Intracavitary potentials in type B ventricular pre-excitation.

In 1945, Rosenbaum and his associates proposed that ventricular pre-excitation should be classified into two types (A and B) according to the configuration of the QRS complexes in the right precordial leads of the scalar electrocardiogram. In type A pre-excitation the delta wave is positive and the S waves are small or absent; in type B the delta wave is negative and the S waves are large. Though somewhat arbitrary, in that occasional cases are difficult to classify because the delta wave and main deflection are of opposite polarity (Rosenbaum, 1957; Tranchesi et al., 1959), this classification has served a useful purpose by suggesting that there are two common sites from which premature excitation of the ventricles may occur, and its value has been broadly confirmed by subsequent vectorcardiographic studies. Bleifer et al. (1959), for example, showed in a series of 38 cases that the delta vectors and QRS sA loops in the horizontal plane fell into two groups that corresponded to Rosenbaum's types A and B. In group A, the delta wave was upright in both the right and left precordial scalar leads, and in the horizontal plane vectorcardiogram the delta vector and the QRS sA loops were orientated anteriorly between + 300 and + 1200. In group B, the delta wave was negative in the right precordial leads and the delta vector and QRS st loops were orientated to the left between -60° and + 30° in the horizontal plane. In general, the angle between the delta vector and the QRS sA loop was narrow, indicating that the delta wave appeared to determine the direction of the remaining portion of the QRS sA loop. Tranchesi et al. (1959) have also made a vectorial analysis, classifying 27 cases into types I and II, according to the orientation of the delta vector. In type I the delta wave was positive in V2 and the