Intramyocardial impedance measurements for diagnosis of acute cardiac allograft rejection

Abstract

Background. Measurements of intramyocardial impedance at high frequencies can indicate alterations in cell membranes and intracellular spaces during acute cardiac allograft rejection. Methods. Fifteen beagle dogs underwent heterotopic heart transplantation and were immunosuppressed with cyclosporine and methyl prednisolone (MP). Impedance was determined twice daily by means of four screw-in electrodes in the right and left ventricle. Transmyocardial biopsies and the intramyocardial electrogram (IMEG) were performed as reference methods. A total of 23 rejection episodes were induced. When acute rejection was recognized histologically and through IMEG readings, the animals were treated with a bolus of 125 mg of methyl prednisolone over 5 consecutive days. Treatment of rejection was controlled by biopsy and IMEG. Results. All hearts showed a uniform decrease in impedance of about 28.3% ± 5.5% immediately after transplantation, which subsequently reached a stable plateau after 7 to 8 days. Impedance values then remained unchanged as long as rejection was absent. Biopsy findings of grades 1A to 1B (ISHLT) were accompanied by a statistically significant increase in impedance of 12.2% ± 2.5%; of grades 2 to 3A of 19.2% ± 3.2% and of grades 3B to 4 of 27.0% ± 2.9%. Sensitivity was 96%, specificity 91%. Successful treatment of rejection led to a decrease of impedance to the initial levels. Conclusions. The amount of increase in impedance of high frequencies is a method to stratify acute cardiac allograft rejection into grades like histologically grading. The effectiveness of rejection treatment can also be monitored through impedance measurement. The method is also applicable for telemetric rejection monitoring by means of an implantable device.