Significance of the Mathematical Model of Cardiac Electrical Field for the Interpretation ofExperimental Data

Overview

Background.
In concurrence of our recent findings of the elevation of QT dispersion (QTd) in the group of pregnantwomen, mathematical approaches were developed aimed to give possible geometrical explanation whether theobserved changes result from the rotation or from the changed position of the heart.Methods and Results. Mathematical model of the cardiac electrical field approximated as a time variable dipole ina homogenous spatial conductor was developed. From the experimental vectocardiographic records, representingtime course of the cardiac dipole, body surface potential maps were calculated on the basis of the model. To validatethe adequacy of the model, the reconstructed electrocardiograms were compared with the empiric data. To determinethe effects of rotation, original empiric VCG data of the control group were transformed accordingly the hypotheticpregnancy related changes. Calculated surface electrocardiograms were then compared with empiric cardiogramsof the pregnant women.Conclusions. Based on the results, several conclusions can be drawn: 1) QT dispersion is associated also with thegeometrical relations between the direction of cardiac vector during the terminal phase of repolarization and thedirection of axes in the given system of leads. The dispersion then has its typical occurrence at the thoracic surface– minimums of the QT duration are found in the plane perpendicular to the axis of the terminal vector lead. 2) Whenthe duration of repolarization is estimated from the classic thoracic leads within the phisiological variations ofterminal–depolarization vector orientations, can exist that in some cases the minimum of QT interval is and in othersit is not recorded by the lead system. Value of QT dispersion between these two extremes will be significantlydifferent. 3) In case of the horizontal declination of the heart, the ECG signal in most of the leads of the body surfacemapping has a higher voltage than in case of vertical declination due to a smaller angle between axes of the terminalvector and most of the leads. Such factwill contribute tomore accurate reading of the T wave end and to the estimationof QT interval, usually with smaller value of QTd. 4) The change of the cardiac electrical field corresponding to thechanged position of the heart (rotation) does not result by itself in QTd changes, if it is evaluated from the recordsfrom the whole thorax. Obversely, horizontalization of the heart contributes more to the evaluation of lower QTdvalues, as it is given above. 5) More then the result of geometrical changes, QT dispersion found in the group womenin high level of pregnancy is an effect of changes in the T loop morphology, which was observed in this group.Another possible explanation of the observed dispersion is the non-dipolar character of the electrical field changesduring pregnancy.

Key words:
computer models, body surface potential mapping, cardiac electrical field.