SPATIAL-DISTRIBUTION OF CARDIAC TRANSMEMBRANE POTENTIALS AROUND AN EXTRACELLULAR ELECTRODE - DEPENDENCE ON FIBER ORIENTATION

Recent theoretical models of cardiac electrical stimulation or defibri llation predict a complex spatial pattern of transmembrane potential ( V-m,) around a stimulating electrode, resulting from the formation of virtual electrodes of reversed polarity. The pattern of membrane polar ization has been attributed to the anisotropic structure of the tissue . To verify such model predictions experimentally, an optical techniqu e using a fluorescent voltage-sensitive dye was used to map the spatia l distribution of V-m around a 150-mu m-radius extracellular unipolar electrode. An S1-S2 stimulation protocol was used, and upsilon(m) was measured during an S2 pulse having an intensity equal to 10x the catho dal diastolic threshold of excitation. The recordings were obtained on the endocardial surface of bullfrog atrium in directions parallel and perpendicular to the cardiac fibers. In the longitudinal fiber direct ion, the membrane depolarized for cathodal pulses (and hyperpolarized for anodal pulses) but only in a region within 445 +/- 112 mu m (and 6 16 +/- 78 mu m for anodal pulses) from the center of the electrode (n = 9). Outside this region, upsilon(m) reversed polarity and reached a local maximum at 922 +/- 136 mu m (and 988 +/- 117 mu m for anodal pul ses) (n = 9). Beyond this point upsilon(m) decayed to zero over a dist ance of 1.5-2 mm. In the transverse fiber direction, the membrane depo larized for cathodal pulses (and hyperpolarized for anodal pulses) at all distances from the electrode. The amplitude of the response decrea sed with distance from the electrode with an exponential decay constan t of 343 +/- 110 mu m for cathodal pulses and 253 +/- 91 mu m for anod al pulses (n = 7). The results were qualitatively similar in both fibe r directions when the atrium was bathed in a solution containing ionic channel blockers. A two-dimensional computer model was formulated for the case of highly anisotropic cardiac tissue and qualitatively accou nts for nearly all the observed spatial and temporal behavior of upsil on(m) in the two fiber directions. The relationships between upsilon(m ) and both the ''activating function'' and extracellular potential gra dient are discussed.