Negative P Waves in Lead I in Dextroversion: Differential Diagnosis from Mirror-Inage Dextrocardia

IN CONGENITAL dextrocardia the problem is frequently whether dextroversion or a mirror-image dextrocardia is present. In the former the atria preserve their usual relationship in the frontal plane but the left ventricle lies anterior to the right. In mirror-image dextrocardia there is atrial inversion and the right ventricle lies anterior to the left. The electrocardiographic findings are generally considered to be extremely useful in the differential diagnosis. A negative P wave in lead I has been repeatedly emphasized as the most constant and valuable electrocardiographic sign of mirror-image dextrocardia.1-8 Positive P waves in lead I are considered to be the characteristic feature of dextroversion ;3, 8-l0 indeed, some authors believe this to be the " sine qua non" that the heart is dextroverted.11 These concepts are based fundamentally on the analysis of the direction of the spread of atrial depolarization in both conditions. Inasmuch as the atria are transposed in mirrorimage dextrocardia, the mean spatial vector of atrial activation is directed to the right, forward, and downward, producing negative P waves in lead I. In dextroversion, on the other hand, the atria are normally located in the frontal plane and the excitatory process, originating on the right side, spreads to the left, forward, and downward; therefore, the P waves are positive in lead I. As early as