An implantable magnetoresistive element for cardiac monitoring after heart transplantation

Rejection episodes after heart transplantation frequently lead to early diastolic distortion and restrictions in left ventricular wall thickening. These changes are usually measured by echocardiographic examination. The authors wanted to determine what functional information could be gleaned through the use of an implanted magnetoresistive element in combination with a cylindrical magnet (1.5 x 2 mm). The magnet was encapsulated in a titanium box in order to make it biocompatible. Six barbed hooks attached to the box ensured that the magnet did not move after implantation. The encapsulated magnet was implanted into the endocardial side of the left ventricular wall of the allograft. The magnetoresistive element-also equipped with a titanium housing-was epicardially attached opposite the magnet. The motion between the magnet and sensor within a cardiac cycle is nearly one dimensional and varies between 8 and 15 mm in healthy individuals. During rejection or hypertrophy the wall can increase to 30 mm in thickness. The size of the magnet and the sensitivity of the sensor were chosen to have a nearly linear relationship between the changes in resistivity of the sensor and the effect of magnetic field on the sensor within the afore-mentioned distance of 7 to 28 mm. The problem of a linear relationship between the distance of the magnet from the sensor and the change in resistivity of the sensor does not play an important role because in this medical application only intra-individual day-to-day changes in wall thickening are of interest. The dimension of motion of the epicardial side of the cardiac wall compared to the endocardial side presents a similar problem