Estrogen Linked To More Efficient Regulation Of A
Woman’s Heartbeat
Age is the “equalizer,” according to a study that
provides new insights into why
women live longer than men
November 5, 2002 (Bethesda, MD) – Men die
earlier than women. This fact leads scientists and medical researchers to
conclude that gender and age are two basic factors continuously affecting
body functions, disease categories and even life expectancy. Previous
research has determined that gender influences brain structure and
functions; however, in considering the cardiac pacemaker, there is still
debate as to whether heart rate dynamics differ between women and men.
Electrocardiogram (ECG) findings offer interesting
arguments for both sides of the issue. There is a similarity in the sexes
regarding the mean and standard deviation of the R-R interval portion of
electrocardiogram (intervals between positive deflection of the QRS complex,
which involves the depolarization of ventricular cardiac cells). On the
other hand, women have been reported to have lower, similar, and higher
high-frequency power (HF) and similar low-frequency power (LF) of heart rate
variability (HRV). (HRV is a strong predictor of mortality adversely
affected by such problems as anxiety, depression, and trauma.)
A team of researchers has speculated whether confusing
findings in heart activity of the sexes could be attributed to the nonlinear
characteristics of pacemaker activity, which may cause large variations when
analyzed by traditional linear methods. Such variations might lead to severe
interference if there are gender differences in the discharge from the
heart’s natural pacemaker, the sinus (SA) node. Accordingly, they set out to
determine whether the complexity or chaos of cardiac pacemaker activity
differs between women and men. Because aging may be a major determinate of
heart rate dynamics, the researchers systemically studied the effect of
aging on nonlinear properties and on gender-related differences. The results
were then compared with standard frequency-domain methods to measure the
nervous system’s parasympathetic and sympathetic regulation of heart rate.
The authors of “Sexual Dimorphism in the Complexity of
Cardiac Pacemaker Activity,” are Terry B. J. Kuo from Tzu Chi University and
Tzu Chi Buddhist General Hospital, Hualien, Taiwan, and Cheryl C. H. Yang,
also from Tzu Chi University. Their findings appeared in the October 2002
edition of the American Journal of Physiology—Heart and Circulatory
Physiology.
Methodology
The study entailed the participation of 480 volunteers
(240 women and 240 men), age 40–79, similar to overall distribution of males
and females. They were distributed into eight groupings based on five year
age intervals, with each age stratum containing 30 women and 30 men.
Subjects excluded had cardiovascular fluctuations: hypotension,
hypertension, diabetic neuropathy, an implanted cardiac pacemaker, frequent
occurrence of atrial fibrillation, premature atrial or ventricular
contractions, or other forms of arrhythmia. Nonlinear analysis of short-term
resting R-R intervals was performed using the correlation dimension (CD),
approximate entropy (ApEn), and largest Lyapunov exponent (LLE). Evidence
of nonlinear structure was obtained by the surrogate data test. CD, ApEn,
and LLE were negatively correlated with age.
Results
The short-term heart rate variability analyses reveal
that a woman’s heart rate is characterized by a higher CD, ApEn, and LLE
when compared with that of a man in the middle age, indicating a more
complex signal broadcasting from a woman’s cardiac pacemaker.
In assessing the brain, a more complex neural signal is
always accompanied with a more involved neural network. On the other hand, a
very simple firing pattern can only be observed in an isolated neuron. Thus
the higher complexity of women’s heart rate dynamics implies that the female
heart is modulated more comprehensively by the autonomic nervous system (ANS),
especially the rapid vagal influence, although such modulation is not strong
enough to produce an evident change in the mean and standard deviation of
R-R intervals. With the effect of age, previous studies have revealed that
aging is accompanied with a decrease of complexity in either cardiac
pacemaker activity or midbrain neural activity. Older subjects, regardless
of gender, had a lower CD, ApEn, and LLE, indicating a lower degree of
neural modulation to the cardiac pacemaker.
The researchers conclude that this lower complexity in
the elderly may be due to a decrease in the ANS potency resulting from the
normal aging process. The higher complexity of heart rate dynamics observed
in women before the age of menopause may be related to the lower cardiac
mortality and longer life expectancy of women. Previous studies regarding
the nonlinear analysis of HR dynamics chiefly focused on its capability to
discriminate pathological states or senile changes from normal conditions.
Because illness and aging may lead to a prominent change in ANS function,
which in turn leads to a prominent change in HR dynamics, such alterations
can usually be detected by traditional linear methods.
Conclusions
In almost every country, women have a longer life
expectancy than men, generally attributed to gender-related differences in
cardiovascular function. The exact mechanism underlying these differences
remains unclear; however, the data in this study indicate that changes in
the nonlinear properties may reflect effects of gender and aging on cardiac
parasympathetic regulation. The gender-related difference in autonomic
regulation function is key: women before menopause have a lower risk of
heart diseases than do men. The “cardioprotective” effect of estrogen has
been proposed for this trend but the linkage between the sex hormone and the
heart is ambiguous. Because a potentiated vagal function may protect the
heart from tachyarrhythmias after ischemic heart disease and decrease the
mortality rate, the dominance of vagal function in middle-aged women may
produce a protective effect against lethal tachyarrhythmias.
The study revealed 71 percent of women were postmenopausal;
the mean age of menopause was 47.7, and only six percent of postmenopausal
women received hormone replacement therapy. Thus the loss of gender-related
differences in the nonlinear indexes after age 50 supports the hypothesis
that estrogen has a facilitating effect on cardiac parasympathetic
regulation as revealed by the nonlinear content of heart rate dynamics.
Nonlinear analysis techniques may reveal new aspects of HR dynamics.
Mechanisms underlying the effects of gender and aging on nonlinear
properties of cardiac pacemaker activity and related clinical applications
warrant further investigations. Because the algorithms may be directly
incorporated into current HRV analysis protocols, CD, ApEn, and LLE have a
high potential for studying gender-related differences in cardiac pacemaker
activity and related autonomic regulation.
Source: October 2002 edition of the
American Journal of Physiology—Endocrinology and Metabolism.
-end-
The American Physiological
Society (APS) was founded in 1887 to foster basic and applied science, much
of it relating to human health. The Bethesda, MD-based Society has more than
10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals
every year.
***
Editor’s Note: To set up
an interview with a member of the research team, please contact Donna Krupa
at 703.527.7357 (direct dial), 703.967.2751 (cell) or
djkrupa1@aol.com.
