Calculation of QTc duration and variability in the presence of sinus arrhythmia

Normal and abnormal QT interval duration and its changes in preadolescents and adolescents practicing sport

Aims

Twelve-lead electrocardiogram (ECG) is an established tool in the evaluation of athletes, providing information about life-threatening cardiovascular diseases, such as long QT syndrome. However, the interpretation of ECG is sometimes challenging in children, particularly for the repolarization phase. The aim of this prospective, longitudinal study was to determinate the distribution of QT interval in children practicing sport and to evaluate changes in QT duration overtime.

Methods and results

A population of 1473 preadolescents practising sport (12.0 ± 1.8 years, 7–15 years) was analysed. Each athlete was evaluated at baseline, mid-term, and end of the study (mean follow-up: 3 ± 1 years). QT interval was corrected with Bazett (B) and Fridericia (F) formulae. At baseline QT interval corrected with the Bazett formula (QTcB) was 412 ± 25 ms and QT interval corrected with the Fridericia formula (QTcF) 387 ± 21 ms, with no changes during follow-up. Ten children (0.68%) had an abnormal QTc. In those with QTcB and QTcF ≥480 ms, QTc duration persisted abnormal during the follow-up and they were disqualified. Conversely, children with 460 ms < (QTcB) <480 ms had a normal QTc interval at the end of the study. These children had also a normal QTcF. Mean difference in the calculation of QT between the two formulae was 25 ± 11 ms (P < 0.0001). For resting heart rate (HR) ≥82 b.p.m., QTcF was independent from HR contrary to QTcB.

Conclusion

Normal QTc interval does not change over time in preadolescents. A minority of them has a QTc ≥480 ms; in these subjects, QTc interval remains prolonged. The use of Bazett and Fridericia correction formulae is not interchangeable and the Fridericia correction should be preferred in preadolescents with a resting HR ≥82 b.p.m.

Mid-Term Follow-up of School-Aged Children With Borderline Long QT Interval

BACKGROUND

There are no definitive diagnostic criteria or follow-up strategies for long QT syndrome (LQTS) in children with a borderline long QT interval (b-LQT).Methods and Results:We retrospectively evaluated the clinical course, genetic testing results, corrected QT interval (QTc), and LQTS score of 59 school-aged children (5-18 years old) with a b-LQT (400≤QTc<500 ms). Syncope, but neither aborted cardiac arrest nor sudden cardiac death, occurred in 2 patients during the follow-up (6±3.4 years) with LQTS scores ≥4.5 points. The genetic testing results were positive in 92%, 57%, and 67% of patients with high, intermediate, and low probabilities of LQTS, respectively. The maximum and mean QTc during the follow-up significantly differed among the categories with a probability of LQTS, but not the minimum QTc. However, the QTc at rest and at the recovery point after exercise stress testing dramatically changed at the last follow-up. Consequently, the probability of LQTS changed in half of the patients.

CONCLUSIONS

The LQTS score is a reasonable indicator for evaluating school-aged children with a b-LQT, and patients with a low LQTS score appear to be at low risk for cardiac events. However, the LQTS score can change during follow-up. Therefore, when there is doubt or concern for patients with a b-LQT, it is preferable to continue following them. Guidelines on follow-up strategies are desired for b-LQT.

The prevalence and diagnostic/prognostic utility of sinus arrhythmia in the evaluation of congenital long QT syndrome

BACKGROUND

Congenital long QT syndrome (LQTS) affects 1 in 2,500 people and can cause syncope and sudden death. Sinus arrhythmia (SA) is nonpathologic baseline respiratory variation of the RR interval.

OBJECTIVE

This study sought to determine the frequency of SA and its clinical significance among patients with LQTS.

METHODS

We performed an institutional review board-approved retrospective review of all patients (N = 571) evaluated in our LQTS clinic from 7/2000 to 3/2008 diagnosed with LQTS (N = 281) or dismissed as otherwise normal (N = 290). Blinded to diagnosis, the first available electrocardiogram for each patient was examined to quantitate RR interval variability.

RESULTS

Overall, 151 of 281 patients (54%) with LQTS (159 female patients, average age 21.8 ± 16.5 years, average QTc 466 ± 43 ms) had SA with an average RR variability of 13% ± 8% compared with 201 of 290 (69%) patients dismissed as normal (178 female patients, average age 21.7 ± 16 years, average QTc 424 ± 30 ms) who demonstrated SA with RR variability of 16% ± 10% (P < .0001). These differences remained significant when patients on concurrent beta-blocker therapy were excluded (P < .001). SA was least common in LQT3 (23%) compared with LQT1 (61%, P < .005) and LQT2 (51%, P = .055). Patients presenting with torsades de pointes or aborted cardiac arrest had lower RR variability (10% ± 7%, P < .03).

CONCLUSION

SA frequency and magnitude of RR variability was lower among patients with LQTS compared with those patients dismissed as otherwise normal. This attenuation in RR interval variability remained when patients on beta-blocker therapy were excluded. Although the presence/absence of sinus arrhythmia is of little diagnostic value due to cohort overlap, LQTS patients with negligible RR interval variation may be at higher risk.

QTc: how long is too long?

Congenital long QT syndrome (LQTS) affects an estimated 1 in 2500 people and typically presents with syncope, seizures or sudden death. Whereas someone exhibiting marked prolongation of the QT interval with QTc exceeding 500 ms who was just externally defibrillated from torsades de pointes while swimming poses negligible diagnostic challenge as to the unequivocal probability of LQTS, the certainty is considerably less for the otherwise asymptomatic person who happens to host a QTc value coined "borderline" (QTc > or = 440 ms). Although a normal QT interval imparts a much lower risk of life-threatening events, it does not preclude a patient from nevertheless harbouring a potentially lethal LQTS-causing genetic mutation. Indeed, genetic testing exerts significant diagnostic, prognostic and therapeutic implications. However, the 12-lead ECG remains the universal initial diagnostic test in the evaluation of LQTS and is subject to miscalculation, misinterpretation and mishandling. This review discusses the components of accurate QTc measurement and diagnosis, re-examines what is known about factors affecting QT interval measurement, and clarifies current recommendations regarding diagnosis of so-called "borderline" QT interval prolongation. The current guideline recommendations for the athlete with LQTS are also summarised.

Autonomic nervous system functions in children with breath-holding spells and effects of iron deficiency

AIM

To analyse the activity of the autonomic nervous system during breath-holding spells, we assessed the ECG changes, including ventricular repolarization parameters before and during the spell. We also analysed the effects of iron deficiency on these ECG parameters.

METHODS

The study group consisted of 37 children with breath-holding spells (30 cyanotic, 7 pallid) (mean age+/-SD: 12.9+/-10.8 mo). Twenty-six healthy children (mean age+/-SD: 14.4+/-8.6 mo) served as a control group. All patients and controls had standard 12-lead simultaneous surface ECG. All patients had ECG recordings during at least one severe breath-holding spell obtained by "event recorder". Traces obtained by "event recorder" were analysed in terms of mean heart rate and the frequency and duration of asystole during the spell.

RESULTS

Respiratory sinus arrhythmia on standard ECGs and asystole frequency during spells were higher in patients with pallid breath-holding spells. Patients with iron deficiency had a lower frequency of respiratory sinus arrhythmia and prolonged asystole time during the spell. There was no difference in terms of ventricular repolarization parameters (QT/QTc intervals and QT/QTc dispersions) between patients and controls and between patient subgroups (cyanotic versus pallid).

CONCLUSION

These results confirmed the presence of autonomic dysregulation in children with breath-holding spells. Iron deficiency may have an impact on this autonomic dysregulation. Ventricular repolarization was unaffected in patients with breath-holding spells.

Mutations in Kir2.1 Cause the Developmental and Episodic Electrical Phenotypes of Andersen's Syndrome

Andersen's syndrome is characterized by periodic paralysis, cardiac arrhythmias, and dysmorphic features. We have mapped an Andersen's locus to chromosome 17q23 near the inward rectifying potassium channel gene KCNJ2. A missense mutation in KCNJ2 (encoding D71V) was identified in the linked family. Eight additional mutations were identified in unrelated patients. Expression of two of these mutations in Xenopus oocytes revealed loss of function and a dominant negative effect in Kir2.1 current as assayed by voltage-clamp. We conclude that mutations in Kir2.1 cause Andersen's syndrome. These findings suggest that Kir2.1 plays an important role in developmental signaling in addition to its previously recognized function in controlling cell excitability in skeletal muscle and heart.

Normal values of signal-averaged electrocardiographic parameters and QT dispersion in infants and children

Ventricular late potentials, and dispersion of the QT interval, are markers for risk of ventricular arrhythmias. Normal values for these parameters are well established in adults, but may not apply for children. This study has investigated the age dependency of signal averaged electrocardiographic parameters and QT dispersion in 111 normal children aged from 5 days to 16 years. The results indicate that parameters change with age: duration of filtered QRS and low amplitude (< 40 microV) terminal signal increase with age, especially in the younger patients. Filtered QRS is 88.9 +/- 7.87 ms in infants, and increases to 108.7 +/- 8.51 in teenagers (p<0.001). Low amplitude terminal signals are 17.0 +/- 3.44 ms in infants, and 24.5 +/- 5.64 ms in teenagers (p<0.001). Root mean square of the last 40 ms decreases with age, but remains stable after the age of 10 years (122.4 +/- 33.30 microV in infants, 60.9 +/- 31.27 in teenagers, p<0.001). QT dispersion, on the other hand, does not change significantly with age. The mean value for the whole group is 36 +/- 13.7 ms. A weak but significant correlation exists between QT dispersion and filtered QRS. Thus, age must be taken into consideration when interpreting signal-averaged electrocardiograms, but not when measuring QT dispersion.

Andersen syndrome autosomal dominant in three generations

Andersen syndrome is a rare entity and comprises potassium sensitive periodic paralysis, ventricular arrhythmia, and an unusual facial appearance; syncope and sudden death have also been reported. The recognition of the characteristic face permits an early diagnosis in order to detect the severe systemic manifestations that are associated with this syndrome. The genetic defect is not linked to any other form of potassium sensitive periodic paralysis nor is it related to that of the long QT syndrome; nevertheless, a prolonged QT interval can be detected in a significant proportion of the cases. Sixteen cases of this syndrome have been described. We report on a three-generation family with 10 affected members. To our knowledge, this is the largest number of cases reported in one family. We noted some additional minor anomalies such as broad forehead and malar hypoplasia. Our patients had variable expression in the classical triad and of the severity of the systemic manifestations. Five of 8 affected studied members did not have a long QTc, which has been suggested as a constant finding in this syndrome.

Dispersion of QT and QTc interval in healthy children, and effects of sinus arrhythmia on QT dispersion

OBJECTIVE

To determine the normal values of QT and QTc dispersion and the effects of sinus arrhythmia on QT dispersion in healthy children.

PATIENTS AND SETTING

The study was carried out in a university hospital on 372 local schoolchildren (200 male, 172 female), aged seven to 18 years.

METHODS

The QT and preceding RR intervals of at least one sinus beat were measured manually in a range of nine to 12 leads on standard 12 lead surface ECGs. The corrected QT interval was computed by the method of Bazett. Dispersion of QT and QTc were defined as (1) the difference between the maximum and minimum QT and QTc intervals occurring in any of the 12 leads (QTD and QTcD), (2) the standard deviation of the QT and QTc interval in the measurable leads (QT-SD and QTc-SD).

RESULTS

There was no significant difference in QT, QTc, and RR dispersion between girls and boys. Overall 53% of children had sinus arrhythmia. Although QTD and QT-SD were not affected by sinus arrhythmia, both QTcD and QTc-SD were significantly greater in children with sinus arrhythmia than in those without (QTcD: 52.9 (17.4) v 40.9 (13.1); QTc-SD: 17.5 (5.9) v 13.2 (4.0); p < 0.001).

CONCLUSIONS

As calculation of QTc dispersion is affected by sinus arrhythmia, which is common in childhood, we suggest that QT dispersion should not be corrected for heart rate in children.

Andersen's syndrome: a distinct periodic paralysis

A previous study of 4 patients defined Andersen's syndrome (AS) as a triad of potassium-sensitive periodic paralysis, ventricular dysrhythmias, and dysmorphic features. AS appears to be distinct in terms of its genetic defect from the alpha-subunit of skeletal muscle sodium channel and the cardiac potassium channel responsible for most long QT syndromes (LQT1). We studied 11 additional patients with AS from 5 kindreds. Spontaneous attacks of paralysis were associated with hypokalemia, normokalemia, or hyperkalemia. All 11 patients had similar dysmorphic features. The QT interval was prolonged in all patients although only 4 were symptomatic. Genetic linkage studies excluded linkage to the alpha-subunit of the skeletal muscle sodium channel and to four distinct LQT loci. In addition, none of the common dihydropyridine receptor mutations responsible for hypokalemic periodic paralysis were present. We conclude that (1) AS is a genetically unique channelopathy affecting both cardiac and skeletal membrane excitability, (2) attacks of paralysis may be either hypokalemic or hyperkalemic, (3) a prolonged QT interval is an integral feature of this syndrome, and (4) a prolonged QT interval may be the only sign in an individual from an otherwise typical AS kindred. This may be confused with more common, potentially lethal LQT syndromes.

Efficacy and proarrhythmia with the use of d,l-sotalol for sustained ventricular tachyarrhythmias

This study prospectively evaluated the clinical efficacy, the incidence of torsades de pointes, and the presumable risk factors for torsades de pointes in patients treated with d,l-sotalol for sustained ventricular tachyarrhythmias. Eighty-one consecutive patients (54 with coronary artery disease, and 20 with dilated cardiomyopathy) with inducible sustained ventricular tachycardia or ventricular fibrillation received oral d,l-sotalol to prevent induction of the ventricular tachyarrhythmia. During oral loading with d,l-sotalol, continuous electrocardiographic (ECG) monitoring was performed. Those patients in whom d,l-sotalol prevented induction of ventricular tachycardia or ventricular fibrillation were discharged with the drug and followed up on an outpatient basis for 21 +/- 18 months. Induction of the ventricular tachyarrhythmia was prevented by oral d,l-sotalol in 35 (43%) patients; the ventricular tachyarrhythmia remained inducible in 40 (49%) patients; and two (2.5%) patients did not tolerate even 40 mg of d,l-sotalol once daily. Four (5%) patients had from torsades de pointes during the initial oral treatment with d,l-sotalol. Neither ECG [sinus-cycle length (SCL), QT or QTc interval, or U wave] nor clinical parameters identified patients at risk for torsades de pointes. However, the oral dose of d,l-sotalol was significantly lower in patients with torsades de pointes (200 +/- 46 vs. 328 +/- 53 mg/day; p = 0.0017). Risk factors associated with the development of torsades de pointes were the appearance of an U wave (p = 0.049), female gender (p = 0.015), and significant dose-corrected changes of SCL, QT interval, and QTc interval (p < 0.05). During follow-up, seven (20%) patients had a nonfatal ventricular tachycardia recurrence, and two (6%) patients died suddenly. One female patient with stable cardiac disease had recurrent torsades de pointes after 2 years of successful treatment with d,l-sotalol. Torsades de pointes occurred early during treatment even with low doses of oral d,l-sotalol. Pronounced changes in the surface ECG (cycle length, QT, and QTc) in relation to the dose of oral d,l-sotalol might identify a subgroup of patients with an increased risk for torsades de pointes. Other ECG parameters before the application of d,l-sotalol did not identify patients at increased risk for torsades de pointes. Recurrence rates of ventricular tachyarrhythmias are high despite complete suppression of the arrhythmia during programmed stimulation. Therefore programmed electrical stimulation in the case of d,l-sotalol seems to be of limited prognostic value.

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.