title: Spatiotemporal effects of syncytial heterogeneities on cardiac far-field excitations during monophasic and biphasic shocks.
abstract: BACKGROUND
  It has recently been postulated that syncytial (anatomic) heterogeneities inherent within cardiac tissue might represent a significant mechanism underlying field-induced polarization of the bulk myocardium. This simulation study examines and characterizes the spatiotemporal excitatory dynamics associated with this newly hypothesized mechanism.
  
  
  RESULTS
  Two-dimensional regions of syncytially heterogeneous cardiac tissue were simulated with active membrane kinetics. Heterogeneities were manifested via random spatial variations of intracellular volume fractions over multiple length scales. Excitation thresholds were determined for uniform rectangular monophasic (M) and symmetric biphasic (B) far-field stimuli, from which strength-duration and strength-interval relationships were constructed. For regions measuring 5.4 x 5.4 mm, baseline diastolic thresholds for longitudinal (L) and transverse (T) shocks of 5-msec total duration averaged (in 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 15% to 25% when the region sizes were increased to 10.8 x 10.8 mm. Strength-duration relationships correlated strongly with the Weiss-Lapicque hyperbolic relationship, with rheobases and chronaxies of 2.33 V/cm and 1.15 msec for M-L stimuli, and 2.28 V/cm and 2.04 msec for B-L stimuli. Strength-interval relationships for M-L and B-L stimuli decreased monotonically with increasing coupling intervals, with similar minimum coupling intervals at absolute refractoriness. However, the B-L thresholds were substantially less sensitive to changes in coupling intervals than their M-L counterparts.
  
  
  CONCLUSIONS
  This study provides strong additional support for and understanding of the syncytial heterogeneity hypothesis and its manifested properties. Furthermore, these results predict that syncytial heterogeneities of even modest proportions could represent a significant mechanism contributing to the far-field excitation process.
authors.affiliation: St. Jude Medical CRMD, Sunnyvale, California 94086, USA. mfishler@ventritex.com
authors.forename: M G
authors.initials: MG
authors.lastname: Fishler
authors.forename: K
authors.initials: K
authors.lastname: Vepa
mesh_terms: D002462:Cell Membrane / Q000502:physiology; D003198:Computer Simulation; D003971:Diastole / Q000502:physiology; D004554:Electric Countershock / Q000379:methods; D004594:Electrophysiology; D018740:Genetic Heterogeneity; D015726:Giant Cells; D006321:Heart / Q000033:anatomy & histology* / Q000502:physiology*; D006801:Humans; D007700:Kinetics; D008955:Models, Cardiovascular; D055585:Physical Phenomena; D010825:Physics; D013997:Time Factors