The spectrum of symptoms and QT intervals in carriers of the gene for the long-QT syndrome

Evaluation of the spatial aspects of T-wave complexity in the long-QT syndrome

BACKGROUND

The duration of the QT interval is only a gross estimate of repolarization. Besides its limited accuracy and reproducibility, it does not provide information on the morphology of the T wave; thus, morphologic alterations such as notches can be only qualitatively described but not objectively quantified.

METHODS AND RESULTS

To measure the complexity of repolarization in the long-QT syndrome (LQTS) patients, we previously applied principal component analysis to body surface mapping and found it useful in distinguishing normal from abnormal repolarization patterns (sensitivity, 87%). In the present study, we applied principal component analysis to 12-lead Holter recordings. The index of complexity of repolarization that we have developed (CR24h) reflects the average 24-hour complexity of repolarization and is mathematically defined as the average ratio between the second and the first eigenvalue. We studied 36 LQTS patients and 40 control subjects. A mean of 22+/-1.3 ECG recordings at 1-hour intervals was used in each patient, and a total of 1655 recordings were analyzed. CR24h was significantly higher in LQTS than in control subjects (34+/-12% versus 13+/-3%; P<.0001). A CR24h exceeding 2 SD above the mean of the control group (>20%) was present in 32 of 36 patients (88%). The negative predictive value of CR24h in LQTS was 88%, and the combination of prolonged QT and abnormal CR24h identified all LQTS patients from normal subjects, including 4 affected symptomatic individuals with a normal QT interval duration, suggesting that CR24h provides information independent of QT duration.

CONCLUSIONS

Our data suggest that principal component analysis applied to 24-hour, 12-lead Holter recording adequately quantifies the complexity of ventricular repolarization and may become a useful noninvasive diagnostic tool in LQTS.

QTc interval (cardiac repolarization): lengthening after meals

Weight reduction, either by dietary or surgical means, is associated with prolongation of the heart rate corrected QT interval (QTc = QT/R-R0.5) and, on occasion, sudden death. Screening subjects with obesity before weight loss for prolonged QTc intervals is an accepted practice, although at present, there are no guidelines for whether subjects should be fasting before electrocardiogram (EKG) evaluation. The aim of this study was to test the hypothesis that EKG QTc interval duration is independent of meal ingestion. The hypothesis was tested in 11 healthy subjects who ingested a 500-kcal formula meal. A small decrease in absolute QT interval and a steady decline in R-R interval were observed for up to 60 minutes after formula ingestion. The QTc interval increased above baseline at 15 minutes (p < 0.007) after meal, a change that persisted for the 1-hour postmeal observation period. Spectral analysis of EKG R-R intervals (low-/high-frequency amplitude ratio) indicated a change in cardiac autonomic flow after meal ingestion. The QTc interval did not lengthen and R-R low-/high-frequency amplitude ratio remained unchanged in eight subjects evaluated in a similar manner but in whom isovolumic amounts of water replaced the meal. These observations suggest that (1) cardiac repolarization changes with fasting and feeding, (2) the QTc interval is influenced by meal intake, and (3) the autonomic nervous system may play a role in meal-related QTc changes. These findings have implications for the evaluation of patients with obesity before starting and during weight loss treatment.

Mode of onset of malignant ventricular arrhythmias in idiopathic ventricular fibrillation

INTRODUCTION

The mode of onset of malignant ventricular arrhythmias (ventricular tachycardia [VT] or ventricular fibrillation [VF]) has been well described in patients with organic heart disease and in patients with the long QT syndromes. Less is known about the mode of onset of VF in patients with out-of-hospital VF who have no evidence of organic heart disease or identifiable etiology.

METHODS AND RESULTS

We reviewed the ECGs of all our patients with idiopathic VF. Documentation of the onset of spontaneous arrhythmias was available for 22 VF episodes in 9 patients (6 men and 3 women; age 41 +/- 16 years). In all instances, spontaneous VF followed a rapid polymorphic VT, which was initiated by premature ventricular complexes (PVCs) with very short coupling intervals. The PVC initiating VF had a coupling interval of 302 +/- 52 msec and a prematurity index of 0.4 +/- 0.07. These PVCs occurred within 40 msec of the peak of the preceding T wave. Pause-dependent arrhythmias were never observed.

CONCLUSION

Cardiac arrest among patients with idiopathic VF has a very distinctive mode of onset. Documentation of a polymorphic VT that is not pause dependent is of diagnostic value.

Dominant-negative KvLQT1 mutations underlie the LQT1 form of long QT syndrome

BACKGROUND

Mutations that map to the KvLQT1 gene on human chromosome 11 account for more than 50% of inherited long QT syndrome (LQTS). It has been discovered recently that the KvLQT1 and minK proteins functionally interact to generate a current with biophysical properties similar to I(Ks), the slowly activating delayed-rectifier cardiac potassium current. Since I(Ks) modulates the repolarization of cardiac action potentials it is reasonable to hypothesize that mutations in KvLQT1 reduce I(Ks), resulting in the prolongation of cardiac action potential duration.

METHODS AND RESULTS

We expressed LQTS-associated KvLQT1 mutants in Xenopus oocytes either individually or in combination with wild-type KvLQT1 or in combination with both wild-type KvLQT1 and minK. Substitutions of alanine with proline in the S2-S3 cytoplasmic loop (A177P) or threonine with isoleucine in the highly conserved signature sequence of the pore (T311I) yield inactive channels when expressed individually, whereas substitution of leucine with phenylalanine in the S5 transmembrane domain (L272F) yields a functional channel with reduced macroscopic conductance. However, all these mutants inhibit wild-type KvLQT1 currents in a dominant-negative fashion.

CONCLUSIONS

In LQTS-affected individuals these mutations would be predicted to result in a diminution of the cardiac I(Ks) current, subsequent prolongation of cardiac repolarization, and an increased risk of arrhythmias.

Clinical management of patients with the long QT syndrome: drugs, devices, and gene-specific therapy

The familial long QT syndrome (LQTS) is now recognized as a genetic channelopathy with a propensity to arrhythmogenic syncope and sudden death. Three genetic mutations have been identified that involve the slow and fast delayed potassium rectifier currents and the sodium current. Distinctive ECG-T wave phenotypes are associated with each of the three genotypes. Current day therapy includes: beta-adrenergic blocking drugs; pacemakers; left cervicothoracic sympathetic ganglionectomy; implanted cardioverter defibrillators; and possibly, drugs that improve mutant ionic channel dysfunction. LQTS has provided unique insight into the complex relationship between ionic channel dysfunction and ventricular tachyarrhythmias.

Risk profiles and cardiovascular preparticipation screening of competitive athletes

There has been heightened interest in the design and role of preparticipation screening for high school and college athletes. An American Heart Association consensus panel, composed of cardiovascular specialists and other physician experts having extensive clinical experience with athletes of all ages as well as a legal expert, assessed the benefits and limitations of preparticipation screening for early detection of cardiovascular abnormalities in competitive athletes. The panel addressed cost-efficiency and feasibility issues as well as the medicolegal implications of screening; and developed consensus recommendations and guidelines for the most prudent, practical, and effective screening procedures and strategies.

Prenatal findings in patients with prolonged QT interval in the neonatal period

OBJECTIVE

To describe prenatal abnormalities of cardiac rhythm in patients with prolonged QT interval in the neonatal period.

DESIGN

A retrospective analysis of the results of fetal echocardiography and the outcome in patients with prolonged QT interval in the neonatal period who had been referred for prenatal evaluation.

SETTING

Two university hospitals.

PATIENTS

Nine patients with prolonged QT interval in the neonatal period who had been referred for prenatal evaluation. Fetal echocardiograms were obtained from 24 to 40 weeks of gestation. Indications were fetal bradycardia (five patients), a family history of long QT syndrome (two patients), and complex arrhythmias (two patients).

RESULTS

Seven fetuses had persistent sinus bradycardia without ventricular arrhythmias (heart rates 70-120 beats/ min). Five patients were treated with propranolol, after the diagnosis had been established by postnatal electrocardiogram (ECG). One of these patients died suddenly at the age of 3 weeks, after the treatment had been stopped because of profound bradycardia. One of the remaining two patients who did not receive propranolol had a syncope at the age of 6 weeks. Two fetuses presented with frequent runs of ventricular tachycardia and intermittent bradycardia caused by intermittent, functional second degree atrioventricular block. Both patients died on the first day of life despite treatment with propranolol and transvenous temporary pacing.

CONCLUSIONS

Sinus bradycardia in an otherwise normal fetus may be a symptom of long QT syndrome. Postnatal ECGs and a family examination are strongly recommended in these children. In fetuses with frequent runs of ventricular tachycardia and intermittent second degree atrioventricular block long QT syndrome should be suspected prenatally. These high risk patients should be delivered in centres with a paediatric cardiology unit.

A practical approach to torsade de pointes

The term torsade de pointes refers to polymorphic ventricular tachycardia that occurs in the setting of an abnormally long QT interval. While the most common cause is treatment with QT prolonging drugs, torsade de pointes also occurs in the congenital long QT syndromes and in the setting of acquired heart block or severe electrolyte disturbance, notably hypokalemia. Among QT prolonging drugs that cause torsade de pointes, both antiarrhythmics and "noncardioactive" drugs have been recognized. The electrocardiographic features of torsade de pointes include labile QT intervals, prominent U waves, and a "pause-dependent" onset of the arrhythmia. Treatment consists of recognition of the syndrome, correction of underlying electrolyte abnormalities, and withdrawal of any offending drugs. Magnesium, isoproterenol, or cardiac pacing provides specific antiarrhythmic therapy in torsade de pointes.

Four novel KVLQT1 and four novel HERG mutations in familial long-QT syndrome

BACKGROUND

Familial long-QT syndrome (LQTS) is characterized by prolonged ventricular repolarization. Clinical symptoms include recurrent syncopal attacks, and sudden death may occur due to ventricular tachyarrhythmias. Three genes responsible for this syndrome (KVLQT1, HERG, and SCN5A) have been identified so far. We investigated mutations of these genes in LQTS families.

METHODS AND RESULTS

Thirty-two Japanese families with LQTS were brought together for screening for mutations. Genomic DNA from each proband was examined by the polymerase chain reaction-single-strand conformation polymorphism technique followed by direct DNA sequencing. In four of the families, comprising 16 patients, mutations were identified in KVLQT1; five other families (9 patients) segregated mutant alleles of HERG. All 25 of these patients carried the specific mutations present in their respective families, and none of 80 normal individuals carried these alleles. Mutations were confirmed by endonuclease digestion or hybridization of mutant allele-specific oligonucleotides. No mutation in SCN5A was found in any family.

CONCLUSIONS

We identified nine different mutations among 32 families with LQTS. Eight of these were novel and account for 25% of all types of mutations reported to date. Such a variety of mutations makes it difficult to screen high-risk groups using simple methods such as endonuclease digestion or mutant allele-specific amplification.

Toward molecular strategies for heart disease--past, present, future

The past two decades of cardiovascular biology and medicine have been based largely upon the consideration of the heart and vasculature as an integrated physiological system, a view that has resulted in major therapeutic advances. With the advent of developments of gene transfer, mouse and human genetics, genetic engineering of intact animals, and molecular and cellular technology, cardiovascular medicine is now on the threshold of a molecular therapeutic era. Major steps have been taken toward unraveling the molecular determinants of complex, integrative, and polygenic cardiovascular disease states, including atherogenesis, hypertension, cardiac hypertrophy and failure, congenital heart disease, and coronary restenosis following balloon angioplasty. Our improved understanding of the fundamental basis of these important cardiovascular disease processes has established a scientific foundation for diagnostic, prognostic, and therapeutic advances in the mainstream of cardiovascular medicine.

Age-gender influence on the rate-corrected QT interval and the QT-heart rate relation in families with genotypically characterized long QT syndrome

OBJECTIVES

We sought to analyze age-gender differences in the rate-corrected QT (QTc) interval in the presence of a QT-prolonging gene.

BACKGROUND

Compared with men, women exhibit a longer QTc interval and an increased propensity toward torsade de pointes. In normal subjects, the QTc gender difference reflects QTc interval shortening in men during adolescence.

METHODS

QTc intervals were analyzed according to age (< 16 or > or = 16 years) and gender in 460 genotyped blood relatives from families with long QT syndrome linked to chromosome 11p (KVLQT1; n = 199), 7q (HERG; n = 208) or 3p (SCN5A; n = 53).

RESULTS

The mean QTc interval in genotype-negative blood relatives (n = 240) was shortest in men, but similar among women, boys and girls. For genotype-positive blood relatives, men exhibited the shortest mean QTc interval in chromosome 7q- and 11p-linked blood relatives (n = 194), but not in the smaller 3p-linked group (n = 26). Among pooled 7q- and 11p-linked blood relatives, multiple regression analysis identified both genotype (p < 0.001) and age-gender group (men vs. women/children; p < 0.001) as significant predictors of the QTc interval; and heart rate (p < 0.001), genotype (p < 0.001) and age-gender group (p = 0.01) as significant predictors of the absolute QT interval. A shorter mean QT interval in men was most evident for heart rates < 60 beats/min.

CONCLUSIONS

In familial long QT syndrome linked to either chromosome 7q or 11p, men exhibit shorter mean QTc values than both women and children, for both genotype-positive and -negative blood relatives. Thus, adult gender differences in propensity toward torsade de pointes may reflect the relatively greater presence in men of a factor that blunts QT prolongation responses, especially at slow heart rates.

QT interval change with age in an overtly healthy older population

BACKGROUND

Prolonged QT interval on the surface electrocardiogram (ECG) is known to be associated with arrhythmias, coronary heart disease, and sudden cardiac death. Increased QT dispersion has also been related to arrhythymias which are more frequent in the elderly.

HYPOTHESIS

This study investigated the relationship between aging, QT interval, and QT dispersion.

METHODS

Normal resting ECGs were recorded from 96 healthy subjects (73 women, age range 40-102 years). No subject had symptoms or signs of heart disease and none was on medication affecting cardiac function. All had normal heart size on chest x-ray and normal electrolytes. Using a digitizing board, the RR and QT intervals were measured on each lead of each ECG, excluding only the leads in which the T wave was not visible. Mean RR, mean QT interval, and heart rate-adjusted QTc interval (standard Bazet's formula) were obtained from these measurements. Further, QT dispersion was calculated for each ECG as (1) the difference between the maximum and minimum QT interval, and (2) as the coefficient of variance of QT interval of all measurable leads.

RESULTS

A significant correlation between aging and prolonged QTc was noted in the total population (r = 0.43, p < 0.05), as well as in men (r = 0.4, p < 0.05) and women (r = 0.23, p < 0.05) separately. There was no association between QT dispersion and increasing age regardless of the method of calculation (r = -0.04, r = -0.08 respectively, both NS).

CONCLUSION

The rate-adjusted QT interval is prolonged with increasing age and may contribute to the increased risk of ventricular arrhythmias and cardiac mortality in elderly patients.

Multiple mechanisms in the long-QT syndrome. Current knowledge, gaps, and future directions. The SADS Foundation Task Force on LQTS

The congenital long-QT syndrome (LQTS) is characterized by prolonged QT intervals, QT interval lability, and polymorphic ventricular tachycardia. The manifestations of the disease vary, with a high incidence of sudden death in some affected families but not in others. Mutations causing LQTS have been identified in three genes, each encoding a cardiac ion channel. In families linked to chromosome 3, mutations in SCN5A, the gene encoding the human cardiac sodium channel, cause the disease, Mutations in the human ether-à-go-go-related gene (HERG), which encodes a delayed-rectifier potassium channel, cause the disease in families linked to chromosome 7. Among affected individuals in families linked to chromosome 11, mutations have been identified in KVLQT1, a newly cloned gene that appears to encode a potassium channel. The SCN5A mutations result in defective sodium channel inactivation, whereas HERG mutations result in decreased outward potassium current. Either mutation would decrease net outward current during repolarization and would thereby account for prolonged QT intervals on the surface ECG. Preliminary data suggest that the clinical presentation in LQTS may be determined in part by the gene affected and possibly even by the specific mutation. The identification of disease genes in LQTS not only represents a major milestone in understanding the mechanisms underlying this disease but also presents new opportunities for combined research at the molecular, cellular, and clinical levels to understand issues such as adrenergic regulation of cardiac electrophysiology and mechanisms of susceptibility to arrhythmias in LQTS and other settings.

Evaluation of autonomic nervous activity in patients with congenital long QT syndrome by an analysis of RR variability

Congenital long QT syndrome (LQTS) is a rare hereditary disease characterized by a prolonged QT interval and lethal ventricular tachycardia (Torsades de Pointes: TdP). The pathogenesis of LQTS and the induction of TdP have been thought to be closely related to autonomic nervous abnormalities. We examined autonomic activity in 13 LQTS patients by analyzing heart rate variability from 24 h Holter ambulatory electrocardiographic recordings without medications. In a frequency-analysis of RR variability, we calculated the power in the low-frequency domain (LF) and the high-frequency domain (HF) over 24 h. The ratio of LF to HF (an index of sympathetic nervous activity) was lower in LQTS patients than in controls, whereas HF (an index of parasympathetic nervous activity) was higher. Moreover, LQTS patients with TdP had lower abnormal sympathetic nervous activity than those without TdP. The index of autonomic nervous activity obtained using this method could be useful for evaluating the severity in LQTS.

Long reflections on the QT interval: the sixth annual Gordon K. Moe Lecture

This review of accumulated knowledge about the long QT syndromes begins with an iteration of the original papers and then proceeds to a broader historic reflection that includes my personal work as well as many studies by others. Next come reflections upon the current status of knowledge on the subject, combined with comments about remaining challenges and questions grouped as follows: (1) morphologic abnormalities of the cardiac conduction system and their pathophysiologic significance in the long QT syndromes; (2) cardioneuropathy in the pathogenesis of long QT syndromes; (3) variability of QT prolongation, including consideration of peripheral and central dysautonomic conditions with certain diagnostic and therapeutic implications; (4) some caveats about the popular use of left cervical sympathectomy to treat symptomatic individuals with long QT syndrome, emphasizing hazards for future electrical instability of the heart; (5) consideration of genetic influences in the long QT syndromes, a field holding great promise as well as fraught with many puzzling dilemmas; and (6) apoptosis and the pathogenesis of the long QT syndromes, based upon personal observations previously made with electron microscopic studies of the sinus node and now including new immunohistochemical evidence expanding the relevance of this novel view-point. These intentionally provocative presentations are made to stimulate physicians and other scientists to consider these several different perspectives in planning future studies aimed at better understanding of one of the most challenging medical entities facing cardiology today.

Evidence of a long QT founder gene with varying phenotypic expression in South African families

We report five South African families of northern European descent (pedigrees 161, 162, 163, 164, and 166) in whom Romano-Ward long QT syndrome (LQT) segregates. The disease mapped to a group of linked markers on chromosome 11p15.5, with maximum combined two point lod scores, all generated at theta = 0, of 15.43 for the D11S922, 10.51 for the D11S1318, and 14.29 for the tyrosine hydroxylase (TH) loci. Recent studies have shown that LQT is caused by an Ala212Val mutation in a potassium channel gene (KVLQT1) in pedigrees 161 to 164. We report that the same mutation is responsible for the disease in pedigree 166. Haplotype construction showed that all the families shared a common haplotype, suggesting a founder gene effect. DNA based identification of gene carriers allowed assessment of the clinical spectrum of LQT. The QTc interval was significantly shorter in both carriers and non-carriers in pedigree 161 (0.48 s and 0.39 s, respectively) than the same two groups in pedigree 161 (0.52 s and 0.42 s, respectively). The spectrum of clinical symptoms appeared more severe in pedigree 162. The possible influence of modulating genetic factors, such as HLA status and sex of family members, on the expression of an LQT founder gene is discussed.

Allgrove syndrome: documenting cholinergic dysfunction by autonomic tests

We describe two Hispanic adolescents with Allgrove syndrome (alacrima, achalasia, and sensorimotor polyneuropathy) in whom we documented cholinergic dysfunction by cardiovascular autonomic tests. Both patients had orthostatic hypotension and decreased heart rate variability.

Molecular genetic insights into cardiovascular disease

The recent application of molecular genetic tools to inherited forms of cardiovascular disease has provided important insight into the molecular mechanisms underlying cardiac arrhythmias, cardiomyopathies, and vascular diseases. These studies point to defects in ion channels, contractile proteins, structural proteins, and signaling molecules as key players in disease pathogenesis. Genetic testing is now available for a subset of inherited cardiovascular diseases, and new mechanism-based therapies may be available in the near future. This remarkable progress and the implications it may have for more common forms of cardiovascular disease are reviewed here.

Triggers for sudden cardiac death in the athlete

Sudden death on the athletic field is usually due to underlying cardiovascular disease. Coronary artery disease is most common in older athletes, and a variety of congenital cardiovascular malformations predominate in young competitive athletes. Of these lesions, the most common in North America is hypertrophic cardiomyopathy. A variety of coronary artery anomalies are next in frequency, with the most important being anomalous origin of left main coronary artery from the anterior sinus of Valsalva.

The long QT syndrome. A review of recent molecular genetic and physiologic discoveries

There is great reason for optimism in the field of research into the long QT syndrome (LQT). We have made considerable progress, but there is much more to be done. We used molecular genetics to identify genes responsible for 2 forms of LQT (cardiac potassium and sodium channel genes HERG and SCN5A, respectively). These discoveries have led to improved mechanistic undertaking of the disorder and created the possibility for genetic testing. We are working to develop genetic tests for autosomal dominant LQT, but this will require identification of additional LQT genes. Specialized research laboratories like ours can provide genetic testing for many families, but these tests are not yet generally available. These tests may be particularly useful for families with LQT, since asymptomatic LQT gene carriers are still at risk for sudden death. Finally, molecular genetic and physiologic studies offer the possibility of new strategies for treatment and prevention of this cardiovascular disease.

Risk of cardiac events in family members of patients with long QT syndrome

OBJECTIVES

This study sought to identify risk factors for cardiac events (syncope, aborted cardiac arrest or sudden cardiac death) in family members of patients with the long QT syndrome.

BACKGROUND

Patients with the long QT syndrome are known to be at high risk for cardiac events. Whenever the first member of a family is identified as having the long QT syndrome (proband), there is concern regarding the likelihood of cardiac events in other family members.

METHODS

A multivariate logistic regression model was used to evaluate the risk of cardiac events in 637 family members who were first- and second-degree relatives of 151 probands with the long QT syndrome and in a subset of 513 family members who were not receiving beta-adrenergic blocking agents. There were 293 first-degree (46%) and 344 second-degree relatives (54%) (293 men [46%], 344 women [54%]). Fifteen percent of the family members had a corrected QT interval (QTc) > 0.44 s, and relative tachycardia and bradycardia were observed in 12% and 25%, respectively.

RESULTS

The risk of cardiac events occurring before age 40 in family members not taking beta-blockers was influenced by the QTc interval (odds ratio [OR] 1.18/0.01 increase in QTc value; 95% confidence interval [CI] 1.12 to 1.24), relative tachycardia (OR 2.21, 95% CI 0.97 to 5.02) or bradycardia (OR 2.24, 95% CI 1.10 to 4.56) and an interaction term combining gender and closeness of the relationship to the proband (OR for female first-degree relative 3.23 vs. all second-degree relatives, 95% CI 1.67-6.22).

CONCLUSIONS

Female first-degree relatives of patients with the long QT syndrome have a higher risk of cardiac events than male first- or second-degree relatives, independent of recorded electrocardiographic findings. Not only bradycardia, but also tachycardia increases risk of cardiac events in family members of patients with the long QT syndrome.

ECG T-wave patterns in genetically distinct forms of the hereditary long QT syndrome

BACKGROUND

The long QT syndrome is an inherited disorder with prolonged ventricular repolarization and a propensity to ventricular tachyarrhythmias and sudden arrhythmic death. Recent linkage studies have demonstrated three separate loci for this disorder on chromosomes 3, 7, and 11, and specific mutated genes for long QT syndrome have been identified on two of these chromosomes. We investigated ECG T-wave patterns (phenotypes) in members of families linked to three genetically distinct forms of the long QT syndrome.

METHODS AND RESULTS

Five quantitative ECG repolarization parameters, ie, four Bazett-corrected time intervals (QTonset-c, QTpeak-c, QTc, and Tduration-c, in milliseconds) and the absolute height of the T wave (Tamplitude, in millivolts), were measured in 153 members of six families with long QT syndrome linked to markers on chromosomes 3 (n = 47), 7 (n = 30), and 11 (n = 76). Genotypic data were used to define each family member as being affected or unaffected with long QT syndrome. Affected members of all six families had longer QT intervals (QTonset-c, QTpeak-c, or QTc) than unaffected family members (P < .01). Each of the three long QT syndrome genotypes was associated with somewhat distinctive ECG repolarization features. Among affected individuals, the QTonset-c was unusually prolonged in those individuals with mutations involving the cardiac sodium channel gene SCN5A on chromosome 3 (lead II QTonset-c [mean +/- SD]: chromosome 3, 341 +/- 42 ms; chromosome 7, 290 +/- 56 ms; chromosome 11, 243 +/- 73 ms; P < .001); Tamplitude was generally quite small in the chromosome 7 genotype (lead II Tamplitude, mV: chromosome 3, 0.36 +/- 0.14; chromosome 7, 0.13 +/- 0.07; chromosome 11, 0.37 +/- 0.17; P < .001); and Tduration was particularly long in the chromosome 11 genotype (lead II Tduration-c: chromosome 3, 187 +/- 33 ms; chromosome 7, 191 +/- 51 ms; chromosome 11, 262 +/- 65 ms; P < .001). Similar ECG findings were observed in leads aVF and V5. A considerable variability exists in the quantitative repolarization parameters associated with each genotype, with overlap in the T-wave patterns among the three genotypes.

CONCLUSIONS

Three separate genetic loci for the long QT syndrome including mutations in two cardiac ionic channel genes were associated with different phenotypic T-wave patterns on the ECG. This study provides insight into the influence of genetic factors on ECG manifestations of ventricular repolarization.

Recent advances in understanding the molecular mechanisms of the long QT syndrome

Competing theories to explain the congenital long QT syndrome have included an imbalance in sympathetic innervation of the heart or a defect in repolarizing ion currents. Recent studies have identified at least four chromosomal loci at which mutations cause the congenital long QT syndrome in different families. The specific genes mutated in affected individuals have been identified at two of these loci, and both encode cardiac ion channels. The affected genes are SCN5A, the cardiac sodium channel gene, and HERG, whose protein product likely underlies IKr, the rapidly activating delayed rectifier. Thus, currently available evidence indicates that the congenital long QT syndrome is a primary disease of cardiac ion channels. Abnormalities in either inward or outward currents can cause the disease. Ongoing studies are evaluating the function of the mutant ion channels and the relationship between individual mutations and the clinical manifestations of the syndrome.

Molecular analysis at the Harvey Ras-1 gene in patients with long QT syndrome

Patients with long QT syndrome (LQTS; MIM 1921500) frequently suffer from syncope and are threatened by sudden cardiac death due to ventricular arrhythmias, typically of the torsade de pointes type. Initial progress in revealing the molecular basis of the disease was made by the observation of genetic linkage of the disease locus to the Harvey Ras-1 gene (HRAS 1) on chromosome 11p15.5. More recently loci on chromosomes 3, 4, and 7 have also been found to be linked to LQTS, thus demonstrating heterogeneity in the causes for this disease. The present study performed sequence analysis on the HRAS 1 gene in patients with congenital and acquired LQTS to determine the frequency of HRAS 1 mutations in patients with this disease. In neither group were no mutations identified in the coding regions or in the splice donor and acceptor sites. Alleles characterized by a T to C transition in exon 1 and an insertion/deletion polymorphism upstream of exon 1 showed no significant difference in their frequencies between LQTS patients and normal controls. No quantitative influence of the such characterized genotypes on the QT duration was observed. These results demonstrate that structural mutations in the HRAS 1 gene are not a frequent cause of LQTS. Also, since there was no association of different alleles at the HRAS 1 locus with changes in QT duration, it appears unlikely that this gene is a major contributor to this disease.

Blunt impact to the chest leading to sudden death from cardiac arrest during sports activities

BACKGROUND

Sudden death from cardiac arrest in a young person may occur during sports play after a blunt blow to the chest in the absence of structural cardiovascular disease or traumatic injury (cardiac concussion or commotio cordis). We studied the clinical features of this apparently uncommon but important phenomenon.

METHODS

We identified cases from the registries of relevant agencies and organizations, as well as newsmedia accounts, and developed a clinical profile of 25 children and young adults, 3 to 19 years of age.

RESULTS

Each victim collapsed with cardiac arrest immediately after an unexpected blow to the chest, which was usually inflicted by a projectile (such as a baseball or hockey puck). Incidents took place during organized competitive sports in 16 cases and in recreational settings at home, at school, or on the playground in 9. In each instance, the impact to the chest was not judged to be extraordinary for the sport involved and did not appear to have sufficient force to cause death. Twelve victims collapsed virtually instantaneously on impact, whereas 13 remained conscious and physically active for a brief time before cardiac arrest. Cardiopulmonary resuscitation was administered within about three minutes to 19 victims, but normal cardiac rhythm could be restored in only 2 (both incurred irreversible brain damage and died shortly thereafter). Seven victims (28 percent) were wearing some form of protective chest padding.

CONCLUSIONS

We speculate that most sudden deaths related to impact to the chest (not associated with traumatic injury) are due to ventricular dysrhythmia induced by an abrupt, blunt precordial blow, presumably delivered at an electrically vulnerable phase of ventricular excitability. This profile of blunt chest impact leading to cardiac arrest adds to our understanding of the range of causes of sudden death on the athletic field and may help in the development of preventive measures.

Genetic approaches to cardiovascular disease. Supravalvular aortic stenosis, Williams syndrome, and long-QT syndrome

BACKGROUND

Although family history can be an important risk factor for cardiovascular disease, relatively little is known about the nature of specific genetic risk factors. One approach to this problem is to identify and characterize genes responsible for inherited disorders in the hope that this information will also provide mechanistic insight into common forms of cardiovascular disease.

METHODS AND RESULTS

Over the last decade, it has become possible to identify genes that cause human disease by use of the techniques of molecular genetics, specifically genetic linkage analysis, positional cloning, and mutational analyses. We have used these techniques to study three inherited cardiovascular disorders: supravalvular aortic stenosis, Williams syndrome, and long-QT syndrome. We have discovered that the vascular pathology of supravalvular aortic stenosis and Williams syndrome results from mutations involving the elastin gene on chromosome 7q11.23. These mutations include intragenic deletions, translocations, and complete deletion of the elastin gene, suggesting that a quantitative reduction in elastin during vascular development is pathogenically important. To date, only the elastin gene has proved important for supravalvular aortic stenosis. By contrast, genetic linkage analyses in families with long-QT syndrome indicate that at least four distinct genes can cause this disorder. We have identified three LQT loci: LQT1 on chromosome 11p15.5, LQT2 on 7q35-36, and LQT3 on 3p21-24. Recently, we demonstrated that mutations in a putative cardiac potassium channel gene, HERG, are responsible for the chromosome 7-linked form of long-QT syndrome, whereas mutations in the cardiac sodium channel gene SCN5A cause the chromosome 3-linked form of this disorder. HERG mutations and potassium channel biophysics suggest a dominant-negative molecular mechanism and reduced repolarization currents. By contrast, SCN5A mutations probably cause subtle alterations of cardiac sodium channel function and prolonged depolarizing currents.

CONCLUSIONS

Molecular genetic analyses of long-QT syndrome, supravalvular aortic stenosis, and Williams syndrome have begun to unravel the mechanisms underlying these inherited disorders. Rapid genetic testing for Williams syndrome is now available using a simple cytogenetic test, fluorescence in situ hybridization, but additional work will be required for long-QT syndrome and autosomal-dominant supravalvular aortic stenosis. Improved diagnosis and mechanistic understanding of these disorders should lead to rational treatment and prevention.

A molecular basis for cardiac arrhythmia: HERG mutations cause long QT syndrome

To identify genes involved in cardiac arrhythmia, we investigated patients with long QT syndrome (LQT), an inherited disorder causing sudden death from a ventricular tachyarrythmia, torsade de pointes. We previously mapped LQT loci on chromosomes 11 (LQT1), 7 (LQT2), and 3 (LQT3). Here, linkage and physical mapping place LQT2 and a putative potassium channel gene, HERG, on chromosome 7q35-36. Single strand conformation polymorphism and DNA sequence analyses reveal HERG mutations in six LQT families, including two intragenic deletions, one splice-donor mutation, and three missense mutations. In one kindred, the mutation arose de novo. Northern blot analyses show that HERG is strongly expressed in the heart. These data indicate that HERG is LQT2 and suggest a likely cellular mechanism for torsade de pointes.

SCN5A mutations associated with an inherited cardiac arrhythmia, long QT syndrome

Long QT syndrome (LQT) is an inherited disorder that causes sudden death from cardiac arrhythmias, specifically torsade de pointes and ventricular fibrillation. We previously mapped three LQT loci: LQT1 on chromosome 11p15.5, LQT2 on 7q35-36, and LQT3 on 3p21-24. Here we report genetic linkage between LQT3 and polymorphisms within SCN5A, the cardiac sodium channel gene. Single strand conformation polymorphism and DNA sequence analyses reveal identical intragenic deletions of SCN5A in affected members of two unrelated LQT families. The deleted sequences reside in a region that is important for channel inactivation. These data suggest that mutations in SCN5A cause chromosome 3-linked LQT and indicate a likely cellular mechanism for this disorder.

Kinetics of cycle length dependence of ventricular repolarization: effect of autonomic blockade

INTRODUCTION

Beat-to-beat adaptation of ventricular repolarization duration to cardiac cycle length and autonomic activity has not been previously characterized in the spontaneously beating human heart.

METHODS AND RESULTS

The ECG of 14 healthy subjects was recorded from the supine and upright positions. Autonomic blockade was accomplished by atropine and propranolol. RR and RT intervals were measured by a computer algorithm, and the impulse response (h) from RR to RT computed. In the supine position the maximal adjustment of the RT interval occurred in the first beat following a change in cycle length (hpeak = 17.8 +/- 1.6 msec/sec), but continued to be detectable for 3.8 seconds (2.9-4.7 sec). Propranolol attenuated the peak impulse response to 15.8 +/- 4.0 msec/sec (P = NS). In the standing position the peak impulse response was increased to 25.2 +/- 5.0 msec/sec (P = 0.004 vs supine), and the impulse response duration (hdur) shortened to 1.4 seconds (1.3-1.6). This was reversed by beta blockade (hpeak = 10.7 +/- 3.6 [P = 0.005 vs standing]; hdur = 5.5 sec [4.8-6.1]). Parasympathetic and combined autonomic blockade resulted in too little residual heart rate variability to estimate the impulse response accurately. The slope of the regression of delta RT and delta RR in the supine position was 0.0177 +/- 0.0016, which was closely correlated with the peak impulse response (r = 0.91).

CONCLUSIONS

System identification techniques can assist in characterizing the cycle dependence of ventricular repolarization and may provide new insights into conditions associated with abnormal repolarization.

Catecholaminergic polymorphic ventricular tachycardia in children. A 7-year follow-up of 21 patients

BACKGROUND

Primary ventricular tachyarrhythmias are rarely seen in children. Among them, catecholaminergic polymorphic ventricular tachycardia has a poor spontaneous outcome. Its diagnosis is often delayed after the first symptoms, which is unacceptable because treatment with the appropriate beta-blocker prevents sudden death.

METHODS AND RESULTS

We observed 21 children (mean +/- SD age, 9.9 +/- 4 years) at the time of the diagnosis who had no structural heart disease and a normal QT interval on routine ECG. They were referred for stress- or emotion-induced syncope related to ventricular polymorphic tachyarrhythmias. The arrhythmia, consisting of isolated polymorphic ventricular extrasystoles followed by salvoes of bidirectional and polymorphic tachycardia susceptible to degeneration into ventricular fibrillation, was reproducibly induced by any form of increasing adrenergic stimulation. There was a familial history of syncope or sudden death in 30% of our patients. On receiving therapy with the appropriate beta-blocker, the patients' symptoms and polymorphic tachyarrhythmias disappeared. During a mean follow-up period of 7 years, three syncopal events and two sudden deaths occurred, probably due to treatment interruption.

CONCLUSIONS

The entity of adrenergic-dependent, potentially lethal tachyarrhythmia with no structural heart disease deserves to be individualized. It may form a variant of the congenital long QT syndrome in which the ECG marker is lacking; this primary ventricular arrhythmia must be looked for in a pediatric patient with stress- or emotion-induced syncope because only beta-blocking therapy can prevent sudden death and therefore must be given for the patient's lifetime.

Heterogeneity in the inherited long QT syndrome

Heterogeneity is present in the inherited long QT syndrome and affects the diagnosis of patients. The evidence of genetic heterogeneity is clear, with at least five genetic loci responsible for the syndrome. Phenotypic heterogeneity is less well defined, but differences in QT prolongation, T wave morphology, and the risk and frequency of syncope and sudden death are very likely. Of particular importance, it is likely that there are differences in the molecular pathophysiology of the syndrome, which are dependent on the genetic substrate present. Elucidation of the specific molecular physiology of each of the genetic subtypes of long QT syndrome will not only allow precise diagnosis and, potentially, treatment of patients with the syndrome, but will enhance our understanding of the pathophysiology of arrhythmias in general, which will extend the benefit of more precise therapy to many patients in addition to those with the long QT syndrome.

A new form of long QT syndrome associated with syndactyly

OBJECTIVES

The purpose of this study was to characterize a possible association between long QT syndrome and syndactyly.

BACKGROUND

Long QT syndrome causes syncope and sudden death from ventricular arrhythmias. Syndactyly is a developmental disorder that causes webbing of the hands and feet. Both disorders can be inherited as isolated, autosomal dominant traits, but an association between them has not been established.

METHODS

We identified three children with long QT syndrome, atrioventricular (AV) block and simple syndactyly. Phenotypic and laboratory data were obtained from families, attending physicians and medical records.

RESULTS

All patients had bilateral cutaneous syndactyly and were diagnosed with long QT syndrome within the 1st 2 years of life. Structural heart disease, particularly a patent ductus arteriosus, was present in all three patients. Analysis of electrocardiograms showed marked prolongation of the QT intervals with rate-corrected QT intervals of 633, 628 and 680 ms, respectively. Transient AV block was also noted. Two of the three children died suddenly despite treatment with beta-adrenergic blocking agents and permanent pacing.

CONCLUSIONS

We postulate that these children have a new form of long QT syndrome associated with syndactyly and a high risk of sudden death. The association of syndactyly with long QT syndrome may provide insight into the mechanisms underlying both disorders. Patients with syndactyly should be evaluated for the presence of long QT syndrome, and if it is found, aggressive treatment may be warranted.

Use of the rate-corrected JT interval for prediction of repolarization abnormalities in children

A prolonged rate-corrected QT interval (QTc) may be associated with an increased risk of developing ventricular arrhythmias and sudden death, particularly in patients with the hereditary long QT syndrome (LQTS), myocardial ischemia, or antiarrhythmic medication toxicity. It is known that there are some patients with LQTS who sometimes have a borderline or normal QTc (< or = 0.45 second). Although the QTc has been the standard measurement of ventricular repolarization, it includes both depolarization and repolarization and may not always be a sensitive indicator of the type of repolarization abnormalities seen in LQTS. Intraventricular conduction abnormalities complicate evaluation of the QTc interval. The rate-corrected JT interval (JTc) is a more accurate measurement of ventricular repolarization, and therefore may be a more sensitive means of assessing abnormalities. The QTc on a resting electrocardiogram was determined in 40 patients with LQTS and in 31 patients with right bundle branch block after tetralogy of Fallot repair. These were compared with 1,000 age-matched control subjects. The right bundle branch block group had normal JT and JTc measurements, despite having prolonged QT and QTc intervals compared with controls. The JTc identified 85% of patients affected with LQTS compared with only 58% identified using only the QTc as a marker for the syndrome. The JTc is a more specific measurement of ventricular repolarization than the QTc by eliminating QRS duration variability. It appears to be a more sensitive predictor of repolarization abnormalities, and may be helpful in identifying patients with LQTS who have borderline or normal QTc measurements on resting electrocardiograms.

Evidence of genetic heterogeneity in Romano-Ward long QT syndrome. Analysis of 23 families

BACKGROUND

The Romano-Ward long-QT Syndrome (LQTS) is an autosomal dominant inherited trait characterized by prolonged QT interval on ECG, life-threatening arrhythmias, syncope, and sudden death in affected individuals. A gene responsible for this disorder has been shown to be linked to the Harvey ras-1 locus (H-ras-1) DNA marker on the short arm of chromosome 11 (11p) in 7 families. The purpose of this study was to determine, by analyzing 23 families with LQTS for linkage to chromosome 11p, whether evidence exists for more than one gene causing LQTS (ie, locus heterogeneity).

METHODS AND RESULTS

Twenty-three families (262 family members) were clinically evaluated using medical histories, ECGs, and Holter recordings. Each corrected QT interval (QTc) were determined using Bazett's formula. Blood for DNA extraction and cell line immortalization was obtained after informed consent. Southern blotting and polymerase chain reaction were performed, and linkage analysis carried out using the LINKAGE computer program (v 5.03). Genetic heterogeneity was determined using the HOMOG 2 (v 2.51) computer program. Twenty-three families were studied for evidence of linkage to chromosome 11p using the H-ras-1 locus probe pTBB-2 and multiple flanking markers, including tyrosine hydroxylase (TH). Two-point linkage analysis using pTBB-2 and TH markers was consistent with linkage in 15 of 23 families, with the maximum single-family LOD score of +3.038 occurring at theta = 0. However, 8 of 23 families had negative LOD scores, with the values in 4 families being less than -2 at theta = 0, consistent with exclusion of linkage. Analysis with the HOMOG program was consistent with genetic heterogeneity (P < .0001). Multipoint linkage data using pTBB-2 and TH were also examined for evidence of heterogeneity. HOMOG analysis of multipoint LOD scores from 100 cM surrounding the H-ras-1 locus also supported heterogeneity (P < .001).

CONCLUSIONS

In the 23 families with LQTS analyzed for linkage to the H-ras-1 locus on chromosome 11p15.5, 15 of 23 families had LOD scores consistent with linkage. The remaining 8 of 23 families had negative LOD scores, 4 of which were definitively excluded from linkage. Thus, genetic heterogeneity is definitively (P < .001) demonstrated for this disorder.

Genetics and molecular biology of the inherited long QT syndrome

Two forms of the inherited long QT syndrome have been known for many years: the autosomal recessive Jervell and Lange-Nielsen form and the autosomal dominant Romano-Ward form. A gene marker at the 11p 15.5 locus has been identified for some, but not all, families with the autosomal dominant form, but as yet the gene has not been identified. It is apparent that mutations of at least four genes, and possibly more, can cause the syndrome. The molecular biology of the syndrome is not yet clarified, but abnormalities of ion channel function are likely, particularly the potassium delayed rectifier current. Proposals for the pathophysiology include an abnormality of a G protein which controls ion channel and adrenergic pathway function, as well as a disturbance of the sympathetic nervous system. The identification of the abnormal gene(s) and the gene products will provide precise information on the molecular physiology of the syndrome.

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.

Two long QT syndrome loci map to chromosomes 3 and 7 with evidence for further heterogeneity

Cardiac arrhythmias cause sudden death in 300,000 United States citizens every year. In this study, we describe two new loci for an inherited cardiac arrhythmia, long QT syndrome (LQT). In 1991 we reported linkage of LQT to chromosome 11p15.5. In this study we demonstrate further linkage to D7S483 in nine families with a combined lod score of 19.41 and to D3S1100 in three families with a combined score of 6.72. These findings localize major LQT genes to chromosomes 7q35-36 and 3p21-24, respectively. Linkage to any known locus was excluded in three families indicating that additional heterogeneity exists. Proteins encoded by different LQT genes may interact to modulate cardiac repolarization and arrhythmia risk.