Relationship of Surface Electrogram Recordings to Activity in the Underlying Specialized Conducting Tissue

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Abstract
Surface electrodes are widely used to study the electrophysiology of the A-V conducting system. The recording of activity from the human in-situ His bundle represents a practical, clinically useful adaptation of surface recording technics. The interpretation of data accumulated by this means must be considered in relation to the extent to which the surface recordings from the specialized conducting system accurately reflect activity in the underlying tissue. A combination of microelectrode technics to record transmembrane action potentials and surface recording technics on isolated preparations of canine conducting tissue were used to approach this question. Under many conditions the surface electrogram was found to be an accurate indicator of the presence and timing of impulses in the conducting system. However, during early premature activity, marked decreases or complete absence of surface electrogram voltage could occur at coupling intervals at which premature impulses were still able to propagate through tissue under the electrodes. In addition, early premature activity could be associated with reversal of polarity of the surface electrogram without reversal of the direction of propagation. Inaccurate relationships between the time of inscription of the surface recording and the time of passage of the propagating wavefront beneath the surface electrodes also occurred with early premature impulses. Finally, there is some evidence that surface electrograms primarily reflect that activity in the most superficial levels of tissue.