Strength-interval curves for cardiac tissue predicted using the bidomain model

Cardiac Strength-Interval Curves. Introduction: Strength-interval curves are predicted for unipolar anodal and cathodal stimulation of cardiac muscle. Methods and Results: Cardiac tissue is represented by the bidomain model, and the active properties of the membrane are described by the Beeler-Reuter model, Two successive stimuli (S-1 and S-2) are delivered through a single extracellular electrode, The S-2 threshold is determined as a function of the S-1-S-2 interval, for anodal and cathodal S-2 stimuli with 2-, 5-, 10-, and 20-msec durations. Each of the resulting cathodal and anodal strength-interval curves is divided into two parts: one section corresponding to make stimulation (long intervals) and the other section corresponding to break stimulation (short intervals), Generally, the cathodal strength-interval curves are decreasing functions of interval, except for an anomalous section of the 20-msec duration cathodal curve in the interval range from 310 to 318 msec, At short intervals, the anodal strength-interval curve contains a deep dip, which is more prominent for longer S-2 durations, The cathodal threshold is less than the anodal threshold for all intervals except those corresponding to the end of the refractory period. Conclusion: The bidomain model predicts complex anodal and cathodal strength-interval curves, with the anodal curve containing a dip (supernormal stimulation), These results resemble the experimental observations of Dekker.