| Introduction:Myocardial ischemia is a major cause of malignant arrhythmia and sudden cardiac death(SCD). Large number of experiments have confirmed that the state of autonomic nervous system has close relationship with cardiovascular disease mortality including SCD. The research on the autonomic nervous activities during myocardial ischemia play an important role in comprehensive understanding of myocardial ischemia. Holter is the most efficient monitoring measure of myocardial ischemia. Based on short term symbolic dynamics analysis and RR and QT interval series, autonomic nervous activity, especially the sympathetic and vagal interaction during myocardial ischemia was investigated in this paper. At the same time, the consistency of heart rate variability(HRV) and QT variability(QTV) and its influencing factors was studied in healthy subjects.Methods:From ECG records in Long-term ST-T (LTST) Database,193 myocardial ischemic episodes with the duration of 54 s-45 min were selected for HRV analysis. Then,145 myocardial ischemic episodes were selected for the joint analysis of RR and QT interval series. Two 5 min ECG episodes before and after each myocardial ischemic episode were selected respectively as the control ones. In symbolic dynamic analysis, each symbol unit was composed of 3 heart beats. There were 4 modes describing the change of heart beat in symbol units. The percentage of each mode in every episode was refered as 0V%,1V%,2LV%,2UV%. Symbolic dyanamic indexes were calculated in the episodes before, during and after ischemia. Repeated measures analysis of variance was applied to compare those indexes in the episodes before and during ischemia, as well as those before and after ischemia. In the analysis of RR time series, we proposed to use the ratio of 0V%to 2UV% to characterize the balance of autonomic nervous system. In addition, linear fitting was performed between the log of ischemic duration and the frequency number of the ischemic episodes in which the ratio of 0V% to 2UV% increased. The distribution of autonomic reflex mode to ischemia duration was analyzed by the linear fitting, and the test of goodness for fitting was conducted.For the indexes of symbolic dymamic analysis in healty subjects, Fantasia Database was used to test the age and gender differences between the old and the young, and Normal Sinus Rhythm Database to test the circadian rhythm difference. From each long term RR interval series (lasting 2 h),2000 of 100 short term RR interval series were selected and the corresponding QT interval series were selected as well. For each short term RR and QT interval series, the symbolic dynamic indexes (0V%,2UV%) were calculated to get the paired indexes of HRV and QTV. Then, the probability density function for these paired indexes was calculated and the mutual information was estimated based on the function. Differences in age, gender and circadian rhythm were analized respectively using pairedt test.Results:Symbolic dynamic analysis revealed an increase in the percentage of non-variable patterns (0V%) and a decrease in the percentage of very variable patterns (2UV %) during the episodes of ischemia, indicating a more sympathetic predominance compared with the baseline. But the trend of the shift in autonomic balance towards sympathetic predominance reduced with the extension of ischemic duration, suggesting a possible protection for heart. The symbolic dyanamic analysis of QT interval series displayed the same trend.For the healthy subjects,0V% of RR interval series increased significantly with age, while 2UV% decreased significantly with age; 0V% of male in elder group was significantly higher than female, the others had no significantly gender differences;0V% weaker in daytime than nighttime,2UV% stronger in daytime than nighttime; 0V% and 2UV% of QT interval series had no significantly age and gender differences. The mutual information of paired HRV and QTV indexes had no significant difference according to age, sex and circadian rhythm.Conclusions:In this study, dynamic changes of RR and QT interval series during myocardial ischemia were investigated using symbolic dynamic analysis. The ratio of 0V% to 2UV% was used to evaluate the balance of autonomic nerve system and study the relationship of ischemic duration and the moving trend of the balance. Symbolic dynamic analysis is capable of identifying changes in autonomic modulation and appears to be appropriate for elucidating the neural pathophysiological mechanisms occurring in myocardial ischemia.Using mutual information, the consistency of paired HRV and QTV indexes was found in healthy subjects with few influence factors, providing a potencial way in weakening the individual differences. |
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