A myocardial infarct occurs when the heart muscle tissues is irreversibly damaged due to a prolonged lack of oxygen. It is usually preceded by ischemic events. The detection of these episodes in daily life and an understanding of their origins could help in anticipating, diagnosing and treating infarcts. Ischemia occurs where there is a discrepancy between demand and supply of oxygen to the myocardium. It usually results in a decrease in the contractility of the injured area and in changes in the ECG signal. Ischemic episodes may or may not be accompanied by chest pain. Since the pain is not necessarily correlated with the severity of the pathological condition of the coronary vessels, it is thus irrelevant for prognosis. During exercise tests, patients begin showing evidence of ischemia each time some typical heart rate (the ischemic threshold) is reached. However, in daily life, ischemic episodes can appear at heart rates lower than this threshold leading to the assumption that these events are not induced solely by oxygen demand increase. We assume that this lowering of the ischemic threshold could be related to a sudden changed in the autonomic control of the heart. Thus a deficiency of the autonomic control could be one of the determinants of myocardial ischemia. However, knowledge about the time-evolution of the autonomic nervous system activity before and during ischemic episodes is very poor. Results, which have been obtained only during the steady state phases, are quite controversial. In our current study, we have been attempting to establish the possible relation between the autonomic control of the heart and the pathological condition of the heart during ischemia by applying a new time-dependent spectral analysis approach (SDA) which we have developed and implemented in our laboratory. This approach, which has been extensively verified on simulated, physiological and clinical signals, is thus a suitable tool for extending the evaluation of the autonomic nervous system to the transient phases of the occurrence of ischemia. Our project consists of four complementary approaches:
- Exercise tests of normal and ischemic subjects
- Effects of percutaneous transluminal coronary angioplasty (PTCA)
- Examination of ischemic events during daily life
- Changes in autonomic control of the heart during reperfusion induced by steptokinase of acute MI patients.
Combining the knowledge of the behavior of autonomic control during demand induced ischemia (exercise tests) and during externally induced ischemia (PTCA - the use of inflatable balloon advanced through the arteries to the lesion site) is enabling us to evaluate the effects of ischemia on autonomic activity. Through this research we may be able to isolate the changes in autonomic activity inducing or preceding ischemic episodes in daily life and therefore to understand the changes related to or caused by ischemia.
