Spatiotemporal effects of syncytial heterogeneities on cardiac far-field excitations during monophasic and biphasic shocks

Far-Field Excitation Via Syncytial Heterogeneities. Introduction: It has re cently been postulated that syncytial (anatomic) heterogeneities inherent w ithin cardiac tissue might represent a significant mechanism underlying fie ld-induced polarization of the bulk myocardium, This simulation study exami nes and characterizes the spatiotemporal excitatory dynamics associated wit h this newly hypothesized mechanism. Methods and Results: Two-dimensional regions of syncytially heterogeneous c ardiac tissue were simulated with active membrane kinetics, Heterogeneities were manifested via random spatial variations of intracellular volume frac tions over multiple length scales. Excitation thresholds were determined fo r uniform rectangular monophasic (M) and symmetric biphasic (B) far-field s timuli, from which strength-duration and strength-interval relationships we re constructed. For regions measuring 5.4 x 5.4 mm, baseline diastolic thre sholds for longitudinal (L) and transverse (T) shocks of 5-msec total durat ion averaged tin V/cm, n = 10) M-L = 2.87 +/- 0.26, M-T = 6.71 +/- 0.83, B- L = 3.22 +/- 0.25, and B-T = 7.93 +/- 0.51. These thresholds decreased by 1 5% to 25% when the region sizes were increased to 10.8 X 10.8 mm, Strength- duration relationships correlated strongly with the Weiss-Lapicque hyperbol ic relationship, with rheobases and chronaxies of 2.33 V/cm and 1.15 msec f or M-L stimuli, and 2.28 V/cm and 2.04 msec for B-L stimuli. Strength-inter val relationships for M-L and B-L stimuli decreased monotonically with incr easing coupling intervals, with similar minimum coupling intervals at absol ute refractoriness. However, the B-L thresholds were substantially less sen sitive to changes in coupling intervals than their M-L counterparts. Conclusion: This study provides strong additional support for and understan ding of the syncytial heterogeneity hypothesis and its manifested propertie s. Furthermore, these results predict that syncytial heterogeneities of eve n modest proportions could represent a significant mechanism contributing t o the far-field excitation process.