Impedance to transthoracic direct current discharge: a model for testing interface material.

An in vitro method for screening the relative impedance of gels has been developed. A nonconductive chamber was built with 1-cm-diameter electrodes placed 1 cm apart. This chamber is filled with electrode gel to be tested. The defibrillator used delivers a half-sinusoidal waveform. When the same stored energy is discharged by this defibrillator into increasing impedance, the resultant delivered current wave decreases while the voltage wave increases. Thus, with increasing impedance, the delivered volt-time interval increases. Twelve electrode gels were studied; 10 measurements were made on each gel. There is a marked variation in the impedance to direct current discharge of commercially available electrode gels. A large number of commercially available electrode gels are not suitable for use as the interface between the paddle electrodes and the chest wall during elective cardioversion and emergency defibrillation.