The P wave in the magnetocardiogram.

The P wave of the magnetocardiogram (MCG) was investigated in normal subjects and in patients with right and left atrial overloading. In normal subjects, the MCG P wave was positive at right lower sternal sites and negative in the other portions. The current source deduced from the MCG and the isomagnetic map was directed inferiorly and to the left throughout the entire phase of atrial activation. In patients with right atrial overloading, the direction of the current source deduced from the MCG was similar to that of normal subjects, but its amplitude was significantly greater. In left atrial overloadings, a negative-positive biphasic P wave was seen more frequently than in normal subjects at left parasternal sites. In the late phase of atrial activation, the MCG could detect the two dipoles, i.e. one directed inferiorly and other directed to the left. These dipoles might correspond to right and left atrial activities, respectively. These results show that the MCG might add useful information on current source to the conventional electrocardiogram.