ECG: vectorcardiography

Principle

Vectorcardiography is the registration, usually by formation of a loop display on a PC, of the direction and magnitude (vector) of the moment-to-moment electric activity of the heart during one complete cycle. The electric activity is generated by one of the major event of the myogenic activity, the auricular depolarization (P waves), waves of ventricular depolarization (QRS complexes) and waves of ventricular repolarization (T waves). The sources of the activity are supposed to be a single electric dipole which position, direction and magnitude changes during the heart cycle. The position describes a single loop during one heart cycle. Because the resultant traces were all loops of variable shapes, the traces were referred to as P loops, QRS loops and T loops. The 3D-vector loops are represented by projections upon the frontal plane. Fig. 1 illustrates how the QRS vector is constructed in the frontal plane at the instant of the R peak.

Fig. 1 Left: Configuration of Einthoven’s leads. Middle: vector representation. Right: the 3 leads in the hexaxial presentation (see ECG: hexaxial reference system).

Application

Its application is mainly in the field of experimental cardiology. Clinical applications are limited. On-Line vectorcardiography has been applied during coronary angioplasty. Monitoring ST (segment) vector magnitude and QRS vector difference by vectorcardiography is used for identifying myocardial ischaemia during carotid endarterectomy.

More Info

Fig. 2 High resolution vector cardiogram (HRVEC) with many loops.

The nomenclature and symbolic representation of the spatially oriented loops is standardized. The various letters denote the various vectors. One of the systems is:

q: the spatial direction of the vector of the QRS loop having the greatest magnitude,
p: the unit vector perpendicular to QRS and to the 'QRS plane' (the plane containing the QRS loop),
t: the spatial direction of the vector of the T loop having the greatest magnitude.