Role of left ventricular geometry in the alteration of initial QRS vectors due to concentric ventricular hypertrophy.

The relation between serial magnitudes of instantaneous spatial vectors, obtained throughout ventricular depolarization, and echocardiographically estimated left ventricular (LV) mass was investigated in 64 patients with congenital aortic stenosis and in 16 patients with coarctation of the aorta. While the correlation was positive between LV mass and vector magnitudes at 50 and 60 ms after QRS onset (r = .530 and .557, P < .01), it was found to be negative with regard to the magnitude of the initial 10 and 20 ms vectors (r = -.285 and -.355, P < .01). Computer simulation of activation propagation in different models of LV enlargement has shown that the decrease of the spatial magnitude of initial vectors, as well as a marked decrease of the area of the anterior portion of the horizontal plane QRS loop and of the Q amplitude of the orthogonal z lead, are characteristic of concentric LV hypertrophy with decreased diastolic volume and were not observed with unchanged or dilated chamber size. Repeated assessment of the magnitude of initial QRS vectors may indicate changes of LV remodeling in patients with LV pressure overload.

Computer simulation of propagated activation in different types of left ventricular enlargement.

An increase of the left ventricular mass does not always have an unambiguous effect on the electro- or vectorcardiographic image of ventricular activation. To analyse this phenomenon, three types of left ventricular enlargement were simulated by a computer model of propagated activation in cardiac ventricles. These were obtained from the reference (normal) left ventricle by an increase of its wall thickness either centripetally or centrifugally, as well as by increase of the ventricular cavity dimensions (dilatation). Two different degrees of mentioned changes, corresponding to an increase of the value of the respective parameters by 50% or 100% of reference left ventricular wall thickness, were analysed. In the case of a centrifugal increase of left ventricular wall thickness, the magnitude of instantaneous cardiac vectors increased only during the last two-thirds of ventricular activation time. A centripetal increase of the wall thickness resulted in a decrease of the resultant cardiac vector magnitudes during the first half of ventricular activation followed by their slight increase. In dilatation of the left ventricle the cardiac vectors were enhanced during the whole activation time.

An interactive computer model of propagated activation with analytically defined geometry of ventricles.

A computer model of propagated activation in cardiac ventricles was developed for simulation experiments in an interactive regime on personal computers. In the model, the geometry of ventricles is defined by parts of "compound quasi-ellipsoids". The parameters specifying these ellipsoids as well as their spatial positions are derived from input data characterizing the heart geometry. Because the ventricles are defined analytically, the model renders a wide range of possibilities to vary their shape and gross dimensions. The activation is propagated from predetermined starting elements in agreement with Huygen's principle. Different propagation velocities for the myocardium and the subendocardial mesh of Purkinje fibers also may be simulated. The results of computer simulations may be evaluated both qualitatively, by isochrones of the activation propagation, and quantitatively, by the resulting cardiac vector computer in any moment of activation process.

[Vectorcardiographic features of right ventricular dilatation]. / Vektorokardiografický obraz dilatácie pravej komory.

Axial McFee-Parungao lead system vectorcardiograms were obtained in 55 patients with type atrial septal defect, aged 3-24 years, prior to and in average 3 years after surgical repair of the defect. Changes of the QRS loop observed after intervention led to the conclusion that the vectorcardiographic signs of right ventricular dilatation consist of a rightward shift of the posteriorly orientated horizontal plane vectors at 50-70 ms of QRS, decrease of the magnitude of vectors around the 40th ms, no changes in the magnitude and orientation of the initial (10-30 ms) QRS vectors as well abnormal departures of the spatial VCG loop from its preferential plane even in the absence of other signs of right ventricular conduction impairment. The above abnormalities vanished after normalization of hemodynamics.

[Vectorcardiography of variations in localization of specific conduction systems in the left ventricle]. / Vektorokardiografický obraz variácie polohy zakoncení specifického vodivého systému v l'avej komore srdca.

A realistic computer model of propagation of ventricular activation was used to study the effects of varying the position of specific conduction system terminations in the left ventricle and the septum, representing the sites of initial activation, on the resulting simulated spatial heart vectors. Three differently localized foci of initial activation, each of them represented by one model element, were considered: in the central part of the left septal surface, posteriorly at about one third of the distance from the apex to the base, and in the upper part of the anterior free wall. During the model experiments, the positions of the initial activation were shifted +/- 5 model units (ca 5 mm) in the vertical and lateral direction either separately or in different mutual combinations. Small variations of the initial activation site in the basal parts of the left ventricle led to significantly smaller changes of the vectorcardiographic loop than variations of the same extent with the initial activation site located more apically.