Equivalent cardiac generator components were determined for a series of excised turtle hearts immersed in Ringer's solution. Relatively large preparations were contained within a specially designed spherical chamber, and electric field potentials were derived from 20 evenly spaced electrodes on the chamber wall. A laboratory computer was used to acquire and store the 20 leads of signal data in digitized form in real time. An eccentric dipole was optimally fitted to the surface potentials for each 2 msec sample; the remaining voltages were used to determine a centric multipolar series through octapolar content. In addition to the purely quantitative parameters which were thus determined, sequential mapping of isopotential distribution over the spherical boundary gave valuable qualitative insights into the behavior of the equivalent generator throughout ventricular depolarization. This activity varied in complexity from predominantly dipolar to strongly nondipolar among different preparations. Peak quadripolar activity ranged from a low of 10 to a high of 6CWP; the corresponding figures for octapole content were 5-61%. The overall technique permits experimental exploration of several theoretical principles which have been advanced since 1954. Pilot studies on rabbit hearts indicate that the method will also be applicable to mammalian hearts.

The electrophysiology of conduction delay was investigated in the semiisolated right bundle branch of 30 large mongrel dogs. Delay was produced by the external application of an electric blocking current to the bundle branch. Multiple recordings with glass capillary microelectrodes revealed two basic deflections associated with the delay phenomenon. An initial, or leading, deflection originated from the proximal or leading edge of the block and was rapidly transmitted through the block with progressive decay of voltage and rise velocity. A second, or trailing, deflection originated from the regenerative response at the distal or trailing edge of the block and was rapidly transmitted retrograde into the block, with progressive decay of voltage and rise velocity. The transmitted potentials appeared to be electrotonic in nature.The leading deflection appeared to be the transmitted event which maintained a conduction ratio of 1:1 through the block. The magnitude of delay was then related to the time required for this deflection to reach the threshold potential and initiate a regenerative response in the trailing portion of the block where the cells were Jess affected by the blocking current.ADDITIONAL KEY WORDS electric blocking current electrophysiology partial block mechanism of block incomplete bundle-branch block

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