[Vectorcardiographic manifestations of atrial enlargements]

Rational interpretation of changes of the P loop due to atrial enlargements must to rely on the magnitude and spatial orientation of main resultant vectors of the activation sequence of the atria. Under normal conditions, these vectors give rise to a mean vector oriented to the left downward and discretely forward with respect to their point of origin. In the presence of right atrial enlargement, the manifestation of the first vector of atrial depolarization, oriented downward and forward, is increased. This one moves in the same direction as the mean vector of atrial depolarization, originating an elongated P loop of more than 100 mcv in the three planes. Nevertheless, in the horizontal plane, increase of the P loop voltage predominates when hypertrophy exists, while augmentation of its area predominates when dilatation exists. In left atrial enlargement, the manifestation of the second vector of atrial depolarization, oriented to the left and backward, is augmented, and it moves in the same direction as the mean vector of atrial depolarization. For this, the PF loop acquires a characteristic aspect of a boxing glove, an the PH loop becomes diphasic, with its posterior area more or less prominent, or with a typical figure-eight conformation. If a biatrial enlargement is present, the manifestation of both the main resultant vectors of atrial depolarization is accentuated. Therefore the voltage of the diphasic P loop increases. Moreover the Ps loop has a triangular configuration, with its base of 30 msc or more, located below its point of origin. Generally disturbances of interatrial and intraatrial conduction coexist owing to myocardial damage.