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The vectorcardiographic study of the P loop in adults with ostium secundum atrial septal defect.

Frank vectorcardiographic P loops with satisfactory signal-to-noise ratio were obtained from 44 patients with ostium secundum atrial septal defect (OSASD), confirmed at operation, by using computer-averaging technique of 8 successive beats. Morphological and quantitative analyses of the P loop were made in order to reveal those characteristics of the P loop in OSASD which seemed most typically to show volumetric or diastolic overloading of the right atrium. Twenty-three automatically measured parameters of the P loop were compared with hemodynamic variables such as the systolic right ventricular pressure (s-RVP) and the left-to-right shunt ratio.A majority of the maximal P-vector directions in 3 projection planes and the P duration were within normal 95 percentile ranges. The magnitude of the maximal horizontal, the magnitude of mean polar and the magnitude of the maximal spatial P-vector increased above the 97.5 percentile values considered as the upper limit of normality in 66%, 55%, and 50% of the cases, respectively. These findings seem to suggest that the vectorcardiographic analysis of the P loop with computer-averaging technology is a valuable screening tool of right atrial diastolic overloading.Although the horizontal P loop of normal subjects showed many different patterns, the horizontal P loop of OSASD could be divided morphologically into 3 groups (Type A, Type B, and Type C). Thirteen cases (30%) out of 44 were of Type A, 22 cases (50%) Type B, and 9 cases (20%) Type C. There was a significant age difference between the individual mean values in the 3 groups (p<0.01), and there was a significant difference between the mean values in the shunt ratio of groups 'Type A and Type B' and group 'Type C' (p<0.01). The horizontal P loop pattern seems to be very important in the vectorcardiographic diagnosis of the presence and degree of the right atrial diastolic overloading.There was a significant correlation between 5 P-loop parameters and s-RVP, and between 3 of the parameters and the shunt ratio. The correlation between magnitude of the maximal horizontal P vector (XmV) and the shunt ratio (Y%) was the highest of all (r=0.426, p<0.01), the regression equation being Y=130.7X+49.8 (SEE=11.5%). These findings seem to suggest that the horizontal P loop can provide valuable information for the vectorcardiographic diagnosis of the presence of right atrial diastolic overloading and for estimating the degree of its severity.