Voltage-sensitive dyes (VSDs) IntroductionIschemic heart disease (or coronary h.d.), the most common cause of death in the industrialized countries [1], covers a range of conditions in which the blood supply to the heart muscle, and consequently the heart functions, become limited. As a rule, it is caused by a narrowing of a branch of the coronary artery (stenosis) by atherosclerotic process.The diagnostic processes of ischemic heart disease consist of invasive and non-invasive methods. The development of non-invasive methods has been required by the tendency to avoid the untoward side effects during instrumentation, risk of iatropathogenic damage (mainly by infection), and last but not least, the economic aspects of cardiovascular diagnostics. Further, the results could help to develop a new noninvasive method needed in cardiology diagnostics.Ischemic heart disease is instrumentally diagnosed by coronary angiography, which represents the reference method. Echocardiographic data offer valuable information on regional or global impairment of cardiac function as do the radionuclid perfusion methods based on thallium 201 or technetium 99 uptake. For more than a half of a century, the most widely used diagnostic test for the detection and evaluation of cardiac ischemia are the electrocardiographic method [2]. The attenuated or stopped coronary perfusion causes a potential difference between the ischemic and normal regions during the ST segment and, eventually, also at rest. While ST depression is the common manifestation of transitory, exercise-induced cardiac ischemia, ST elevation is related to severe transmural ischemia, acute myocardial infarction. Myocardial ischemia affects also other features than the ST segment [3] including an increase of QRS amplitude, subtle prolongation of QRS duration and shifts of QRS axis [4]. These changes reflect abnormal conduction due to irregular membrane depolarization. There is ample evidence that cardiac ischemia may change the QRS spectrum as the expression of fragmentation of ventricular depolarization.The recent findings show that acute myocardial ischemia in early stages can be detected using the analysis of intra-QRS changes. These changes reveal local ischemia-induced propagation changes earlier than traditional ECG-based indexes [5], [6]. Recent findings specify the changes as low-amplitude short-time events within a window of ventricular depolarization. The actual time and frequency localization of the changes depend on the heart status.Myocardial ischemia studies using optical mapping consists of four experimental phases: control period, dye loading, wash-out, myocardial ischemia. The use of VSDs may negatively influenced electrophysiology of the examined heart. Therefore, dynamic changes in electrocardiograms during particular phases of the experiment must be studied.A number of method based on deterministic or stochastic theory can be applied.