Detecting of the manifestation of myocardium anisotropy by the magnetocardiographic method

Over the last years, highly sensitive magnetic field sensors, SQUIDs, have been employed to measure the magnetocardiogram (MCG), which are generated by ionic currents flowing in myocardium when it is active. The MCG method provides additional information in comparison to surface electric potential distribution. The obtained information is complementary in nature because the magnetic field is related to the curl of the impressed current sources into the human heart while the electric field is related to its divergence [1]. It is known that the myocardium has many kinds of anisotropy. One of them is determined by an anisotropy of conductivity (uniform anisotropy), which plays a key role in realistic propagation of the electrical excitation in the myocardium. This was indicated by computer simulation of normal MCG based on a propagation model of an excitation wave front in threedimensional arrays of cardiac cells within the cellular automata theory [2]. Animal studies and theoretical calculations suggest that magnetic recordings can provide new information about the electrical anisotropy of cardiac tissue [3,4,5] and the problem only exists to reveal and use in clinical practice. The uniform anisotropy is due to two properties, which are microscopic in nature; the first being the difference in electrical conductivity in the axial and transverse direction of the fibers. Such anisotropy is described by the bidomain theory and characterized by the common fiber orientation defined by global angles in standard spherical co-ordinates [6]. This theory explains experimental potential distributions measured from isolated canine hearts [7] and it was found that the conduction velocity parallel to the fiber axis was greater (3:1) than that of the transverse. Secondly, uniform anisotropy is caused by directional differences in cell-to-cell coupling on a macroscopic level. This anisotropy is described within the framework of the models of discontinuous conduction [8]. Today ECG plays an important role in clinical diagnostics and cardiac research, and an equivalent electric dipole is used as the model of the cardiac electric source. The said dipole, in fact, is the vector of the electro-motive force (EMF), i.e. electric potential, which is also called the electric heart vector (EHV). The most used dipole within the MCG-method model is an equivalent magnetic dipole which is also called as magnetic heart vector (MHV) and equivalent current dipole (ECD) which is the vector of the impressed current. It was shown that angle between EMF and MHV at the QRS peak is very close to 90° [9]. It should be mentioned that if this angle is exactly equal to 90°, there would be no new information in the MCG compared to ECG. However, it has been found that this angle varies considerably during QRS complex, both from patient to patient [10] and within various cardiac disorders [11]. The above-mentioned results show that the angle between MHV and EHV should contain new information about the myocardium conductivity. However, in our opinion MHV does not reflect completely electrophysiological phenomena because the physiological sense only has the multipole expansion of the cardiac generator [12]. It is sum of the equivalent current generators-multipoles where ECD is the equivalent current generator of the lowest order. In the overall anisotropic case a myocardium conductivity is a tensor [13]. However, at the axial symmetry with respect to fiber direction one can consider only two components transversal (S⊥) and axial (S0) conductivity. The present study is based on the statement that the degree of anisotropic conductivity manifestation is characterized by an angle γ between a direction (α) of a EMF vector and direction (β) of maximal current density (MCD) vector in the human heart at the same time