ventricular tachyarrhythmias. We assessed the relation of QT intervals and QT dispersion to echocardiographically determined left ventricular (LV) mass and geometry in a large population of hypertensive patients with ECG evidence of LVH.

METHODS

QT intervals and QT dispersion were determined from baseline 12-lead ECGs in 577 (57% male; mean age 65 +/- 7 years) participants in the LIFE study. LV mass index (LVMI) and geometric pattern were determined by echocardiography and QT interval duration and QT dispersion were assessed in relation to gender-specific LVMI quartiles.

RESULTS

In both genders, increasing LVMI was associated with longer rate-adjusted QT intervals. QT dispersion measures showed a weaker association with LVMI quartiles. Both concentric and eccentric LVH were associated with increased QT interval duration and QT dispersion. These relations remained significant after controlling for relevant clinical variables.

CONCLUSIONS

In hypertensive patients with ECG evidence of LVH, increased LVMI and LVH are associated with a prolonged QT interval and increased QT dispersion. These findings suggest that an increased vulnerability to repolarization-related ventricular arrhythmias might in part explain the increased risk of sudden death in hypertensive patients with increased LV mass.

Transient QT interval prolongation with inverted T waves indicates myocardial salvage on dual radionuclide single-photon emission computed tomography in acute anterior myocardial infarction

In patients with acute myocardial infarction (AMI), transient QT interval prolongation with a prominent negative T wave is frequently observed in cases of early spontaneous reperfusion and often indicates a good prognosis. Additionally, in nuclear cardiac imaging, technetium-99m/thallium-201 overlap on dual single-photon emission computed tomography (dual SPECT) in AMI patients indicates the presence of viable myocardium and early recanalization. To elucidate the clinical significance of this transient QT interval prolongation, 34 patients (64 +/- 8 years) admitted within 24h of the onset of anterior AMI were enrolled and classified into 2 groups according to the presence (group A, n=24) or absence (group B, n=10) of scintigraphic overlap on simultaneous dual SPECT imaging. The maximal QTc interval was 0.59 +/- 0.06 s in group A and 0.52 +/- 0.06 s in group B (p<0.01). The peak creatine kinase was lower in group A (2650 +/- 2160 IU/L) than in group B (3490 +/- 2060 IU/L). The left ventricular ejection fraction (LVEF) at discharge was 62 +/- 11% in group A and 49 +/- 14% in group B (p<0.01). The scintigraphic overlap group had a smaller infarct and better LVEF, which suggests that the QT interval prolongation that appears transiently in the acute phase of AMI indicates scintigraphically the presence of salvaged myocardium.

Cardiovascular disease in chinese type 2 diabetic women is associated with a prolonged QTc interval

We studied the relationships between QT interval and cardiovascular disease status in 192 Chinese type 2 diabetic patients. Of these 192 subjects, 132 (68.8%) were women and 60 (31.2%) were men. The mean age (+/-S.D.) was 56.6+/-12.9 years (range: 23-84, median: 58.0 years). Women had longer QTc interval compared to men (0.402+/-0.030 s vs. 0.387+/-0.026 s, P<0.01). Of the 192 subjects, 18 women and two men had prolonged QTc interval (QTc >0.433 s). Women with prolonged QTc interval have a 2.8-fold greater rate of cardiovascular disease as compared to those with normal QTc interval (38.9% vs. 14.0%, P<0.05). Using multiple regression analysis (stepwise) to assess the relationship with QTc interval with age, sex, body mass index, waist-hip ratio, blood pressure, fasting plasma glucose, glycated haemoglobin, lipid profiles, smoking and duration of diabetes as independent variables (R(2)=0.146, F=8.88, P<0.001), systolic blood pressure (beta=0.198, P=0.017), age (beta=0. 189, P=0.023) and female gender (beta=0.157, P=0.037) were found to be independently associated with QTc interval. In conclusion, we have shown a significant association between prolonged QTc interval, ischaemic heart disease and cardiovascular disease in Chinese type 2 diabetic women. Age, systolic blood pressure and female gender are independently correlated to QTc interval.

QT interval lengthening in cardiac disease relates more to left ventricular systolic dysfunction than to autonomic function

BACKGROUND

There are multiple influences on the QTc interval, including the autonomic nervous system. Which influence is the principal determinant of the variation in QTc interval between different cardiac diseases is not yet clear, though some studies have suggested that the QTc interval primarily reflects sympatho-vagal balance. This study investigated this claim further.

AIM

To determine if autonomic tone was the prime determinant of variation in the QTc interval between subjects with different cardiac diseases.

METHODS

Subjects with different cardiac diseases were studied, QTc interval determined and correlated with three different measures of the autonomic nervous system, that of baroreflex sensitivity, catecholamine levels (epinephrine and norepinephrine) and sympatho-vagal balance as determined by power spectrum analysis.

RESULTS

47 subjects were studied, comprising 17 subjects with heart failure, 14 subjects with left ventricular hypertrophy and 16 control subjects. For the group as a whole there was no relationship between QTc interval and any measure of the autonomic nervous system function, but there was a reasonable relationship between fractional shortening and QTc interval (r=0.47, P<0.003). For subjects with an echocardiographic fractional shortenings less than 0.35 (which correlates with an ejection fraction of <50%), a strong relationship between fractional shortening and QTc interval remained (r=0.57, P<0.002), but in addition a relationship between QTc interval and catecholamine levels developed (for epinephrine: r=0.67, P<0.002; and for norepinephrine: r=0.62, P<0.005). Multiple regression analysis showed that fractional shortening and epinephrine levels were independently related to QTc interval.

CONCLUSION

In subjects with a variety of cardiac diseases, the prime determinant of QTc interval is left ventricular systolic performance rather than the autonomic nervous system, though in subjects with low normal and less fractional shortenings catecholamine levels are independently related to QTc interval.

QTc interval and scintigraphically assessed myocardial perfusion in newly diagnosed and long-term type 1 diabetes mellitus

In diabetes mellitus, heart rate corrected QT interval (QTc) has been suggested to be related to ischemic heart disease and increased risk of sudden cardiac death. The aim of the study was to analyze the length of QTc interval with regard to global and regional myocardial perfusion in type 1 diabetic patients. Myocardial perfusion was investigated in 20 newly diagnosed and 40 long-term type 1 diabetic patients without clinical evidence for coronary artery disease by means of Tc-99-methoxyisobutylisonitrile (Tc-99m-MIBI)-scintigraphy (myocardial uptake (MU) score: 1-6). Five consecutive RR and QT intervals of resting electrocardiogram (ECG) tracing were measured and corrected for the previous cycle length. ECG-based cardiac autonomic neuropathy (CAN) was assessed with five cardiac reflex tests. Length of QTc interval was 423+/-29 ms in newly diagnosed and 433+/-26 ms in long-term type 1 diabetic patients. Nine (45%) newly diagnosed and 18 (45%) long-term diabetic patients demonstrated a prolonged QTc interval (>440 ms). Both newly diagnosed and long-term diabetic patients did not display significant global or regional myocardial perfusion defects (mean MU scores<3). In newly diagnosed diabetic patients, the length of QTc interval was related to global, posterior and septal Tc-99m-MIBI uptake (p<0.05, respectively). In long-term diabetic patients, the length of QTc interval was associated with apical Tc-99m-MIBI uptake (p<0.05). Two (10%) newly diagnosed and 19 (48%) long-term type 1 diabetic patients demonstrated ECG-based CAN. In long-term type 1 diabetic patients, global myocardial Tc-99m-MIBI uptake did not differ significantly between patients with and without CAN. QTc interval was not significantly different between diabetic patients with and without ECG-based CAN (433+/-19 ms vs. 428+/-17 ms). Long-term diabetic patients, of whom 10 (25%) patients had microalbuminuria and seven (18%) patients had macroalbuminuria, demonstrated an association between QTc interval and albuminuria (p<0.05). The results somewhat suggest an association between QTc interval and vascular factors in type 1 diabetes mellitus. Future investigations are required to analyze the role of QTc interval in the pathogenesis of abnormalities of myocardial perfusion.

Electrophysiological basis of QT dispersion measurements

Dispersion of ventricular repolarization is a now widely used term describing nonhomogeneous recovery of excitability or heterogeneity of ventricular repolarization. It is usually expressed as the difference or the range of various repolarization measurements obtained from a heart. Experimentally, an increased dispersion of ventricular repolarization was found to be tightly associated with increased propensity for ventricular arrhythmias, and, therefore, is considered an important arrhythmogenic mechanism. Noninvasively, this arrhythmogenic substrate was approached using multilead body surface potential mapping, but also QT interval dispersion (QTd) and similar electrocardiogram (ECG) variables from the 12-lead surface ECG. Standard QTd from the ECG correlates significantly with dispersion of repolarization measured from the myocardium. A causal relationship is, however, still unclear, and there are 2 main hypotheses to explain the electrophysiological basis of QTd. The local hypothesis explaining QTd with spatial differences in action potential duration mirrored in the various QT intervals competes with the global hypothesis explaining the variation in surface ECG measurements with different projections of a common T-wave vector. Notwithstanding the final explanation for QTd, and particularly for technical reasons, new markers like advanced T-wave loop variables may best reflect the abnormal repolarization substrate on the surface ECG.

Assessment of QT interval and QT dispersion for prediction of all-cause and cardiovascular mortality in American Indians: The Strong Heart Study

BACKGROUND

Both a prolonged QT interval and increased QT interval dispersion (QTD) have been proposed as surface ECG markers of vulnerability to ventricular arrhythmias and potential predictors of mortality.

METHODS AND RESULTS

The predictive values of QT prolongation and QTD were assessed in 1839 participants in the Strong Heart Study, a prospective study of cardiovascular disease in American Indians. ECGs were acquired at 250 Hz; QT intervals were measured by computer in all 12 leads and corrected for heart rate (QTc) by use of Bazett's formula. QTD was calculated as the difference between the maximum and minimum QTc. After a mean follow-up of 3.7+/-0.9 years, there were 188 deaths from all causes, including 55 cardiovascular deaths. In univariate Cox analyses, prolonged QTc and increased QTD were significant predictors of all-cause mortality (chi(2)=53.0, P<0.0001; chi(2)=11.3, P=0.0008) and cardiovascular mortality (chi(2)=14.7, P=0.0001; chi(2)=26.5, P<0.0001). In multivariate Cox regression analyses controlling for risk factors, QTc remained a strong predictor of all-cause mortality (chi(2)=16.5, P<0.0001) and a weaker predictor of cardiovascular mortality (chi(2)=5.8, P=0.016); QTD remained a significant predictor of cardiovascular mortality only (chi(2)=12.5, P=0.0004).

CONCLUSIONS

These findings support the value of computerized measurements of QTc and QTD in noninvasive risk stratification and suggest that these surface ECG variables may reflect different underlying abnormalities of ventricular repolarization.

Mechanisms of the spatial distribution of QT intervals on the epicardial and body surfaces

INTRODUCTION

The role of QT dispersion as a predictor of arrhythmia vulnerability has not been consistently confirmed in the literature. Therefore, it is important to identify the electrophysiologic mechanisms that affect QT duration and distribution. We compared the spatial distributions of QT intervals (QTI) with potential distributions on cardiac and body surfaces and with recovery times on the cardiac surface. We hypothesized that the measure of QTI is affected by the presence of the zero potential line in the potential distribution, as well as the sequence of recovery. We also investigated use of the STT area as a possible indicator of recovery times on the cardiac surface.

METHODS AND RESULTS

High-resolution spatial distributions of QTI and potentials were determined on the body surface of human subjects and on the surface of a torso-shaped tank containing an isolated canine heart. Additionally, spatial distributions of QTI, recovery times, and STT areas were determined on the surface of exposed canine hearts. Unipolar electrograms were recorded during atrial and ventricular pacing for normal hearts and cases of myocardial infarction. Regions of shortest QTI always coincided with the location of the zero potential line on the cardiac and body surfaces. On the cardiac surface, in regions away from the zero line, similarities were observed between the patterns of QTI and the sequence of recovery. STT areas and recovery times were highly correlated on the cardiac surface.

CONCLUSION

QTI is not a robust index of local recovery time on the cardiac surface. QTI distributions were affected by the position of the zero potential line, which is unrelated to local recovery times. However, similarities in the patterns of QTI and recovery times in some regions may help explain the frequently reported predictive value of QT dispersion. Preliminary results indicate STT area may be a better index of recovery time and recovery time dispersion on the epicardium than QTI.

QT dispersion. Is it an independent risk factor for in-hospital mortality in patients with intracerebral hemorrhage?

Electrocardiographic repolarization changes, comprising QT prolongation, are most commonly seen after intracerebral hemorrhage. In this study in patients with intracerebral hemorrhage (ICH), QT dispersion and its daily changes were examined and the relation between QT dispersion and in-hospital mortality assessed. In 28 patients with intracerebral hemorrhage, diagnosed by computerized tomographic scanning, an ECG was obtained on the day of admission to hospital and then serial ECGs were recorded on the following four consecutive days. Blood electrolytes (K, Ca, Mg) were also analysed. The patients with intracerebral hemorrhage were followed until discharge or death (mean 14 +/- 4 days). QT, QT peak, and QT-QT peak dispersion were measured on simultaneous twelve lead electrocardiograms. Also, in 29 healthy subjects as a control group, five consecutive day serial electrocardiograms were recorded. There were no statistically significant differences between the study and control groups in terms of gender and age. During the five days, QT, QT peak, and QT-QTpeak dispersion values were significantly higher in patients with intracerebral hemorrhage than in the control subjects (p < 0.001). There were no statistically significant differences in two patient groups with intracerebral hemorrhage who died and who were discharged in terms of mean QT, QTpeak, and QT-QTpeak dispersion values. In conclusion, QT, QT peak, and QT-QTpeak dispersion values were significantly greater in patients with intracerebral hemorrhage than in the control subjects, but QT, QT peak, and QT-QT peak dispersions were not independent risk factors for in-hospital mortality in patients with intracerebral hemorrhage.

Effects of heart rate on vulnerability to fibrillation in a computer model

Effects of heart rate on refractory period (RP) duration and disparity have opposing actions on vulnerability to fibrillation. Both bradycardia and tachycardia have been reported to increase vulnerability to fibrillation, and the role of their effects on RP duration and disparity in producing that effect is uncertain. That role has been investigated with a computer model of propagated excitation having nonuniform, cycle length-dependent refractoriness and slow propagation during incomplete recovery of excitability. Vulnerability was assessed as fibrillation threshold (FT), defined as the duration of train stimulation required to initiate simulated fibrillation. When measured as a function of train onset time during a cycle, FT initially decreased to a minimum and then increased to the original level. Slower rates shifted that curve upward and to the right, so that the FT was higher during early portions of the cycle but lower in later portions. Longer mean duration of RPs increased FT during all portions of the cycle, increased the difference of FT at various rates during early portions of the cycle, and decreased differences later in the cycle. Greater RP range reduced the FT and decreased the difference of FT with varied rate in early portions of the cycle, while increasing the difference in later portions. Accelerating rate had additional effects on FT-related to nonuniform propagation of responses prior to train stimulation. The findings defined mechanisms based on established effects of rate on RP, by which either tachycardia or bradycardia could increase vulnerability to fibrillation, and demonstrated the effects of RP range and duration on the mechanisms.