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-

of atrial and brain natriuretic peptides (ANP and BNP) in patients with essential hypertension (7-9). Accumulating evidence suggests that insulin resistance plays an important role in the development of essential hypertension (10, 11). Therefore, we hypothesized that LVH with concomitant increase in plasma concentrations of ANP and BNP may occur more easily in nondipper essential hypertensive patients than in dippers, and that insulin resistance may play an essential role in these processes. In this study, we analyzed 24-h ambulatory blood pressure (ABP), echocardiographic findings, plasma ANP and BNP levels, and insulin resistance in untreated patients with essential hypertension.

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