Objectives:The aim of this study was to investigate the effects of the menstrual cycle on QT interval dynamics and the autonomic tone in healthy women.Methods: Holter ECGs were recorded in 11 healthy women aged 18-32 years during the follicular and luteal phases of their regular menstrual cycle. The interval from QRS onset to the apex (QaT) and to the end of the T-wave (QeT), the interval between the apex and the end of the T-wave (Ta-e), and RR intervals were measured automatically in the course of 24 hours by Holter ECGs. The QeT/RR, QaT/RR, and Ta-e/RR relationships were evaluated in each subject. The autonomic tone was assessed by the serum catecholamine level at rest and heart rate variability was measured by Holter ECGs.Results: (1) The follicular and luteal phases did not differ significantly with respect to the slopes of the QeT/RR, QaT/RR, and Ta-e/RR relationships. However, QeT and QaT intervals were significantly shorter for all RR intervals in the luteal than in the follicular phase (P < 0.0001). (2) The serum progesterone concentration was significantly higher in the luteal than in the follicular phase (P < 0.001). (3) Noradrenaline was significantly higher in the luteal than in the follicular phase (P < 0.05). There was no significant difference in the follicular and luteal phases with respect to heart rate variability measurements.Conclusions: Our results suggest that the menstrual cycle affects the QT intervals. The observed shorter QT interval during the luteal than the follicular phase may be attributable to the increase in serum progesterone and sympathetic tone. (PACE 2006; 29:607-613) QT interval, menstrual cycle, sex hormone, gender difference, QT/RR relationship, autonomic tone IntroductionPrevious studies have reported a gender difference in the incidence of various types of ventricular arrhythmias. Torsades de pointes associated with long QT syndrome is more common in women than in men, 1-3 whereas the incidence of Brugada syndrome 4 or sudden death 5 is higher in men than in women. We reported that genderspecific differences exist in the incidence of various types of idiopathic ventricular tachycardia (VT) and that VT originating from the right ventricular outflow tract (RVOT) is more common in women than in men. 6 It is important to consider the potential significance of endogenous, menstrual cycle-related sex hormones in various cardiovascular diseases. 7,8 The early follicular phase, during which serum estrogen is at its lowest level in the menstrual cycle, might be a time when premenopausal women with coronary artery disease are particularly susceptible to ischemic events. 7,8 On the other hand, Marchlinski et al. 9 reported that the hormonal flux that occurs during pre-

Gender-specific differences exist in the incidence and age distribution of the various types of idiopathic VT. Studies on gender-specific differences in arrhythmia will lead to a better understanding of its mechanism(s) and provide valuable information for the development of optimal treatment strategies.

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Objective-To examine the circadian variation in the signal averaged electrocardiogram (saECG) and heart rate variability and investigate their relations in healthy subjects. Methods-24 hour ECGs were obtained with a three channel recorder using bipolar X, Y, and Z leads in 20 healthy subjects. The following variables were determined hourly: heart rate, filtered QRS (f-QRS) duration, low and high frequency components of heart rate variability (LF and HF), and the LF/HF ratio. Results-Heart rate, f-QRS duration, HF, and the LF/HF ratio showed significant circadian rhythms, as determined by the single cosinor method. Heart rate and the LF/HF ratio increased during daytime, and f-QRS duration and HF increased at night. f-QRS duration was negatively correlated with heart rate (r = 0.95, p < 0.001) and the LF/HF ratio (r = 0.94, p < 0.001) and positively with HF (r = 0.93, p < 0.001). Conclusions-f-QRS duration has a significant circadian rhythm in healthy subjects and is closely related to the circadian rhythm of autonomic tone. (Heart 1998;79:493-496) Keywords: signal averaged ECG; f-QRS duration; circadian rhythm; heart rate variability Signal averaged electrocardiography (saECG) is a non-invasive method of detecting the substrate for malignant ventricular arrhythmias and is widely used to identify individuals at high risk for such arrhythmias and for sudden cardiac death. [1][2][3][4][5] Although the saECG has been assumed to measure fixed parameters, several studies have shown that these are altered by changes in factors such as heart rate and autonomic tone.6-8 Analysis of heart rate variability has been used to evaluate cardiac autonomic tone. 9-11Heart rate, heart rate variability, cardiac autonomic tone, and ventricular arrhythmias are known to have a circadian rhythm, [12][13][14][15][16] but information on the possibility of a circadian rhythm of the saECG and on the contribution of circadian fluctuations in autonomic function to the saECG is limited. We investigated whether the saECG has a circadian rhythm in healthy subjects and examined the relation between circadian fluctuations of the saECG and heart rate variability. Methods STUDY POPULATIONWe analysed 24 hour ECG recordings obtained in 44 healthy volunteers (28 men and 16 women, mean age (SD): 28.2 (3.3) years) in whom no abnormalities were detected by the medical history, physical examination, biochemical profile, 12 lead ECG, echocardiogram, or chest x ray. No subjects were on drug treatment. ANALYSIS OF 24 HOUR ECG RECORDINGSThe 24 hour ECG recordings were made with a three channel recorder (model 459, Del Mar Avionics, Irvine, California, USA) using bipolar leads X, Y, and Z. saECG analysisSignal averaging was performed on the data obtained from the first 500 seconds of every hour over the 24 hour period and filtered with a bidirectional filter at 40 to 250 Hz. The filtered leads were combined to form a vector as: √X 2 +Y 2 +Z 2 . The QRS onset and oVset were determined by a computer algorithm. The computer determined QRS oVset point w...

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