Identification of Ischemic Lesions Based on Difference Integral Maps, Comparison of Several ECG Intervals

Abstract

Ischemic changes in small areas of myocardium can be detected from difference integral maps computed from body surface potentials measured on the same subject in situations with and without manifestation of ischemia. The proposed method for their detection is the inverse solution with 2 dipoles. Surface potentials were recorded at rest and during stress on 10 patients and 3 healthy subjects. Difference integral maps were computed for 4 intervals of integration of electrocardiographic signal (QRST, QRSU, STT and STU) and their properties and applicability as input data for inverse identification of ischemic lesions were compared. The results showed that better (more reliable) inverse solutions can be obtained from difference integral maps computed either from QRST or from STT interval of integration. The average correlation between these maps was 97%. The use of the end of U wave instead of the end of T wave for interval of integration did not improve the results. YOCARDIAL ISCHEMIA manifests itself also by a changed shape of action potentials (AP) of myocytes in the ischemic area during the repolarization phase. The idea to find the affected area (lesion) using difference integral maps (DIMs) of the QRST interval and computing the inverse solution to one dipole was introduced in (1). DIMs were computed by subtraction of QRST integral maps obtained under normal conditions from QRST integral maps obtained during the manifestation of ischemia. These maps can be measured on patients in relaxed state and under stress conditions (e.g. physical exercise). In our previous study, an inverse solution with one dipole using DIMs was applied to find the position of a single small local lesion (2). Recently we reported a method for identification of local ischemic lesions by computing an inverse solution with two dipoles (3) for recognition of 1 or 2 simultaneously affected areas on simulated data. The decision to use the whole QRST interval that represents both, depolarization and repolarization of the myocardium (3) was based on the assumption that ischemic changes could affect also the depolarization phase of myocytes' AP by reduced AP amplitude and its rate of rise (4). The other possibility would be to evaluate only the STT interval in electrocardiogram (ECG) reflecting the repolarization phase of the myocardium activation. Another issue when selecting the ECG interval for evaluation of repolarization changes is the determination of the end of repolarization. In many real signals small U wave appears after the T wave in ECG. Although it was observed already by Einthoven, there is no unique (generally acceptable) explanation of its origin. The U-wave is explained in (5) by the presence of after-potentials on the cardiac action potentials (caused by mechanical stress of cardiac cells during systolic phase) associated with ventricular wall motion. According to other studies (6), (7) T and U waves together represent the repolarization period. In such case, inclusion of the U wave into the evaluated interval would be desirable. In (3), only repolarization changes (influencing only STT interval) without the U wave were modeled on simulated data. In the present study the differences between DIMs computed from QRST and STT intervals of real measured data were analyzed and the results obtained for identification of small ischemic lesions were compared. Also the influence of the inclusion of U wave into the evaluated interval representing the myocardium repolarization was investigated. 2. METHODS & MATERIAL