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Abstract
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) v40.9 (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.
- QT dispersion
- heart rate
- children
- sinus arrhythmia
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QT interval dispersion is an indirect measure of the heterogeneity of ventricular repolarisation.1 ,2 A potential application of this interlead difference from standard 12 lead electrocardiograms (ECGs) was first proposed by Dayet al in 1990.3 Since that time, this concept has been applied to ECGs in various clinical populations ranging from healthy volunteers4 ,5 to patients with the long QT syndrome,6 survivors of acute myocardial infarction,7 and patients with hypertrophic8 and dilated cardiomyopathies,9as well as many other well defined groups.10-12 As there are no published data on normal values of QT interval dispersion in the paediatric age group, we planned this study to determine the normal values of QT and QTc dispersion in healthy children.
Methods
Electrocardiograms were obtained from 372 schoolchildren (200 male, 172 female) seven to 18 years of age (mean (SD):12.4 (2.6) years). Children having acute or chronic illness by either history or physical examination were excluded. The study was approved by the ethics committee of our institution.
Measurements were carried out from standard 12 lead ECGs recorded at a speed of 25 mm/s at rest. A three channel electrocardiographic recorder (Hewlett Packard, model 4745 A; Hewlett Packard Inc, Camas, Washington, USA) was used. The QT and preceding RR intervals of at least one sinus beat (range one to three) were measured in a range of nine to 12 leads, and the mean QT and RR intervals were calculated. The corrected QT interval was calculated by the method of Bazett (QTc = QT/(RR)).13 The mean QT and the mean RR were used to calculate the mean QTc for each lead. Heart rate was derived from the mean of the RR intervals. The QT and RR intervals were measured manually with calipers by a single observer. Repeatability of the QT and QTc measurements was tested by a second observer, who measured the same intervals of lead II in 50 randomly selected electrocardiograms. QT intervals were measured from the onset of the QRS complex to the end of the T wave. The end of the T wave was defined as the point of return to the isoelectric line. When a U wave was present, the QT interval was measured to the nadir of the curve between the T and U wave.10
Dispersion of the QT and QTc were defined in two ways: (1) the difference between the maximum and minimum QT and QTc intervals occurring in any of the 12 leads (QTD, QTcD), and (2) the standard deviation of the QT and QTc interval in the leads that could be measured (QT-SD, QTc-SD).14 RR variation was also calculated in a same manner (RRD and RR-SD, respectively). Sinus arrhythmia was defined as RRD of ⩾ 80 ms, without significant change in P wave or frontal plane axis.15
All data are expressed as mean (SD). Paired and unpaired Student’st tests were used where appropriate. Correlations were assessed by Pearson’s coefficients. A two tailed p value < 0.05 was considered significant.
Results
All analyses were done by sex, age, heart rate, and the presence of sinus arrhythmia. Heart rate and ECG data of normal children are shown in table 1. The QT intervals were similar for boys and girls. Both the mean and the 95th centile QTc intervals were significantly greater for girls than for boys. There was no significant difference in QT, QTc, and RR dispersion parameters between boys and girls. We found that neither QT nor QTc dispersion parameters were correlated with age. There was a significant positive correlation between RR and QT intervals (r = 0.78). We found that QTD and QT-SD did not vary significantly with RR, RRD, or RR-SD, but both QTcD and QTc-SD varied significantly with RRD and RR-SD (table 2). Sinus arrhythmia was present in 198 children (53%). QTD and QT-SD were not affected by sinus arrhythmia, but both QTcD and QTc-SD were significantly greater in children with sinus arrhythmia than those without (table 3).
Heart rate, RR, QT, and QTc intervals, and QT, QTc, and RR dispersion parameters for normal boys and girls
Correlations of QT and QTc dispersion parameters with age and other ECG measurements
ECG parameters in children with and without sinus arrhythmia
In the 50 selected ECGs, the mean of QT intervals measured, and the mean of QTc intervals calculated, were similar between observer 1 and observer 2 (QT: 334.6 (26.1) ms v 335.6 (23.8) ms, p = 0.5, r = 0.92; QTc: 391.9 (25.4) ms v 393.7 (27.7) ms, p = 0.31,r = 0.90).
Discussion
In a large normal adult population (n = 1555), Macfarlaneet al showed that the upper limit of QT dispersion using an automated method was 40 ms.16 In addition, data for matched controls in most studies including adults suggest that QTD and QTcD in normal subjects are usually in the range of 30–50 ms and 40–60 ms, respectively4-6 ,9 ,10 ,12 As far as is known, there are no published limits of normal QT interval dispersion in the paediatric age group, and it is possible that the data in our study are the first in which such results are presented. QT interval dispersion results of our population study, which included a relatively large number of children, are correlated with adults results. Dispersion of QT and QTc intervals were found to be 29.9 (10.2) ms and 47.3 (16.6) ms, respectively. We also used standard deviation of QT, QTc, and RR intervals as a measure of dispersion, to minimise probable errors resulting from non-measurable leads. We found that QT-SD, QTc-SD, and RR-SD correlated significantly with the corresponding values of QTD, QTcD, and RRD.
In a recent study, Pearl showed that QTc intervals were significantly greater for girls than for boys in the 14 to 18 year age range.17 We also showed this sex difference in our study group.
In a small number of healthy adult subjects (n = 25), Feiet al showed that the difference in the QT dispersion between men and women was significant.4However, we did not find that sex had a significant impact on QT or QTc dispersion parameters in children. Macfarlane et al also showed that there was no difference between men and women for QT interval dispersion.16 In our study, age and heart rate were not found to have an effect on QT and QTc dispersion in children either.
Nearly half of the children in our study group had sinus arrhythmia. We therefore determined whether sinus arrhythmia had an effect on QT and QTc dispersion in children. Although QT dispersion parameters (QTD and QT-SD) did not vary significantly with RR variation (for example, with RRD and RR-SD), we observed that QTc dispersion parameters did vary significantly with RR dispersion. As a result, children with sinus arrhythmia not surprisingly have greater QTcD and QTc-SD values than children without sinus arrhythmia. Indeed, Anderson showed that despite a large variation in RR interval, the QT interval remained relatively stable in sinus arrhythmia, and QTc consequently showed considerable beat to beat variation.18 Fei et al also found that there was no significant relation between QT dispersion and RR variation in patients with congestive heart failure.19
We have not been able to analyse the beats all through the respiratory cycle and this is the limitation of our results. We might have found a smaller variation in QTc in children with sinus arrhythmia if we had been able to analyse the beats in this way.
In conclusion, as QT interval changes are not correlated significantly with beat to beat RR variation, the computation of QTc dispersion is seriously affected by the presence or absence of sinus arrhythmia if simultaneous 12 lead ECG recordings are not used. Thus we suggest that, since sinus arrhythmia is a common condition in childhood, QT dispersion should not be corrected for heart rate in children.