Mlcek M, Kittnar O
Fyziologický ustav 1. LF UK, Praha.
Cas Lek Cesk. 2004;143(9):608-13.
In concurrence of our recent findings of the elevation of QT dispersion (QTd) in the group of pregnant women, mathematical approaches were developed aimed to give possible geometrical explanation whether the observed changes result from the rotation or from the changed position of the heart.
Mathematical model of the cardiac electrical field approximated as a time variable dipole in a homogenous spatial conductor was developed. From the experimental vectocardiographic records, representing time course of the cardiac dipole, body surface potential maps were calculated on the basis of the model. To validate the adequacy of the model, the reconstructed electrocardiograms were compared with the empiric data. To determine the effects of rotation, original empiric VCG data of the control group were transformed accordingly the hypothetic pregnancy related changes. Calculated surface electrocardiograms were then compared with empiric cardiograms of the pregnant women.
Based on the results, several conclusions can be drawn: 1) QT dispersion is associated also with the geometrical relations between the direction of cardiac vector during the terminal phase of repolarization and the direction 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) When the duration of repolarization is estimated from the classic thoracic leads within the phisiological variations of terminal-depolarization vector orientations, can exist that in some cases the minimum of QT interval is and in others it is not recorded by the lead system. Value of QT dispersion between these two extremes will be significantly different. 3) In case of the horizontal declination of the heart, the ECG signal in most of the leads of the body surface mapping has a higher voltage than in case of vertical declination due to a smaller angle between axes of the terminal vector and most of the leads. Such fact will contribute to more accurate reading of the T wave end and to the estimation of QT interval, usually with smaller value of QTd. 4) The change of the cardiac electrical field corresponding to the changed position of the heart (rotation) does not result by itself in QTd changes, if it is evaluated from the records from the whole thorax. Obversely, horizontalization of the heart contributes more to the evaluation of lower QTd values, as it is given above. 5) More then the result of geometrical changes, QT dispersion found in the group women in 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 changes during pregnancy.
鉴于我们最近发现孕妇组中QT离散度(QTd)升高,我们开发了数学方法,旨在从几何学角度解释观察到的变化是源于心脏的旋转还是位置改变。
建立了一个将心脏电场近似为均匀空间导体中随时间变化的偶极子的数学模型。根据代表心脏偶极子随时间变化过程的实验向量心电图记录,基于该模型计算体表电位图。为验证模型的适用性,将重建的心电图与经验数据进行比较。为确定旋转的影响,对照组的原始经验向量心电图数据根据假设的与妊娠相关的变化进行相应转换。然后将计算得到的体表心电图与孕妇的经验心电图进行比较。
基于这些结果,可以得出以下几个结论:1)QT离散度还与复极末期心脏向量方向与给定导联系统轴方向之间的几何关系有关。这种离散度在胸壁表面有典型表现——QT间期的最小值出现在与终末向量导联轴垂直的平面上。2)当从经典胸导联估计复极持续时间时,在终末去极化向量方向的生理变化范围内,在某些情况下,QT间期的最小值可能被导联系统记录到,而在其他情况下则可能记录不到。这两种极端情况之间的QT离散度值将有显著差异。3)在心脏水平倾斜的情况下,由于终末向量轴与大多数导联之间的夹角较小,体表标测大多数导联中的心电图信号电压比垂直倾斜时更高。这一事实将有助于更准确地读取T波终点并估计QT间期,通常QTd值较小。4)如果从整个胸部的记录进行评估,与心脏位置改变(旋转)相对应的心脏电场变化本身不会导致QTd变化。相反,如上文所述,心脏水平化对较低QTd值的评估贡献更大。5)妊娠晚期女性组中发现的QT离散度更多是T环形态变化的结果,该组中观察到了这种变化。观察到的离散度的另一种可能解释是妊娠期间电场变化的非偶极特性。