Control of high common mode voltage during transthoracicdefibrillation

A high common mode voltage (V_cm) relative to earth ground is produced on the myocardium during the delivery of a defibrillator pulse and can generate a differential error signal when potential gradients are recorded with bipolar electrodes and isolation amplifiers. The error signal is proportional to V_cm, and therefore, a reduction in V_cm improves the accuracy of the potential gradient data. Experiments were conducted on 5 dogs to determine whether V_cm can be controlled using a bridge circuit. The bridge circuit consisted of a 5 kΩ power rheostat in parallel with the transthoracic resistance of the dog. The variable contact of the rheostat was connected to earth ground, and by adjusting the rheostat, V_cm on the myocardium could be varied. In each dog, 20 A shocks were delivered through stainless steel transthoracic electrodes. Point contact electrodes sutured to the epicardium were used to measure V_cm. It was determined that V_cm could be reduced to approximately zero at a given electrode on the heart. In addition, for the 5 dogs studied, the maximum measured V_cm on the heart was only 10% of the transthoracic voltage when the bridge circuit was balanced for an interior point in the heart