In vivo myocardial excitability studied by simultaneous electrocardiogram and electrogram

In open-chest dogs with complete A-V block unipolar ventricular stimulation was applied at various current levels and at different time intervals in the relative refractory period. The silver-silver chloride electrode, used for stimulation, was also used for recording of electrograms. In other experiments an EG needle electrode was utilized; thereby stimulation and recording were carried out at different but very close sites. It was found that progressively increased current levels from zero to a few milliamperes cause distinct and well defined local electrical responses, the magnitude of which is related to the current level. The local response is not detectable in a simultaneously recorded conventional electrocardiogram until a certain magnitude is obtained, at which stage a generalized response of the heart is inscribed in both recording methods. A local response preceding the QRS complex is clearly seen in the electrogram prior to the generalized response. It is assumed that in the in vivo beating heart, current levels insufficient to stimulate the whole heart may evoke a local and detectable electrical response of a number of adjacent myocardial fibers. The lack of propagation of local responses of different magnitude may be explained by the various degrees of refractoriness offered by the ventricular and Purkinje fibers. The data presented in this study are in accordance with and offer a reasonable explanation for previously described marked variations in the threshold of excitability of the in situ heart.