Comparative study of the normal vector magnetocardiogram and vector electrocardiogram.

The normal vector magnetocardiogram and its statistical variation are described in detail. The magnetic heart vector of 18 healthy subjects is detected with the corrected unipositional lead system and analysed together with the simultaneously recorded Frank vector electrocardiogram. The mean values and standard errors for the magnitudes and directions of the magnetic QRS and T waves are calculated with a digital computer. The interindividual variability of the normal magnetic and electric heart vectors was found to be about the same. The inaccuracy in the detector placement at the fourth intercostal space gives rise to only a small error in the direction of the detected magnetic heart vector. This is due to the self-centering effect of the lead fields in the heart's region, which arises from the difference in conductivity between the well-conducting heart and the intracardiac blood mass and the low-conducting lungs. The shape of the magnetic heart vector remains unchanged when it is measured at various distances from the chest surface on the line which is perpendicular to the frontal plane and joins with the beginning of the fourth intercostal space. The relationship between the normal magnetic and electrical heart vectors was also studied. The angle between the maximum magnetic and electrical heart vectors was shown to differ slightly from the theoretically predicted value of 90 degrees. Our results support the theoretically known difference in the relative sensitivities of the magnetocardiographic and electrocardiographic leads: the magnetic heart vector is more sensitive than the electrical vector to the terminal phase of the ventricular depolarization, when the activation in the heart appears in a more tangential direction than at the initial phase.