Strength‐Interval Curves for Cardiac Tissue Predicted Using the Bidomain Model

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Abstract
Strength-interval curves are predicted for unipolar anodal and cathodal stimulation of cardiac muscle. 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 (S1 and S2) are delivered through a single extracellular electrode. The S2 threshold is determined as a function of the S1-S2 interval, for anodal and cathodal S2 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 S2 durations. The cathodal threshold is less than the anodal threshold for all intervals except those corresponding to the end of the refractory period. 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.