Figure 7. Representative arrhythmia recordings illustrating repetitive responses induced by (A) an extrastimulus (ES) and (B) spontaneous arrhythmias. Traces show action potentials (AP) recorded simultaneously in normal zone (NZ) and altered zone (AZ). In panel A, a single ES (closed circle), applied 115 ms after the stimulus (open circle) in NZ, induced one response in AZ and NZ and two additional extrasystoles in AZ. The ES-induced arrhythmias might be a result of reentry movements: The ES applied in the NZ elicited a response first in AZ, probably caused by the refractory period in NZ. The signal then propagated in NZ, which in turn reexcited the AZ (first abnormal extrasystole). Considering the action potential duration dispersion between both regions, out of its refractory period the AZ would be reexcited by the depolarization maintained in the NZ (second abnormal extrasystole in the AZ). In panel B, note that stimulation was stopped just after the onset of arrhythmia, although sustained spontaneous activity persisted. These spontaneous arrhythmias probably can be attributed to abnormal automatic activities (see the discussion for more details).