A normal range of HRV in the healthy asymptomatic population has still not been identified[2]. Without a normal range, changes in HRV are difficult to interpret and use in evaluating disease adjustments.
The purpose is to evaluate the gender and postural effects in HRV parameters between symptomatic Mitral Valve Prolapse Syndrome (MVPS) patients and an apparently healthy population.
1.2.1 Normal Range
If a set of amplitudes is obtained from a group of apparently healthy individuals within a well-defined age range, there will be a spread of measurements obtained with a predominance of values in the middle of the group and a smaller number at either side. In a classical situation, where there is an even spread of measurements around a central value with the distribution, there is said to be a normal distribution[27].
A point on the bell-shaped curve simply indicates the number of people in the group
who have an amplitude of a certain value[27]. When the distribution is symmetrical, the mean of the values will be in the middle of the range. Given the set of values, it becomes possible to calculate a mean and standard deviation (SD) for the parameter of interest. With a classical normal distribution, the mean plus or minus twice the standard deviation delineates approximately 95% of the range of values. Hence, if it could be shown that a set of values possessed a normal distribution, then one method of defining the normal range would simply be to calculate the mean and standard deviation and proceed to derive the upper and lower limits of the normal 95 percentile range. By taking such limits, where 2.5% of the values are excluded at either end of the distribution, so-called outliers can be excluded.
With respect to ECG measurements, however, it was pointed out by Simonson [28]
many years ago that the range of measurements for most ECG parameters is not normal, it tends to have a skewed distribution, for the R-wave amplitude in V5. The longer tail of the distribution is toward the higher values with the shorter tail being toward the lower values. To avoid this difficulty, an alternative approach for deriving the 95 percentile limits can be adopted.
The principal measurements of the ECG waveform note that these amplitudes and
durations are referred to well-defined onsets and terminations of the component waves[27]. In practice, the onset of the first wave in one lead does not need necessarily coincide with the apparent onset in other leads. This leads to the concept of introducing isoelectric segments within the QRS complex. The diagnostic significance of these segments has not yet been evaluated, but they have a relationship to vector orientations in a certain sense.
While the human eye at a glance may be able to say whether or not an ST segment is concave or convex upward, it requires several measurements to establish this when using a computer program. For this reason, Pipberger and co. [25] introduced the concept of time normalization. It was suggested [26] that the P wave be divided into four equal time-normalized segments. This approach has certain advantages, but it can suffer from errors in determining the onset and termination of the components[27].
1.2.2 Gender Difference
HRV might be effect by a lot of physical factors, some like illness, age, gender, racial or exercise. But nowadays we still do not have a HRV database all belonging to Taiwanese. Because of the differences in environment and life styles, Taiwanese
shouldn‟t use the same HRV normal range data with other racial. We try to build the
HRV normal range of Taiwanese in this research.
Gender differences in the autonomic nervous system may be present because of developmental differences or due to the effects of prevailing levels of male and female sex hormones[14]. Differences in the autonomic system may be due to differences in afferent receptor stimulation, in central reflex transmission, in the efferent nervous system and in post synaptic signaling. At each of these potential sites of difference, there may be effects due to different size or number of neurons, variations in receptors, differences in neurotransmitter content or metabolism as well as functional differences in the various components of the reflex arc[14].
In human beings, resting plasma concentrations and urinary excretion of Noradrenaline(NA) and adrenaline are generally not different between males and females[15–17]. However, males have been found to have higher resting sympathetic nerve activity to muscles, as determined by micro-neurography [17]. About in HRV, the majority of studies have found women to have a lower LF/HF power ratio than men, suggesting a preponderance of vagal over sympathetic responsiveness [18].
Higher LF power in men has been found in several studies [19–21]. These data
suggest that males have a preponderance of sympathetic over vagal control of cardiac function compared with females. Sato and Miyake[3] found that the male subjects were more sympathetic dominant than the female subjects. Our group has presented in 2010 ICE Lund meeting that gender difference will affect the result of HRV data[12].
1.2.3 Postural Changes
Autonomic Nervous System has a great effect in HRV. In this research we ask our case in three postures: lying, sitting and standing. These three postures mean the activity of sympathetic and parasympathetic nerves, and help us know how ANS work in different situation. In normal conditions, parasympathetic nerves are more active in lying position, and sympathetic nerves is much stimulated than parasympathetic nerves in standing position[12].
1.2.4 Mitral Valve Prolapse Syndrome and Normals
Patients with mitral valve prolapse syndrome (MVPS) may have a variety of cardiac and non cardiac abnormalities in addition to the characteristic valvular lesion with its mid-systolic click and late systolic murmur[9]. Symptoms such as atypical chest pain,
easy fatigability, abnormal cardiovascular and electrocardiographic responses to exercise, ST- and T wave changes on resting ECGs, and a variety of atrial and ventricular arrhythmias[10][11].
The presence of chest pain, ST-T-wave abnormalities, and arrhythmias suggests the possibility of a functional disorder involving the autonomic nervous system[9]. Many of the clinical features of the MVPS are found in other conditions which have been attributed to some type of autonomic dysfunction, e.g., neuro-circulatory or vaso-regulatory asthenia[13].
1.2.5 Age
Under the age of 50, sympathetic nerve activity was significantly greater in men than women [22]. In the study of Kuo et al.[23], the percentage LF power was significantly higher in the younger males than the younger females whilst the percentage HF power was significantly higher in the younger females than the younger males. Yamasaki et al.[18] also found a decline with age for both HF and LF power. The decline with age was more marked for men than for women.
Because of the data is not enough, we did not discuss about aging problem in this study.