[The P loop in atrioventricular septal defect. Embryologico-vectorcardiographic correlation].

Atrial vectorcardiograms (vcg) were analysed in 23 patients with partial (n = 15) or complete (n = 8) forms of atrioventricular septal defect (AVSD). The rotation and the projection of the maximum left atrial vector (MLAV) on the frontal plane (FP) and on the horizontal plane (HP) were noted. To assess the possible influence of the degree of left-to-right shunting and the right atrial and right ventricular pressures on the rotation and MLAV projection, these vcg data were correlated with hemodynamic values obtained at the time of preoperative cardiac catheterization. Vcg characteristics of the patients were also compared with those of 25 healthy individuals. No significant differences were noted between the vcg of patients with the partial form and those with the complete form of AVSD. Also, there was no apparent influence of the hemodynamic values on the loop rotation or the MLAV projection in either plane among the patients. However, the MLAV showed a more posteriorly and superior location in the FP and HP than in normal subjects (p = 0.0001). Moreover, 67% of the cases showed a clockwise rotation of the loop in the HP and 33% in the FP; in contrast, normal subjects always showed a counterclockwise rotation in both planes. A more posterior and superior MLAV is consistent with direct observations in human embryos with AVSD which have shown that deficiencies of the posterior portion of the interventricular septum are the basic feature in all cases. The adaptation of the conductive tissue to such deficiency occurs in early phases of cardiac development.