Differential response of QTU interval to exercise, isoproterenol, and atrial pacing in patients with congenital long QT syndrome

The Effects of Cycle Ergometer Versus Treadmill Exercise Stress Testing on QTc Interval Prolongation in Patients With Long QT Syndrome: A Systematic Review and Meta-analysis

OBJECTIVE

The safest and most effective exercise stress tests (EST) modalities for long QT syndrome (LQTS) are currently unknown. The main objective was to explore the effects of EST on the corrected QT interval (QTc) in patients with LQTS, and to compare the effects of different EST modalities (cycle ergometer vs treadmill).

DATA SOURCES

Systematic searches were performed in September 2022 in accordance with the PRISMA statement through PubMed, Medline, EBM Reviews, Embase, and Web of Science.

MAIN RESULTS

A total of 1728 patients with LQTS, whether congenital or acquired, without any age restrictions (pediatric age ≤18 years and adult age >19 years), and 2437 control subjects were included in the 49 studies. The QT interval data were available for 15 studies. Our analyses showed that the QT interval prolonged in a similar manner using either a cycle ergometer or a treadmill (standardized mean difference [SMD] = 1.89 [95% CI, 1.07-2.71] vs SMD = 1.46 [95% CI, 0.78-2.14], respectively). Therefore, it seems that either modality may be used to evaluate patients with LQTS.

CONCLUSIONS

The methodology for the measurement of the QT interval was very heterogeneous between studies, which inevitably influenced the quality of the analyses. Hence, researchers should proceed with caution when exploring and interpreting data in the field of exercise and LQTS.

Spatiotemporal repolarization dispersion before and after exercise in patients with long QT syndrome type 1 versus controls: probing into the arrhythmia substrate

Congenital long QT syndrome (LQTS) carries an increased risk for syncope and sudden death. QT prolongation promotes ventricular extrasystoles, which, in the presence of an arrhythmia substrate, might trigger ventricular tachycardia degenerating into fibrillation. Increased electrical heterogeneity (dispersion) is the suggested arrhythmia substrate in LQTS. In the most common subtype LQT1, physical exercise predisposes for arrhythmia and spatiotemporal dispersion was therefore studied in this context. Thirty-seven patients (57% on β-blockers) and 37 healthy controls (mean age, 31 vs. 35; range, 6-68 vs. 6-72 yr) performed an exercise test. Frank vectorcardiography was used to assess spatiotemporal dispersion as Tampl, Tarea, the ventricular gradient (VG), and the Tpeak-end interval from 10-s signal averages before and 7 ± 2 min after exercise; during exercise too much signal disturbance excluded analysis. Baseline and maximum heart rates as well as estimated exercise intensity were similar, but heart rate recovery was slower in patients. At baseline, QT and heart rate-corrected QT (QTcB) were significantly longer in patients (as expected), whereas dispersion parameters were numerically larger in controls. After exercise, QTpeakcB and Tpeak-endcB increased significantly more in patients (18 ± 23 vs. 7 ± 10 ms and 12 ± 17 vs. 2 ± 6 ms; P < 0.001 and P < 0.01). There was, however, no difference in the change in Tampl, Tarea, and VG between groups. In conclusion, although temporal dispersion of repolarization increased significantly more after exercise in patients with LQT1, there were no signs of exercise-induced increase in global dispersion of action potential duration and morphology. The arrhythmia substrate/mechanism in LQT1 warrants further study.NEW & NOTEWORTHY Physical activity increases the risk for life-threatening arrhythmias in LQTS type 1 (LQT1). The arrhythmia substrate is presumably altered electrical heterogeneity (a.k.a. dispersion). Spatiotemporal dispersion parameters were therefore compared before and after exercise in patients versus healthy controls using Frank vectorcardiography, a novelty. Physical exercise prolonged the time between the earliest and latest complete repolarization in patients versus controls, but did not increase parameters reflecting global dispersion of action potential duration and morphology, another novelty.

Provocation testing in congenital long QT syndrome: A practical guide

Congenital long QT syndrome (LQTS) is a hereditary cardiac channelopathy with an estimated prevalence of 1 in 2500. A prolonged resting QT interval corrected for heart rate (QTc interval) remains a key diagnostic component; however, the QTc value may be normal in up to 40% of patients with genotype-positive LQTS and borderline in a further 30%. Provocation of QTc prolongation and T-wave changes may be pivotal to unmasking the diagnosis and useful in predicting genotype. LQTS provocation testing involves assessment of repolarization during and after exercise, in response to changes in heart rate or autonomic tone, with patients with LQTS exhibiting a maladaptive repolarization response. We review the utility and strengths and limitations of 4 forms of provocation testing-stand-up test, exercise stress test, epinephrine challenge, and mental stress test-in diagnosing LQTS and provide some practical guidance for performing provocation testing. Ultimately, exercise testing, when feasible, is the most useful form of provocation testing when considering diagnostic sensitivity and specificity.

The influence of exercise and postural changes on ventricular repolarization in the long QT syndrome: a systematic scoping review

Current exercise recommendations make it difficult for long QT syndrome (LQTS) patients to adopt a physically active and/or athletic lifestyle. The purpose of this review is to summarize the current evidence, identify knowledge gaps, and discuss research perspectives in the field of exercise and LQTS. The first aim is to document the influence of exercise training, exercise stress, and postural change interventions on ventricular repolarization in LQTS patients, while the second aim is to describe electrophysiological measurements used to study the above. Studies examining the effects of exercise on congenital or acquired LQTS in human subjects of all ages were included. Systematic searches were performed on 1 October 2021, through PubMed (NLM), Ovid Medline, Ovid All EBM Reviews, Ovid Embase, and ISI Web of Science, and limited to articles written in English or French. A total of 1986 LQTS patients and 2560 controls were included in the 49 studies. Studies were mainly case-control studies (n = 41) and examined exercise stress and/or postural change interventions (n = 48). One study used a 3-month exercise training program. Results suggest that LQTS patients have subtype-specific repolarization responses to sympathetic stress. Measurement methods and quality were found to be very heterogeneous, which makes inter-study comparisons difficult. In the absence of randomized controlled trials, the current recommendations may have long-term risks for LQTS patients who are discouraged from performing physical activity, rendering its associated health benefits out of range. Future research should focus on discovering the most appropriate levels of exercise training that promote ventricular repolarization normalization in LQTS.

Utility of Provocative Testing in the Diagnosis and Genotyping of Congenital Long QT Syndrome: A Systematic Review and Meta-Analysis

Background Diagnosis is particularly challenging in concealed or asymptomatic long QT syndrome (LQTS). Provocative testing, unmasking the characterization of LQTS, is a promising alternative method for the diagnosis of LQTS, but without uniform standards. Methods and Results A comprehensive search was conducted in PubMed, Embase, and the Cochrane Library through October 14, 2021. The fixed effects model was used to assess the effect of the provocative testing on QTc interval. A total of 22 studies with 1137 patients with LQTS were included. At baseline, QTc interval was 40 ms longer in patients with LQTS than in controls (mean difference [MD], 40.54 [95% CI, 37.43-43.65]; P<0.001). Compared with the control group, patients with LQTS had 28 ms longer ΔQTc upon standing (MD, 28.82 [95% CI, 23.05-34.58]; P<0.001), nearly 30 ms longer both at peak exercise (MD, 27.31 [95% CI, 21.51-33.11]; P<0.001) and recovery 4 to 5 minutes (MD, 29.85 [95% CI, 24.36-35.35]; P<0.001). With epinephrine infusion, QTc interval was prolonged both in controls and patients with QTS, most obviously in LQT1 (MD, 68.26 [95% CI, 58.91-77.60]; P<0.001) and LQT2 (MD, 60.17 [95% CI, 50.18-70.16]; P<0.001). Subgroup analysis showed QTc interval response to abrupt stand testing and exercise testing varied between LQT1, LQT2, and LQT3, named Type Ⅰ, Type Ⅱ, and Type Ⅲ. Conclusions QTc trend Type Ⅰ and Type Ⅲ during abrupt stand testing and exercise testing can be used to propose a prospective evaluation of LQT1 and LQT3, respectively. Type Ⅱ QTc trend combined epinephrine infusion testing could distinguish LQT2 from control. A preliminary diagnostic workflow was proposed but deserves further evaluation.

An explainable algorithm for detecting drug-induced QT-prolongation at risk of torsades de pointes (TdP) regardless of heart rate and T-wave morphology

Torsade de points (TdP), a life-threatening arrhythmia that can increase the risk of sudden cardiac death, is associated with drug-induced QT-interval prolongation on the electrocardiogram (ECG). While many modern ECG machines provide automated measurements of the QT-interval, these automated QT values are usually correct only for a noise-free normal sinus rhythm, in which the T-wave morphology is well defined. As QT-prolonging drugs often affect the morphology of the T-wave, automated QT measurements taken under these circumstances are easily invalidated. An additional challenge is that the QT-value at risk of TdP varies with heart rate, with the slower the heart rate, the greater the risk of TdP. This paper presents an explainable algorithm that uses an understanding of human visual perception and expert ECG interpretation to automate the detection of QT-prolongation at risk of TdP regardless of heart rate and T-wave morphology. It was tested on a large number of ECGs (n=5050) with variable QT-intervals at varying heart rates, acquired from a clinical trial that assessed the effect of four known QT-prolonging drugs versus placebo on healthy subjects. The algorithm yielded a balanced accuracy of 0.97, sensitivity of 0.94, specificity of 0.99, F1-score of 0.88, ROC (AUC) of 0.98, precision-recall (AUC) of 0.88, and Matthews correlation coefficient (MCC) of 0.88. The results indicate that a prolonged ventricular repolarisation area can be a significant risk predictor of TdP, and detection of this is potentially easier and more reliable to automate than measuring the QT-interval distance directly. The proposed algorithm can be visualised using pseudo-colour on the ECG trace, thus intuitively 'explaining' how its decision was made, which results of a focus group show may help people to self-monitor QT-prolongation, as well as ensuring clinicians can validate its results.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Dental treatment of a patient with long QT syndrome under moderate sedation with target-controlled infusion of propofol

Long QT syndrome (LQTs) is a rare congenital disorder of the heart's electrical activity. Patients with LQTs are at increased risk of developing fatal ventricular arrhythmias. Elevated levels of sympathetic stimulation can exacerbate this risk. Successful behavior management is indispensable in the treatment of patients with LQTs. However, many drugs involved in pharmacologic behavior management are known to adversely affect the QT interval. Therefore, careful selection of a sedative drug is essential in avoiding such incidences. A 10-year-old boy with a known diagnosis of LQTs required restorative treatment due to dental caries at the permanent molar. He required sedation since treatment was painful and dental phobia can trigger sympathetic stimulation, creating a dangerous situation for patients with LQTs. Therefore, the treatment was performed over two sessions under moderate sedation involving propofol combined with nitrous oxide. Restorative treatment was successful without any complications under sedation with a target-controlled infusion (TCI) of propofol. There was no significant QT prolongation during pulpal treatment. Propofol TCI may be a good candidate for sedation in patients with LQTs.

Clinical and genetic diagnosis for inherited cardiac arrhythmias

Molecular genetic studies in the last 2 decades have revealed a link between several inherited cardiac arrhythmias and genes encoding for ion channels or other membrane components. Two recent international expert consensus statements endorsed by 3 continental electrophysiology societies have updated the clinical and genetic diagnoses and management in patients with inherited arrhythmia syndromes, including congenital long QT syndrome (LQTS) and Brugada syndrome. Thirteen genotypes have been identified in 50% to 80% of clinically affected patients with congenital LQTS. Therefore, genotype-phenotype correlations have been investigated, especially, in the 3 major genotypes--LQT1, LQT2 and LQT3 syndromes--enabling genotype-specific management and therapy. On the other hand, less than half of patients with Brugada syndrome can be genotyped, and mainly for the sodium channel gene, SCN5A. However, recent advances in molecular genetic testing include genome-wide association studies using gene arrays and targeted, whole-exome and whole-genome next-generation sequencing techniques. In this article, I will review the clinical and genetic diagnoses in congenital LQTS and Brugada syndrome.

Long-QT syndrome

Acquired and hereditary long-QT syndromes are important causes of sudden cardiac death. Both categories are characterized by abnormally prolonged cardiac repolarization arising from a complex interaction between genetic and environmental factors. This produces a potentially dangerous substrate for polymorphic ventricular tachycardia and sudden cardiac death. In this review, the pathophysiologic, diagnostic, and prognostic features of long-QT syndromes, as well as recommendations regarding therapy, are reviewed.

Derivation and Validation of a Simple Exercise-Based Algorithm for Prediction of Genetic Testing in Relatives of LQTS Probands

Background—

Genetic testing can diagnose long-QT syndrome (LQTS) in asymptomatic relatives of patients with an identified mutation; however, it is costly and subject to availability. The accuracy of a simple algorithm that incorporates resting and exercise ECG parameters for screening LQTS in asymptomatic relatives was evaluated, with genetic testing as the gold standard.

Methods and Results—

Asymptomatic first-degree relatives of genetically characterized probands were recruited from 5 centers. QT intervals were measured at rest, during exercise, and during recovery. Receiver operating characteristics were used to establish optimal cutoffs. An algorithm for identifying LQTS carriers was developed in a derivation cohort and validated in an independent cohort. The derivation cohort consisted of 69 relatives (28 with LQT1, 20 with LQT2, and 21 noncarriers). Mean age was 35±18 years, and resting corrected QT interval (QTc) was 466±39 ms. Abnormal resting QTc (females ≥480 ms; males ≥470 ms) was 100% specific for gene carrier status, but was observed in only 48% of patients; however, mutations were observed in 68% and 42% of patients with a borderline or normal resting QTc, respectively. Among these patients, 4-minute recovery QTc ≥445 ms correctly restratified 22 of 25 patients as having LQTS and 19 of 21 patients as being noncarriers. The combination of resting and 4-minute recovery QTc in a screening algorithm yielded a sensitivity of 0.94 and specificity of 0.90 for detecting LQTS carriers. When applied to the validation cohort (n=152; 58 with LQT1, 61 with LQT2, and 33 noncarriers; QTc=443±47 ms), sensitivity was 0.92 and specificity was 0.82.

Conclusions—

A simple algorithm that incorporates resting and exercise-recovery QTc is useful in identifying LQTS in asymptomatic relatives.

β-blockers protect against dispersion of repolarization during exercise in congenital long-QT syndrome type 1

INTRODUCTION

β-Blocker therapy reduces syncope and sudden death in long-QT syndrome type 1 (LQT1), but the mechanism of protection is incompletely understood. This study tested the hypothesis that β-blockade reduces QT prolongation and dispersion of repolarization, measured as the T peak-to-end interval (T(pe) ), during exercise and recovery in LQT1 patients.

METHODS AND RESULTS

QT and T(pe) were measured in 10 LQT1 patients (33 ± 13 years) and 35 normal subjects (32 ± 12 years) during exercise tests on and off β-blockade. In LQT1 patients, β-blockade reduced QT (391 ± 25 milliseconds vs 375 ± 26 milliseconds, P = 0.04 during exercise; 419 ± 41 milliseconds vs 391 ± 39 milliseconds, P = 0.02 during recovery) and markedly reduced T(pe) (91 ± 26 milliseconds vs 67 ± 19 milliseconds, P = 0.03 during exercise; 103 ± 26 milliseconds vs 78 ± 11 milliseconds, P = 0.02 during recovery). In contrast, in normal subjects, β-blockade had no effect on QT (320 ± 17 milliseconds vs 317 ± 16 milliseconds, P = 0.29 during exercise; 317 ± 13 milliseconds vs 315 ± 14 milliseconds, P = 0.15 during recovery) and mildly reduced T(pe) (69 ± 13 milliseconds vs 61 ± 11 milliseconds, P = 0.01 during exercise; 77 ± 19 milliseconds vs. 68 ± 14 milliseconds, P < 0.001 during recovery).

CONCLUSION

In LQT1 patients, β-blockers reduced QT and T(pe) during exercise and recovery, supporting the theory that β-blocker therapy protects LQT1 patients by reducing dispersion of repolarization during exercise and recovery.

Beta1-adrenergic receptor polymorphisms, QTc interval and occurrence of symptoms in type 1 of long QT syndrome

BACKGROUND

The most prevalent LQT1 form of inherited long QT syndrome is caused by mutations of the KCNQ1 gene resulting repolarizing I(Ks) potassium current to decrease and the QT interval to prolong. As abrupt sympathetic activation triggers ventricular arrhythmias that may cause syncopal attacks and sudden death in LQT1 patients, we investigated whether two known beta1-adrenergic receptor polymorphisms were associated with the duration of QT interval or history of symptoms in LQT1.

METHODS

We determined beta1-adrenergic receptor polymorphisms (Ser49Gly and Arg389Gly) in 168 LQT1 patients. We also reviewed each patient's clinical records on the history of long QT syndrome-related symptoms and measured QT intervals from baseline ECG in each subject and from an exercise test ECG in 55 LQT1 patients.

RESULTS

Patients with the homozygous Arg389Arg genotype tended to have shorter and those with the Ser49Ser genotype longer QT intervals than patients with other genotypes, but neither polymorphism studied alone affected the risk of symptoms. In contrast, adjusted odds ratio for the history of symptoms was 4.9 (95% CI 1.18 to 20.3) in patients homozygous for both Ser49 and Arg389. These double homozygous patients showed similar QT intervals as the rest of the LQT1 cohort.

CONCLUSIONS

In this relatively small study, double homozygosity for Arg389 and Ser49 of the human beta1-adrenergic receptor associated with the risk of symptoms in LQT1. The association between these beta1-adrenergic receptor polymorphisms and the symptom history in LQT1 is not mediated via QT interval duration.

Sympathomimetic Infusion and Cardiac Repolarization: The Normative Effects of Epinephrine and Isoproterenol in Healthy Subjects

INTRODUCTION

Catecholamines are known to affect cardiac repolarization, and provocation with either isoproterenol or epinephrine has been proposed as a tool for uncovering latent repolarization abnormalities. This study systematically compares the effects of isoproterenol and epinephrine infusions on QT interval (QT), T waves and U waves in normal subjects.

METHODS AND RESULTS

Twenty-four normal subjects (29 +/- 8 years) were evaluated during graded infusions of up to 0.30 microg/kg/minute epinephrine and 5.0 microg/minute isoproterenol. Heart rates at peak doses were 81 +/- 13 bpm at 0.28 +/- 0.04 microg/kg/minute epinephrine and 104 +/- 5 bpm at 2.4 microg/minute isoproterenol. The longest absolute QT increase was 4 +/- 5 msec above baseline during isoproterenol (P < 0.001) and 12 +/- 23 msec during epinephrine (P = 0.07), while the longest corrected QT interval (QTc) increase was 67 +/- 28 msec (P < 0.0001) and 79 +/- 40 msec (P < 0.0001) above baseline during isoproterenol and epinephrine, respectively (P = 0.12 for difference). There was a 2-fold increase in U-wave amplitude during each intervention (P < 0.001). The specificity of paradoxical QT prolongation (>or=30 msec at 0.05 microg/kg/minute or >or=35 msec at 0.10 microg/kg/minute epinephrine) and an increase in QTc >or=600 msec at any dose epinephrine were 100%. However, the specificity of other proposed criteria that utilized QTc measurement (>or=30 msec at 0.10 microg/kg/minute or >or=65 msec at any dose) was poor whether all leads or only lead II were assessed.

CONCLUSION

Both epinephrine and isoproterenol are associated with QTc prolongation and amplification of the U wave in normal subjects. The specificity of proposed criteria for epinephrine provocation in diagnosis of the long-QT syndrome is variable; however, paradoxical QT prolongation at low-dose epinephrine or a QTc >or=600 msec is highly specific.

Use of autonomic maneuvers to probe phenotype/genotype discordance in congenital long QT syndrome

Patients with congenital long QT syndrome due to potassium channel mutations (LQT1 and LQT2) may elude diagnosis due to normal electrocardiographic findings at rest, yet remain at risk of sudden death during bradycardia or sympathetic stimulation. To test the hypothesis that autonomic maneuvers can unmask long QT syndrome in genetically abnormal subjects with a normal phenotype (QTc < or =450 ms), we exposed 13 controls (33 +/- 9 years; 5 men), 5 patients with LQT1 (32 +/- 12 years; 3 men), and 5 patients with LQT2 (30 +/- 11 years; 5 men) to phenylephrine bolus, exercise, and epinephrine infusion. The QT interval was measured at baseline and after each intervention. A substantial overlap was found in QTc among the groups at baseline and after phenylephrine. In contrast, QTc was significantly and consistently longer in subjects with LQT1 compared with controls during and after exercise (492 +/- 40 vs 407 +/- 14 ms, p <0.0001, at peak exercise; 498 +/- 30 vs 399 +/- 20 ms, p <0.0001, at 1 minute into recovery) or epinephrine (623 +/- 51 vs 499 +/- 51 ms, p <0.001, at peak epinephrine; 604 +/- 36 vs 507 +/- 54 ms, p <0.01, at 1 minute into recovery) but not in subjects with LQT2. In conclusion, sympathetic stimulation can reveal the LQT1 phenotype even in subjects with normal baseline electrocardiographic findings.

Burst bicycle exercise facilitates diagnosis of latent long QT syndrome

OBJECTIVES

The aim of this study was to develop a diagnostic technique to detect latent long QT syndrome.

BACKGROUND

Asymptomatic patients with genetically diagnosed long QT syndrome (LQTS) may have a normal resting QT interval, yet remain at risk for cardiac events. We hypothesized that the QT response during a novel burst exercise protocol simulating clinical events might distinguish patients with "latent LQTS" from healthy subjects.

METHODS

A burst bicycle protocol was performed on 31 healthy subjects and 31 patients with LQTS (13 LQT2, 3 LQT1, 15 unknown genotype). The bicycle exercise protocol involved sudden maximal exertion against a fixed workload (200 W) for 1 minute. Digitized 12-lead eletrocardiograms were acquired every 10 seconds at baseline for 1 minute, during 1 minute of burst exercise, and for 5 minutes during recovery. Patients with LQTS were segregated according to whether the baseline QTc was normal (< or = 440 milliseconds, n = 13) or abnormal (> 440 milliseconds, n = 18).

RESULTS

During exercise, the QTc increased to a greater extent in the group with latent LQTS (DeltaQTc 98 +/- 36 milliseconds) in comparison with controls (DeltaQTc 65 +/- 19 milliseconds, P < .01) and those with baseline QTc prolongation (DeltaQTc 17 +/- 70 milliseconds, P < .01). In patients with a normal baseline QTc, a DeltaQTc > 85 milliseconds had a sensitivity of 85% and a specificity of 86% for LQTS (P = .0004).

CONCLUSION

Marked QTc prolongation during burst bicycle ergometry provides potentially diagnostic information for patients with normal baseline QTc and suspected LQTS.

Sudden Intense Exercise Increases QT Heart Rate Slope and T Wave Complexity in Long QT Syndrome and Normal Subjects

Sudden intense physical activity is a trigger for ventricular arrhythmias in long QT syndrome (LQTS), and beta-blockers (B) reduce the risk of ventricular arrhythmias in LQTS. We compared the effect of graded (gradual intensity) versus burst (sudden intensity) exercise on QT-rate adaptation and T wave complexity in LQTS + B (n = 21), LQTS - B (n = 5), and normal controls (n = 20). Graded exercise consisted of symptom-limited bicycle ergometry (30 W, increment 20 W/min). Burst exercise involved a fixed 200 W load for 1.0 minute. ECGs were digitally recorded every 10 seconds during exercise and a 10 minute recovery period. QT-rate adaptation was quantified using the slope of the QT cycle length relationship fit to a quadratic function. Principle component analysis (PCA) was used to quantify T wave complexity. The QT-rate slope was two-fold greater with burst exercise than graded exercise for LQTS + B (-82E +/- 40E vs -36E +/- 40E, P = 0.0016), LQTS - B (-85E +/- 60E vs -30E +/- 50E, P = 0.011) and controls (-100E +/- 60E vs -48E +/- 100E, P = 0.0011) (E = x10(-5)). For each exercise protocol, there was no difference in QT-rate slope between the three groups. In contrast, the QT-rate slope during the 10 minute recovery period was similar between the burst and graded protocol for LQTS + B (25E +/- 40E vs 30E +/- 50E), LQTS - B (81E +/- 80E vs 85E +/- 70E) and controls (90E +/- 80E vs 82E +/- 80E). The coefficient of variability of PCA (T wave complexity variability) during burst exercise was greater than that during graded exercise for LQTS + B (41 +/- 15 vs 30 +/- 10, P = 0.017), LQTS - B (47 +/- 25 vs 26 +/- 4, P = 0.016), and control (46 +/- 14 vs 33 +/- 19, P = 0.012). For each exercise protocol, no difference in T wave complexity variability was seen between the three groups. In conclusion, QT heart rate slope and T wave complexity variability are greater during sudden intense exercise than graded exercise in LQTS patients (on and off beta-blockers) and normal subjects, with similar findings among the three groups of patients.

Management of ventricular fibrillation or unstable ventricular tachycardia in patients with congenital long-QT syndrome: a suggested modification to ACLS guidelines

Prolongation of the QT interval is a known risk factor for syncope, seizures and sudden cardiac death. Most patients with QT prolongation have an acquired cause, but congenital forms of QT prolongation are being increasingly recognized. However, existing advanced cardiac life support (ACLS) treatment algorithms for prolonged QT mediated ventricular fibrillation pertains to acquired long-QT syndrome (LQTS). Here, a young patient with out-of-hospital cardiac arrest secondary to congenital LQTS illustrates critical exceptions to the current ACLS treatment algorithms for ventricular fibrillation and unstable ventricular tachycardia when QT prolongation is congenital in origin. A clarified ACLS algorithm is proposed.

Autonomic nervous system influences on QT interval in normal subjects

OBJECTIVES

We sought to determine whether the relationship between heart rate (HR) and QT interval (QT) differs as HR increases in response to exercise, atropine and isoproterenol.

BACKGROUND

Autonomic nervous system influences on repolarization are poorly understood and may complicate the interpretation of QT measurements.

METHODS

Twenty-five normal subjects sequentially underwent graded-intensity bicycle exercise, atropine injection and isoproterenol infusion. Serial 12-lead electrocardiograms were recorded at steady state during each condition and analyzed using interactive computer software. The HR-QT data were modeled linearly and the slopes (quantifying QT adaptation to HR) as well as the QT intervals at 100 beats/min for each intervention were compared by repeated-measures analysis of variance.

RESULTS

As HR increased, QT was longer for isoproterenol in comparison to exercise or atropine, which were similar. The HR-QT slope (ms/beats/min) was less steep for isoproterenol (-0.83 +/- 0.53) than for atropine (-1.45 +/- 0.21) or exercise (-1.37 +/- 0.23) (p < 0.0001). In comparison to men, women had more negative HR-QT slopes during all interventions. At 100 beats/min, the QT was 364 ms during isoproterenol, which was significantly longer than that during exercise (330 ms) or atropine (339 ms) (p < 0.0001). Isoproterenol produced a dose-dependent increase in U-wave amplitude that was not observed during exercise or atropine.

CONCLUSIONS

In comparison to exercise and atropine, isoproterenol is associated with much less QT shortening for a given increase in HR and, therefore, greater absolute QT intervals. Our findings demonstrate that autonomic conditions directly affect the ventricular myocardium of healthy subjects, causing differences in QT that are independent of HR.

Rate dependency of delayed rectifier currents during the guinea-pig ventricular action potential

1. The action potential clamp technique was exploited to evaluate the rate dependency of delayed rectifier currents (I(Kr) and I(Ks)) during physiological electrical activity. I(Kr) and I(Ks) were measured in guinea-pig ventricular myocytes at pacing cycle lengths (CL) of 1000 and 250 ms. 2. A shorter CL, with the attendant changes in action potential shape, was associated with earlier activation and increased magnitude of both I(Kr) and I(Ks). Nonetheless, the relative contributions of I(Kr) and I(Ks) to total transmembrane current were independent of CL. 3. Shortening of diastolic interval only (constant action potential shape) enhanced I(Ks), but not I(Kr). 4. I(Kr) was increased by a change in the action potential shape only (constant diastolic interval). 5. In ramp clamp experiments, I(Kr) amplitude was directly proportional to repolarization rate at values within the low physiological range (< 1.0 V s(-1)); at higher repolarization rates proportionality became shallower and finally reversed. 6. When action potential duration (APD) was modulated by constant current injection (I-clamp), repolarization rates > 1.0 V s(-1) were associated with a reduced effect of I(Kr) block on APD. The effect of changes in repolarization rate was independent of CL and occurred in the presence of I(Ks) blockade. 7. In spite of its complexity, the behaviour of I(Kr) was accurately predicted by a numerical model based entirely on known kinetic properties of the current. 8. Both I(Kr) and I(Ks) may be increased at fast heart rates, but this may occur through completely different mechanisms. The mechanisms identified are such as to contribute to abnormal rate dependency of repolarization in prolonged repolarization syndromes.

Sinus node function and ventricular repolarization during exercise stress test in long QT syndrome patients with KvLQT1 and HERG potassium channel defects

OBJECTIVES

This study was performed to evaluate the QT interval and heart rate responses to exercise and recovery in gene and mutation type-specific subgroups of long QT syndrome (LQTS) patients.

BACKGROUND

Reduced heart rate and repolarization abnormalities are encountered among long QT syndrome (LQTS) patients. The most common types of LQTS are LQT1 and LQT2.

METHODS

An exercise stress test was performed in 23 patients with a pore region mutation and in 22 patients with a C-terminal end mutation of the cardiac potassium channel gene causing LQT1 type of long QT syndrome (KVLQT1 gene), as well as in 20 patients with mutations of the cardiac potassium channel gene causing LQT2 type of long QT syndrome (HERG gene) and in 33 healthy relatives. The QT intervals were measured on electrocardiograms at rest and during and after exercise. QT intervals were compared at similar heart rates, and rate adaptation of QT was studied as QT/heart rate slopes.

RESULTS

In contrast to the LQT2 patients, achieved maximum heart rate was decreased in both LQT1 patient groups, being only 76 +/- 5% of predicted in patients with pore region mutation of KvLQT1. The QT/heart rate slopes were significantly steeper in LQT2 patients than in controls during exercise. During recovery, the QT/heart rate slopes were steeper in all LQTS groups than in controls, signifying that QT intervals lengthened excessively when heart rate decreased. At heart rates of 110 or 100 beats/min during recovery, all LQT1 patients and 89% of LQT2 patients had QT intervals longer than any of the controls.

CONCLUSIONS

LQT1 is associated with diminished chronotropic response and exaggerated prolongation of QT interval after exercise. LQT2 patients differ from LQT1 patients by having marked QT interval shortening and normal heart rate response to exercise. Observing QT duration during recovery enhances the clinical diagnosis of these LQTS types.

The autonomic control of the transmural dispersion of ventricular repolarization in anesthetized dogs

INTRODUCTION

The initiation of ventricular arrhythmias is in part associated with autonomic nervous tone. We investigated the effects of sympathetic and parasympathetic stimulation on the monophasic action potentials (MAPs) of the epicardium (EPi), mid-myocardial (M) region, and endocardium (Endo) in vivo.

METHODS AND RESULTS

In 12 mongrel open chest anesthetized dogs, both sides of the cervical vagus nerves and stellate ganglia were crushed with a tight ligature. Right atrial pacing at 600 msec cycle length was begun after the sinus nodal area had been crushed. MAPs from the M region were measured by two needle electrodes that were supported by a W-shaped plastic frame. The epicardial, M region, and endocardial MAP durations at 90% repolarization (MAPD90) were 287+/-7, 315+/-7, and 290+/-8 msec, respectively. The MAPD90 from M cells was longer than that from Epi or Endo. Sympathetic stimulation shortened MAPD90 more in the M region (53+/-4 msec) than that in the Epi (27+/-3 msec) or Endo (26+/-4 msec). The transmural dispersion of repolarization during sympathetic stimulation was shorter than that of the control. Parasympathetic stimulation did not significantly affect any of the MAPD90 values. Simultaneous sympathetic and parasympathetic stimulation produced changes not significantly to those produced by sympathetic stimulation alone.

CONCLUSION

Our results suggest that sympathetic activity can reduce transmural dispersion of repolarization under autonomic control in the canine heart under baseline conditions.

Highly abnormal thermotests in familial dysautonomia suggest increased cardiac autonomic risk

OBJECTIVE

Patients with familial dysautonomia have an increased risk of sudden death. In some patients with familial dysautonomia, sympathetic cardiac dysfunction is indicated by prolongation of corrected QT (QTc) interval, especially during stress tests. As many patients do not tolerate physical stress, additional indices are needed to predict autonomic risk. In familial dysautonomia there is a reduction of both sympathetic neurons and peripheral small nerve fibres which mediate temperature perception. Consequently, quantitative thermal perception test results might correlate with QTc values. If this assumption is correct, quantitative thermotesting could contribute to predicting increased autonomic risk.

METHODS

To test this hypothesis, QTc intervals were determined in 12 male and eight female patients with familial dysautonomia, aged 10 to 41 years (mean 21.7 (SD 10.1) years), in supine and erect positions and postexercise and correlated with warm and cold perception thresholds assessed at six body sites using a Thermotest.

RESULTS

Due to orthostatic presyncope, six patients were unable to undergo erect and postexercise QTc interval assessment. The QTc interval was prolonged (>440 ms) in two patients when supine and in two additional patients when erect and postexercise. Supine QTc intervals correlated significantly with thermal threshold values at the six body sites and with the number of sites with abnormal thermal perception (Spearman's rank correlation p<0.05). Abnormal Thermotest results were more frequent in the four patients with QTc prolongation and the six patients with intolerance to stress tests.

CONCLUSION

The results suggest that impaired thermal perception correlates with cardiac sympathetic dysfunction in patients with familial dysautonomia. Thus thermotesting may provide an alternative, albeit indirect, means of assessing sympathetic dysfunction in autonomic disorders.

Hysteresis of the RT interval with exercise: a new marker for the long-QT syndrome?

BACKGROUND

The diagnosis of the long-QT syndrome (LQTS) may be difficult to establish in patients with normal or borderline prolongation of the QT interval. Noninvasive markers are needed to identify patients with LQTS.

METHODS AND RESULTS

Fourteen patients with known LQTS, 9 unaffected family members, and 40 control subjects underwent modified Bruce protocol exercise testing. The RT interval (peak of R wave to peak of T wave) and rate-corrected RT interval (RTc) were measured during exercise and recovery. The RT interval at 1 minute into recovery was subtracted from the RT interval at a similar heart rate during exercise (deltaRT). The RTc shortened by 61 milliseconds (ms) in the LQTS patients compared with 23 to 26 ms in the other two groups (P=.003 by ANOVA). The RT interval shortened in a linear fashion in all patients but demonstrated persistent shortening during recovery in the LQTS patients. This was manifested as a hysteresis loop in the curve relating the RT interval to cycle length. The hysteresis loop was present in 13 of 14 LQTS patients and only 4 of 40 control subjects. DeltaRT >25 ms had a sensitivity of 73%, a specificity of 92%, a positive predictive value of 79%, and a negative predictive value of 90% for LQTS.

CONCLUSIONS

Hysteresis of the RT interval with exercise may be useful for the diagnosis of LQTS.

QT and JT dispersion in children with long QT syndrome

INTRODUCTION

Abnormalities of ventricular repolarization leading to ventricular arrhythmias place children with long QT syndrome at high risk for sudden death. Dispersion of the QT (QTd) and JT (JTd) intervals, as markers of cardiac electrical heterogeneity, may be helpful in evaluating children with long QT syndrome and identifying a subset of patients at high risk for development of critical ventricular arrhythmias (ventricular tachycardia, torsades de pointes, and/or cardiac arrest).

METHODS AND RESULTS

The QTd and JTd intervals in 39 children with long QT syndrome were compared to those of 50 normal age-matched children. In the long QT syndrome group, QTd measured 81 +/- 70 msec compared to 28 +/- 14 msec in the control group (P < 0.05), and JTd in the long QT syndrome group was 80 +/- 69 msec compared to 25 +/- 15 msec in the control group (P < 0.05).

CONCLUSION

Children with long QT syndrome have an increased QTd and JTd when compared to normal controls. A QTd or JTd > or = 55 msec correlates with the presence of critical ventricular arrhythmias. These ECG measures of dispersion can be useful in stratifying children with the long QT syndrome who are at higher risk for developing critical ventricular arrhythmias.

Exercise-induced ventricular arrhythmias and sudden cardiac death in a family

Members of a family have been investigated because of three sudden deaths among them. Two young sisters, aged 12 and 16, died suddenly while swimming and running, while their 19-year-old brother died suddenly during emotional stress. In no case did autopsies reveal any structural abnormalities. Their 39-year-old mother and her 19-year-old daughter gave a history of syncopes, while having a normal physical examination and normal ECGs. During a treadmill test, multiple ventricular extrasystoles and bursts of polymorphic ventricular tachycardia were provoked. Patient-members of this family have undergone echocardiography, catheterization of the left and right sides of the heart, endomyocardial biopsy, and electrophysiologic studies. A differential diagnosis of an inherited long QT interval syndrome, catecholamine-induced arrhythmias, and arrhythmogenic right ventricular dysplasia have been suggested. Patients were given atenolol and were followed up for 18 months. This therapy has greatly reduced the exertional arrhythmias as assessed by serial treadmill tests.

Effect of heart rate on QT interval in children and adolescents

OBJECTIVE

To study the effect of sympathetic stimulation and increase in heart rate on the QT and QTc intervals.

DESIGN

Prospective non-randomised study of eight consecutive patients.

SETTING

Electrophysiology laboratory at a tertiary centre.

PATIENTS

Eight patients aged 10-20 years (median 12.5) undergoing repeat electrophysiological study after previously successful catheter ablation (n = 6) or presumed supraventricular tachycardia (n = 2) with negative studies.

INTERVENTIONS

Electrocardiograms were obtained (a) at baseline, (b) during atrial pacing at 450 ms cycle length, (c) during isoprenaline infusion at 0.025 microgram/kg/min, (d) adding atrial pacing (450 ms cycle length) to isoprenaline at 0.025 microgram/kg/min, and (e) isoprenaline at 0.05 microgram/kg/min.

MAIN OUTCOME MEASURES

QT and QTc intervals at each of the above mentioned stages.

RESULTS

The QT interval was reduced from a mean value of 350 ms to around 315-325 ms by each of the above manoeuvres. Correspondingly, the QTc increased from a mean of 407 ms to around 445-470 ms. Pacing was as effective as isoprenaline in shortening the QT interval and prolonging the QTc intervals.

CONCLUSIONS

Heart rate directly influences QT and QTc intervals in children and adolescents. The QT is shortened, but QTc is prolonged. Hence, reliance on the QTc alone could lead to mistaken diagnosis of long QT syndrome.

Frequency-dependent electrophysiologic properties of ventricular repolarization in patients with congenital long QT syndrome

OBJECTIVES

This study was performed to evaluate the frequency dependency of ventricular repolarization and the effect of epinephrine in patients with congenital long QT syndrome (LQTS).

BACKGROUND

The efficacy of pacemakers in addition to antiadrenergic therapy in the treatment of congenital LQTS has been reported.

METHODS

Monophasic action potentials were recorded from right and left ventricular endocardium during atrial pacing at heart rates from 70 to 140 beats/min at baseline and from 100 to 140 beats/min during epinephrine infusion (0.1 microgram/kg body weight per min) in 11 patients with congenital LQTS and 10 control patients. The response of monophasic action potential duration at 90% repolarization (MAPD90) and the dispersion of MAPD90 were examined.

RESULTS

At baseline, both the MAPD90 and the dispersion of MAPD90 were significantly (p < 0.001) longer in the congenital LQTS group than the control group. The differences in these variables between the two groups significantly decreased (MAPD90: from 105 to 31 ms; dispersion of MAPD90: from 55 to 13 ms, p < 0.001) at heart rate was increased. Epinephrine prolonged the MAPD90 and increased the dispersion of MAPD90 significantly (p < 0.001) at all paced heart rates in the congenital LQTS group without frequency dependency but did not change in the control group. Thus, epinephrine increased the differences in these variables between the two groups.

CONCLUSIONS

The repolarization abnormalities in congenital LQTS were attenuated by increasing the heart rate, which supported the efficacy of pacemaker therapy. However, during sympathetic stimulation, the effects of increased heart rate on these repolarization abnormalities were limited.

Genetically defined therapy of inherited long-QT syndrome. Correction of abnormal repolarization by potassium

BACKGROUND

Many members of families with inherited long-QT (LQT) syndrome have mutations in HERG, a gene encoding a cardiac potassium channel that is modulated by extracellular potassium. We hypothesized that an increase in serum potassium would normalize repolarization in these patients.

METHODS AND RESULTS

We studied seven subjects with chromosome 7-linked LQT syndrome and five normal control subjects. Repolarization was measured by ECG and body surface potential mapping during sinus rhythm, exercise, and atrial pacing, before and after serum potassium increase. Potassium administration improved repolarization in the LQT syndrome. At baseline, LQT subjects differed from control subjects: resting corrected QT interval (QTc, 627 +/- 90 versus 425 +/- 25 ms, P = .0007), QTc dispersion (133 +/- 62 versus 36 +/- 9 ms, P = .009), QT/RR slope (0.35 +/- 0.08 versus 0.24 +/- 0.07, P = .04), and global root-mean-square QT interval (RMS-QTc; 525 +/- 68 versus 393 +/- 22, P = .002). All LQT subjects had biphasic or notched T waves. After administration of potassium, the LQT group had a 24% reduction in resting QTc interval (from 617 +/- 92 to 469 +/- 23 ms, P = .004) compared with a 4% reduction among control subjects (from 425 +/- 25 to 410 +/- 45 ms, P > .05). The reduction was significantly greater in LQT subjects (P = .018). QT dispersion became normal in LQT subjects and did not change in control subjects. The slope of the relation between QT interval and cycle length (QT/RR slope) decreased toward normal. T-wave morphology improved in six of seven LQT subjects. The LQT group had a greater reduction in RMS-QTc than control subjects (P = .04).

CONCLUSIONS

An increase in serum potassium corrects abnormalities of repolarization duration, T-wave morphology, QT/ RR slope, and QT dispersion in patients with chromosome 7-linked LQT.

Abnormal response to exercise, face immersion, and isoproterenol in children with the long QT syndrome

The present study was performed to observe the change of QT interval by sympathetic stimulations in patients with the long Qt syndrome (LQTS). The study group consisted of 6 children with LQTS and 6 healthy children without QT prolongation. All LQTS patients had syncopal episodes. The QTc and delta QTc% ([QTc interval after examination-QTc interval at rest]/ QTc interval at rest x 100) by treadmill testing, face immersion, and isoproterenol were examined. One minute after peak exercise of treadmill testing, the changes in the QTc interval were not significant in either group, but delta QTc% was larger in the LQTS group than in the control group (+ 11.0 +/- 12.1% vs -2.6 +/- 3.2%; P = 0.02). The QTc interval at the shortest RR interval during face immersion was prolonged in the LQTS group (0.47 +/- 0.01 s to 0.51 +/- 0.04 s; P = 0.02), but there were no significant changes in the control group (0.40 +/- 0.03 s to 0.41 +/- 0.03 s; P = NS). delta QTc% was larger in the LQTS group than in the control group (+ 10.0 +/- 7.3% vs +1.1 +/- 5.5%; P = 0.04). In the LQTS group, the RR interval was shortened (P = 0.009) and QTc interval was prolonged (P = 0.0008) after isoproterenol infusion. These sympathetic stimulations amplified the TU abnormality in the LQTS group. By observing the TU changes caused by face immersion, we hoped to find a possible new method with which to diagnose LQTS. The combination of these examinations may be helpful in screening the borderline cases of TU abnormalities.

Effects of verapamil and propranolol on early afterdepolarizations and ventricular arrhythmias induced by epinephrine in congenital long QT syndrome

OBJECTIVES

This study used monophasic action potentials to investigate the effects of verapamil and propranolol on epinephrine-induced repolarization abnormalities in congenital long QT syndrome.

BACKGROUND

Early afterdepolarizations have been suggested to play a significant role in QT prolongation and ventricular arrhythmias in congenital long QT syndrome. Calcium channel blocking as well as beta-adrenergic blocking agents are reported to be effective in the management of this syndrome.

METHODS

Monophasic action potentials from 2 to 4 sites were recorded simultaneously in eight patients with the long QT syndrome (22 sites) and in eight control patients (23 sites) and were obtained during constant atrial pacing 1) before epinephrine infusion; 2) during epinephrine infusion (0.1 microgram/kg body weight min); 3) after verapamil injection (0.1 mg/kg) during epinephrine infusion; and 4) after both propranolol (0.1 mg/kg) and verapamil injections.

RESULTS

Early afterdepolarizations were recorded in two of the eight patients (2 of 22 sites) during the control state. During epinephrine infusion, early afterdepolarizations were recorded in six patients (six sites), and ventricular premature complexes were induced in three and torsade de pointes in one. Epinephrine prolonged 90% monophasic action potential duration from 348 +/- 48 (mean +/- SD) to 381 +/- 49 ms (22 sites, p < 0.0005) and increased the dispersion of action potential duration (difference between the longest and shortest action potential duration) from 36 +/- 20 to 64 +/- 34 ms (p < 0.005). Verapamil eliminated (two sites) or reduced (four sites) early afterdepolarizations and abolished ventricular premature complexes in two of the three patients as well as suppressing torsade de pointes. Verapamil shortened the action potential duration to 355 +/- 28 ms (p < 0.01 vs. epinephrine) and decreased the dispersion to 44 +/- 19 ms (p < 0.05 vs. epinephrine). Propranolol further eliminated (two sites) or reduced (two sites) early after depolarizations, abolished ventricular premature complexes in the remaining one patient and further shortened the action potential duration to 337 +/- 32 ms (p = 0.09 vs. verapamil). In the control patients, none of the early afterdepolarizations, ventricular arrhythmias or marked prolongations of action potential duration were induced by epinephrine, and neither verapamil nor propranolol changed repolarization variables.

CONCLUSIONS

These results indicate that both verapamil and propranolol can improve repolarization abnormalities induced by epinephrine in congenital long QT syndrome.

Dynamic relationship between the Q-aT interval and heart rate in patients with long QT syndrome during 24-hour Holter ECG monitoring

The purpose of this study was to investigate the dynamic relationship between heart rate and the Q-aT interval (the interval from the Q wave to the T wave apex) in patients with long QT syndrome. The QT to heart rate relation is useful for evaluating abnormalities of the ventricular repolarization, but its clinical application to the long QT syndrome requires accurate computer aided measurement of the QT interval and the sampling of a large number of beats. Therefore, the Q-aT interval was used on the basis of some reports that the heart rate dependency of the QT interval was concentrated in the Q-aT interval. Recent advances in the computer technology have allowed analysis of the relationship between the Q-aT and RR intervals on Holter ECG recordings. However, in addition to a prolonged QT interval, most patients with long QT syndrome have bizarre and variable T waves and the influence of this T wave morphology on the Q-aT to heart rate relation has not been clarified. We investigated the dynamic relationship between the Q-aT interval and heart rate in 10 patients with long QT syndrome and 11 control subjects using our original computer algorithm for the analysis of 24-hour Holter ECG recordings. The patients showed morphological T wave changes associated with heart rate changes during Holter recordings and these affected the Q-aT interval. The patients showed the following characteristics in the relationship between the major T wave peak and the RR interval: (1) a modestly decreased correlation between Q-aT and RR than in the control subjects (a median r value of 0.87 vs 0.93; P = 0.001); and (2) a steeper Q-aT/RR slope than in controls (a median slope of 0.24 vs 0.16; P < 0.05). Abnormal and variable T wave morphology in the long QT patients was closely related to a modestly decreased correlation between Q-aT and RR than in the control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine induced torsades de pointes in a child with congenital long QT syndrome

Torsades de pointes is a rare arrhythmia characterized by its bradycardia dependence and increased adrenergic discharge, whether it occurs as a congenital anomaly or as an acquired problem resulting from drug intoxication or other conditions. There are no reliable tests to assess the propensity toward torsades de pointes or evaluate the efficacy of treatment in these patients. Adenosine can result in marked slowing of sinus and ventricular rate and leads to increased sympathetic discharge when given intravenously. We induced torsades de pointes in a child with congenital long QT syndrome (Jervell-Lange-Nielsen syndrome) using 200 micrograms/kg IV adenosine bolus. Higher dosage of adenosine (600 micrograms/kg) did not lead to torsades de pointes after beta blockade. Adenosine may induce torsades de pointes in patients with the long QT syndrome and may be used as a test to reproduce the clinical arrhythmia. Whether adenosine proves to be useful for assessing the efficacy of treatment will require extensive investigation in larger series of patients.

Diagnostic value of recovery time measured by body surface mapping in patients with congenital long QT syndrome

The QT interval of the resting 12-lead electrocardiogram is normal or borderline in some patients with congenital long QT syndrome (LQTS). Recently, several in vivo experimental studies have shown that the time of maximum dV/dt in the ST-T segment is correlated with the time of local ventricular recovery. The purpose of this study was to examine the value of the body surface recovery time measured by 87-lead body surface mapping for detecting LQTS. Body surface mapping and 12-lead electrocardiography were performed simultaneously in 18 patients with LQTS and 40 controls of similar age and sex. The recovery time (RT), that is, the interval between QRS onset and the time of maximum dV/dt in the ST-T segment, was measured automatically by computer from each of the 87 mapping leads, and the corrected RT (RTc) was calculated by Bazett's method. The QT interval was measured from each of the 12 standard electrocardiographic leads, and the corrected QT (QTc) interval was also calculated. The maximum RT and RTc, the minimum RT and RTc, and the RT and RTc dispersions (difference between maximum and minimum RT and RTc in each patient) were significantly longer in the LQTS group than in the control group. In addition, a maximum RT of 390 msec, a maximum RTc of 430 msec 1/2, an RT dispersion of 170 msec, and an RTc dispersion of 170 msec1/2 separated the 2 groups completely (i.e., no overlap). The maximum QT and QTc, the minimum QT and QTc, and the QT and QTc dispersions (difference between maximum and minimum QT and QTc in each patient) were also significantly longer in the LQTS group than in the control group. However, the maximum QTc was normal (< or = 440 msec1/2) or borderline (< or = 460 msec1/2) in 5 of the 18 LQTS patients, and none of these parameters clearly separated the 2 groups. These results suggest that measurement of RT by 87-lead body surface mapping is useful for diagnosing latent or borderline LQTS.

Epiglottitis and torsade de pointes tachycardia

Severe throat infection is not usually associated with a higher risk of paroxysmal attacks of torsade de pointes tachycardia. A patient is reported in whom epiglottitis was associated with the sudden development of cardiac syncope caused by an acquired adrenergic-dependent long QT syndrome and the development of ventricular arrhythmias.

Effect of beta-adrenergic stimulation on the QT interval of children with syncope

The effect of intravenous bolus doses (0.25, 0.5, 1.0 microgram) of isoproterenol on the QT and RR intervals was reviewed in a group of 34 children undergoing autonomic testing for syncope. Twenty-one patients had a positive orthostatic test and 13 were negative. The two groups (positive and negative) were compared. Baseline QT and RR intervals were similar. The RR interval was shortened by isoproterenol in both groups. Isoproterenol shortened the QT interval in the negative group (as seen in normal persons), but produced QT prolongation in the positive group, although neither reached statistical significance when compared to baseline within the respective group. Comparing the values for RR and QT at each dose of isoproterenol (including baseline) between the two groups showed a significant difference in the QT interval after the 1.0-microgram dose of isoproterenol. Thus children with orthostatic positive neurocardiogenic syncope showed a different QT response to beta-adrenergic stimulation. This lends support to the theory of altered beta-adrenergic sensitivity being present in children with neurocardiogenic syncope.