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Figure 3 Effects on Purkinje fibers APD₉₀ of an abrupt change in PCL from 300 to 1,000 ms (A) and from 1,000 to 300 ms (B), as illustrated above the graph (horizontal bar, 1 s). Action potential duration₉₀ (Y-axis) is plotted versus time (X-axis), 0 corresponding to the change in PCL. Action potential duration₉₀ values are expressed as the percentage of the steady-state value. For each sample, the APD₉₀/time curve was fitted using a monoexponential function (see Methods section). Function parameters were averaged, and the resulting curves are shown in dotted lines for unaffected dogs and solid lines for affected ones. Corresponding equations are in (A) y = (255 + 73·exp[1-0.0194·x])/328·100 for unaffected dogs (n = 17) and y = (284 + 75·exp[1–0.0240·x])/359·100 for affected dogs (n = 11), in (B) y = (184 + 48·exp [–0.7130·x] + 34·[–0.0686·x])/195·100 for affected dogs (n = 10). Inserts: Average time-constants ( in A and ’ in B) in unaffected (white) and affected dogs (black). *p < 0.05 (Student t test). APD = action potential duration; PCL = pacing cycle length.