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Cardiovascular Research 2003 58(3):549-554; doi:10.1016/S0008-6363(03)00319-5
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Copyright © 2003, European Society of Cardiology

Sex differences in the rate dependence of the T wave descending limb

Peter Smetana, Velislav Batchvarov, Katerina Hnatkova, A John Camm and Marek Malik*

Department of Cardiological Sciences, St.George's Hospital Medical School, Cranmer Terrace, London SW17 0RE, UK

m.malik{at}sghms.ac.uk

* Corresponding author. Tel.: +44-20-8725-5316; fax: +44-20-8725-0846.

Received 25 October 2002; accepted 14 January 2003


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 5 Summary
 Acknowledgments
 References
 
Objective: The interval from the peak to the end of the T wave (TpTe) has been proposed to reflect the heterogeneity of action potential durations within the ventricular wall. Several studies have previously described TpTe to be independent of heart rate, which contradicts the in vitro observation of marked changes in transmural repolarisation heterogeneity due to cycle length changes. Because of this inconsistency, we investigated heart rate related changes of TpTe interval. Methods: During 24-h recordings (SEER MC, Marquette GE) in healthy young women (n = 25, 26±7 years) and men (n = 25, 27±8 years), a 10-s 12-lead ECG was obtained every 30 s. Recordings were repeated after 1 day, 1 week, and 1 month and results in each subject were pooled together and grouped for women and men. The QT and QTpeak intervals were obtained automatically using QT Guard software (Marquette) and TpTe was computed as the difference between QT and QTpeak. In each subject TpTe values were averaged over 10-ms RR interval bands from 550 to 1150 ms. Results: In both sexes, TpTe interval showed marked rate dependence with prolongation at long RR intervals. TpTe intervals in men were significantly longer over the entire range of investigated RR intervals (P = 1.4x10–25). However, whereas the difference between sexes was marked at short cycle length (RR interval bin 540–550 ms: women 87±5 vs. men 95±9, P = 5.1x10–4) it decreased at long cycle lengths (RR interval bin 1140–1150 ms: women 99±5 vs. men 106±6, P = 9.3x10–4). Conclusion: There is a marked rate dependence of TpTe interval, which differs between women and men. The finding is consistent with the TpTe interval being an approximate surrogate of the intraventricular repolarisation gradient. The rate dependent increase in transmural repolarisation heterogeneity might be one of the reasons for the increased propensity of torsades de pointes in women.

KEYWORDS Electrophysiology; Arrhythmia (mechanisms); ECG; Gender; Heart rate; Repolarisation


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 5 Summary
 Acknowledgments
 References
 
The terminal part of the T wave gained interest already in the 1980s [1,2]. The changes in the Tpeak–Tend interval (TpTe) were considered to reflect ischemia caused alterations of myocardial repolarisation. The description of M cells in the 1990s [3] led to an improved understanding of the electrocardiographic repolarisation patterns. It was suggested that transmural dispersion of repolarisation might be estimated from the TpTe interval [4]. Although the extent of transmural gradients of action potential duration (APD) in vivo is still to be established [5,6], findings of increased TpTe duration in patients with hypertrophic cardiomyopathy [7], inducible VT [8,9], and LQTS [10] suggest that prolonged TpTe is arrhythmogenic.

Clinical observations suggesting that TpTe is relatively independent of heart rate [1,7,11], contradict experimental studies. It has been shown that differences in heart rate dependent changes of action potential duration (APD) are a major discriminant of different myocardial layers. It has indeed been proposed that these differences lead to the increase of transmural dispersion of repolarisation with long cycle lengths [12].

Because of this inconsistency, this study was designed to investigate heart rate related changes of TpTe in 24-h ECG recordings in healthy women and men.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 5 Summary
 Acknowledgments
 References
 
2.1 Study population
The study population consisted of 60 healthy volunteers, 27 men aged 26.7±7.3 years and 33 women aged 27.1±9.6 years recruited among employees and students of St. George's Hospital Medical School. All subjects had negative medical histories, normal physical examination, and a normal 12-lead electrocardiogram. During the study, participants were not taking any medication. All subjects had a normal day–night activity profile without any workload in unusual h of the day. The study was approved by the local Ethics Committee and all subjects gave a written informed consent.

2.2 Data acquisition
Digital ECGs (24 h, 12 lead) were obtained in each subject using SEER MC recorders (GE Medical Systems, Milwaukee, WI, USA) and repeated after 1 day, 1 week, and 1 month. During each 24-h recording, standard 10-s 12-lead ECG samples were obtained every 30 s.

2.3 Electrocardiographic measurements
Within each ECG sample, individual cardiac cycles were identified, individual RR intervals computed, and a mean RR interval obtained. Using linear regressions between RR interval durations and their consecutive order, slope values were calculated quantifying systematic acceleration or deceleration of heart rate within the ECG sample.

Using the research version of ECG software by GE Medical Systems, median beats of all leads of each ECG sample were constructed and processed with six different algorithms for the QT interval measurement: least-square-line-fitting with six (method 1) and twelve (method 2) samples around the maximum down-slope of T wave, and the threshold method based on 5% (method 3) and 15% (method 4) of the maximum T peak, and on 5% (method 5) and 15% (method 6) of the maximum T wave differential. For each of these methods, the median QT interval of all measurable leads was calculated, and the results of the six methods were averaged to obtain the representative QT interval. The same commercial software was used to compute the median QRS and the median QTpeak (QTp) interval in all measurable leads. The difference between the QT and QTp intervals was taken as the TpTe interval in each ECG sample.

2.4 Exclusion criteria
Stability of the automatic measurement of the QT interval was used as surrogate of data quality. It is well appreciated that the parameter setting of the algorithm used for determination of the end of the T wave influences its results and can lead to significant intra-algorithmic variability [13]. To minimise any possible biasing effect we used 6 different algorithms and excluded not only ECG samples in which the QT interval was measurable in <6 leads but also those in which the results of the six different algorithms differed >40 ms. These limits were based on previous experience with the research ECG software. ECGs were also excluded if recorded from episodes of nonstable heart rate (systematic significant acceleration or deceleration >5 ms per RR interval through the whole 10-s sample). Finally only subjects with >500 valid ECG samples in each of the four 24-h recordings were considered.

2.5 Statistical analysis
To compare QRS durations, QT, QTp, and TpTe intervals occurring at the same heart rate, individual ECG samples were averaged over RR interval bins ranging from 550 to 1150 ms in 10-ms steps. This sorting according to RR interval bins was performed separately for each recording. In each subject repeated recordings were pooled together and results grouped for women and men.

In each individual the TpTe/QTp ratio and the ratio TpTe/(QTp–QRS), called here the TpTe/STp ratio, were computed for each RR interval bin to eliminate the influence of QRS duration changes.

It has been recently shown [14] that QT/RR relationship significantly differs between subjects. Consequently, QRS, QT, QTp, and TpTe intervals were fitted into both a linear and hyperbolic regression model: {Phi} = β+{alpha}xRR (linear), {Phi} = β+{alpha}/RR (hyperbolic) where {Phi} is one of the QRS, QT, QTp, and TpTe and RR is the RR interval; with all measurements in seconds. Slopes of the regression lines {alpha} in each recording were averaged for each subject and compared between women and men.

Data are presented as means±S.D. unless otherwise stated. Results in women and men were compared by nonparametric U test. A P value <0.05 was considered statistically significant.


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 5 Summary
 Acknowledgments
 References
 
3.1 Data availability
After applying all the exclusion criteria, we excluded two female subjects aged 49 and 59 years. In this way, we obtained a comparable age distribution of both sex groups and the investigated population consisted of 25 women (aged 25.9±6.8 years, range 18–45) and 25 men (aged 26.5±7.5 years, range 18–41). In accepted recordings, the mean number of analysable ECG samples was 1453±385.

The differences in the rate relationship of QRS durations, QT, QTp, and TpTe intervals in women and men are shown in Fig. 1. Since it is obvious from the figure that the sex differences were different at slow and fast heart rates, Table 1 compares QRS, QTp, STp, TpTe intervals, and TpTe/QTp, and TpTe/STp ratios in women and men at two heart rate bins 540–550 ms and 1140–1150 ms, respectively. Table 2 shows slope values of the regression models of QRS, QT, QTp, and TpTe intervals in women and men.


Figure 1
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  		Fig. 1 Mean QRS duration, QT interval, QTp interval, and TpTe interval at the same RR interval in women (open circles) and men (closed circles). Values are given as mean±S.E.M. (ms).

 

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  		Table 1 Sex differences at RR interval bins of 540–550 and 1140–1150 ms

 

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  		Table 2 Sex differences in linear and hyperbolic slopes of regression models between repolarisation measures and RR intervals

 
3.2 QTp interval
QTp intervals showed a very similar rate dependency as QT intervals. However, the difference between sexes was more marked than that of QT intervals, especially at fast heart rates.

3.3 TpTe interval
TpTe intervals in both sexes showed a marked rate dependence. At the same RR interval, men had consistently longer TpTe intervals than women. The difference was very marked at high heart rates and became less distinct as the RR interval lengthened. Accordingly, linear slopes of TpTe/RR were significantly steeper in women than in men.

3.4 TpTe/QTp and TpTe/STp ratios
In both sexes, TpTe/QTp and TpTe/STp ratios decreased significantly with lengthening of RR interval. At all investigated RR intervals both ratios were significantly higher in men. However, whereas the difference was more marked at short RR intervals it became smaller at slower heart rates.


   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
5 Summary
 Acknowledgments
 References
 
As repeatedly described before [11,15,16], our study found that women have a longer QT interval than men, especially at slow heart rates. Also in consistence with previous reports [17,18], QRS duration was longer in men than in women and in both sexes, it increased with lengthening of the cardiac cycles.

However, using a detailed analysis of this massive data-set, we cannot confirm previous suggestions [1,7,11] that TpTe is heart rate independent. We have found significant rate relationship of TpTe in both sexes. Moreover, the sex difference of TpTe is the opposite of that of QT interval. Whereas QTp intervals like QT intervals are longer in women TpTe intervals are longer in men. Rate changes in women are steeper than in men for both QTp and TpTe (and QT) resulting in an increase of the sex difference in QT and QTp interval but a decrease of the sex difference in TpTe interval at slower heart rates. Whereas former studies used either data from exercise tests [1] or resting ECGs [7,11] we were able to investigate changes in repolarisation over the wide range of physiologic heart rates during 24 h in a large number of 12-lead ECGs. This aspect together with methodological differences, e.g. the average of six different algorithms compared to just one algorithm [1,7,11] used for determination of the end of the T wave, may well explain the differences between our findings and previous studies. The precision of any electrocardiographic study of repolarisation intervals is substantially dependent not only on the methodological accuracy but also on the number of investigated ECG recordings.

There is evidence from experimental studies [4,12], that in tissue slabs the QTp interval reflects epicardial APD and the QT interval M cells APD. Their difference, i.e. TpTe, reflects the transmural dispersion in APD and thus the transmural heterogeneity of repolarisation. It was also demonstrated repeatedly [3,12,19,20] that APD in M cells increases significantly more with lengthening of cycle length than APD of epicardial cells resulting in an increase of transmural heterogeneity of repolarisation at long RR intervals.

TpTe interval has been willingly accepted as an easy assessable measure of transmural heterogeneity of repolarisation and was shown to be increased in settings of expected higher repolarisation heterogeneity [8–10]. However, reports on relative heart rate independence of TpTe interval [1,7,11] contradicted one of the main principles of TpTe reflecting transmural heterogeneity of repolarisation. If it were true that TpTe interval stays the same at different heart rates, then the difference in the QT interval due to changes in cycle length equals the changes in QTp interval. This would suggest that epicardial and M cells increase their APD by the same amount at slow heart rates and transmural heterogeneity of repolarisation remains unchanged by heart rate changes.

In good agreement with the evidence from experimental studies we found a marked rate dependence of TpTe interval. Assuming that TpTe reflects transmural heterogeneity of repolarisation and is related to arrhythmogenesis, this suggests that at short cycle lengths men, despite a shorter QT interval, have higher transmural heterogeneity of repolarisation than women and hence have a higher arrhythmic risk. However, with lengthening of cycle length transmural heterogeneity, i.e. arrhythmic risk in women increases to a larger extent than in men.

This is in good agreement with reports on differences in arrhythmic risk between women and men. Whereas men are known to have a higher incidence of sudden cardiac death [21] women are more prone to develop bradycardia related torsades de pointes [22]. Longer QTc [15], respectively QTp intervals in women [11,16] cannot fully explain this sexual disparity in arrhythmic risk. However, marked differences in TpTe intervals and the TpTe/QTp ratio between women and men were not described before and might be responsible for sex differences in the propensity to arrhythmias. Differences in global as well as local repolarisation heterogeneity between women and men were described recently [23]. Women were reported to follow more closely the pattern of inverse sequence between depolarisation and repolarisation (i.e. cells that depolarise last repolarise first) than men [23]. This is in good agreement with the finding of longer TpTe intervals in men in this study reflecting larger differences in APD in men than in women.

Differences in APD within the myocardium reflect differences in ion channel density and expression [24]. There is increasing evidence from animal models that sex hormones may influence repolarisation properties [25–29]. Although evidence from humans is still missing, it is tempting to hypothesise that the marked sex differences in transmural heterogeneity of repolarisation might be explained by differences in ion channel pattern and/or activity due to differences in circulating hormones in women and men.

4.1 Limitations of the study
The extrapolation of results of experimental studies of myocardial blocks to human surface electrocardiograms is only approximate. In surface electrocardiograms, the TpTe interval does not only express the transmural dispersion of repolarisation but is also contributed by the apex to base differences in APD and by the differences between the left and right ventricle. At the same time, it is plausible to speculate that the transmural and apex–base repolarisation heterogeneities have similar proarrhythmic properties.

The noninvasive concept of this study did not allow to directly investigate APD and correlate these findings with electrocardiographic measures of QTp and TpTe interval.

Difficulties in determination of the end of the T wave are well appreciated. Determination of the peak of the T wave, especially with flat T waves is also known to be as problematic. Since TpTe interval is influenced by inaccuracies in both determination of the peak as well as the end of the T wave its reliability might be questioned. However, TpTe measures were shown to be well reproducible [30] and by using six different algorithms for determination of the QT measures we further minimised this problem. Also, any inaccuracy in the measurement of QTp and TpTe intervals applies to both women and men and therefore the described differences should not be affected.

Since the study involved almost 700 000 individual 10-s ECG samples, it was not possible to review all ECGs visually and/or to check them manually. To optimise the accuracy of the automatic measurements used, we had to introduce some arbitrary limits and based them on our experience with the ECG processing software. It is unlikely that the particular settings (e.g. QT interval measured in >6 leads) had any impact on the findings of the study.

Because the age distribution of our population was not sufficiently broad, we were not able to investigate the effects of ageing that may affect transmural dispersion of repolarisation by a number of mechanisms including changes in sex hormone levels, autonomic control, and myocardial histology. Our preliminary observations (results not shown here) indeed indicate that TpTe interval increases with increased age (mainly at slower heart rates) but this need to be properly investigated in a different population with a sufficient age span.

The autonomic influences may contribute to the differences in QT/RR and TpTe/RR relationship. It would be therefore appropriate to group the ECG samples according to the underlying levels of high- and low-frequency modulations of heart rate variability. Unfortunately, this was not possible in this study since the ECG data were not fully continuous and heart rate variability could not have been assessed.

Finally, our study was performed in healthy young subjects and our findings may therefore not be directly applicable to patients with cardiac abnormalities.


   5 Summary
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 5 Summary
Acknowledgments
 References
 
Substantial differences exist between women and men in both QTp and TpTe intervals. Men have shorter QTp but longer TpTe intervals over the entire range of investigated cycle lengths. However, with lengthening of cycle length TpTe interval in women increases to a much larger extent than in men. The observed rate dependence of TpTe interval agrees with experimental studies showing different rate dependencies of APD in different myocardial layers. Thus, it seems reasonable to use TpTe interval, carefully measured in surface 12-lead ECGs as approximate surrogate of intraventricular repolarisation gradient.

Time for primary review 24 days.


   Acknowledgments
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 5 Summary
 Acknowledgments
References
 
Supported in part by: the Primarärzteverein des Wilhelminenspitals, Vienna, Austria the Wellcome Trust, London, England, and the British Heart Foundation, London, England.


   References
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 5 Summary
 Acknowledgments
 References
 

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