The Relationship Between Myocardium Responsiveness And Its Firing Dynamics

One of the characteristic features in myocardium cells is its autorhythmicity. The cardiac impulse originates in a relatively small group of primary pacemaker cells. Typical primary pacemaker cells have the highest intrinsic frequency in firing. Following its firing, there is a slow depolarization called pacemaker potential. The heart rate is determined by the slope of depolarization. In physiological state heart rate is mediated by various physical, chemical and neural processes that take place in different, linked organs and organ systems. Above influences are conformed by central nervous system and transmitted to sinoatrial node by sympathetic and parasympathetic nerves. In physiological conditions the parasympathetic nerve modulation to sinoatrial node play more important roles than sympathetic nerve and is the main source of the heart rate variability.Since 1980s, the research on heart rate variability had made great progresses. It is considered that heart rate variability is the marker of a healthy heart. The loss of complexity and alterations of long-range (fractal) organization maybe associated with many heart diseases. The predictive value of heart ratevariability is independent of other noninvasive measures commonly used to predict prognosis of cardiac diseases, low left ventricular ejection fraction, and so on. Like other risk predictors, measures of heart rate variability have modest positive predictive accuracy when used alone. However, combining measures of heart rate variability with other risk predictors yields a positive predictive accuracy of about 50%. But until now it is still not clear why heart rate variability can be used to predict the occurrence and development of cardiac diseases. It is generally recognized that the reduction of vagal nerve modulation leads to lower values of heart rate variability. No one has considered the relationship between the loss of heart rate variability and the development of heart diseases from the myocardium responsiveness point.In sensory nerve cells we have found that responsiveness of the nerve cell was higher in nonperiod than in period firing dynamic state. The fact has been verified in many kinds of nerve cells. Based on above results we postulated the hypothesis that responsiveness of excitable cells changed following its firing dynamic states.Myocardium cell belongs to excitable cells and its firing obey the nonlinear laws. But it is not clear whether its responsiveness has inherent relationship with its firing dynamics. If the dynamic responsiveness hypothesis could be proved in myocardium cell it is not only the important supplement to nonlinear research on myocardium cell but provide a new idea on the mechanism of lower heart rate variability predicting cardiac diseases prognosis.In order to investigate the relationship between myocardium responsiveness and its firing dynamic states, bilateral vagotomy, M receptor blocking, acetylcholine (Ach) perfusion and continuously autonomic nerve ending stimulating methods were employed to change the rabbit sinoatrial node (SA) firing dynamics. Isoprenaline, noradrenalin, Ach, sympathetic or vagal nerve ending stimulation was used individually to measure the SA responsiveness indifferent dynamic state. Approximate entropy (ApEn) and nonlinear forecasting means were used to differentiate the SA firing dynamics. Response rate was calculated by the formula: Response rate = baseline IBI - response IBI / baseline IBI.The main results as followsResults from Sinoatrial node in vivo1, Under bilateral vagi intact condition the value of HF (high frequency power) component, LF (low frequency power) component and ApEn value of IBIs was the highest, but the LF/HF ratio was the lowest; Unilateral vagotomy decreased ApEn value of IBIs, right vagotomy increased LF/HF ratio but left vagotomy did not. The LF/HF ratio increased while ApEn value of IBIs decreased significantly as bilateral vagotomy.2, Atropine decreased the ApEn(approximate entropy) value of IBIs(interbeat intervals) fr...