Figure 8.

Coexistence of TD and PD block of slow responses in a depressed dog cardiac Purkinje fiber exposed to 20 mM KCl and 9 mM CaCl₂ and stimulated at a BCL of 2000 ms. Premature stimuli were applied every tenth beat with an increasing delay to scan the diastolic interval. The superimposed records in all three panels were recorded within 3 minutes. The upper record (labeled P) in each of them corresponds to a site proximal to the bipolar stimulating electrode; the lower record (labeled D), to a site 12 mm apart. A: Early premature slow responses are blocked before they reach the distal recording. B: Slow responses initiated at intermediate intervals propagate throughout the fiber. C: Slow responses initiated very late in diastole are unable to propagate through the fiber. See text for details.