Circulation, Vol 69, 1013-1025, Copyright © 1984 by American Heart Association
BP Damiano and MR Rosen
Early afterdepolarizations (EADs) are depolarizing potentials that occur
during phase 2 or phase 3 of repolarization. They can induce triggered
activity and have been proposed as a cause for arrhythmias in the heart in
situ. To determine the response of EADs and triggered activity to
interventions analogous to those used in the clinic for identifying
arrhythmogenic mechanisms, we used a pacing protocol to study the response
of EADs to sustained drive and extrastimuli. Cesium chloride (5 to 20 mM),
which induces triggered arrhythmias in the intact dog, was used to induce
EADs in isolated canine Purkinje fibers, and these were studied by
microelectrode techniques. Cesium prolonged action potential duration and
induced two types of EADs. At a potassium concentration of 4 mM, EADs
occurred at membrane potentials of -3 to - 30 mV. They were initiated 240
to 680 msec after the action potential upstroke and were 2 to 30 mV in
amplitude. Their amplitude increased as drive cycle length increased, but
their coupling interval to the action potential did not change with drive
cycle length. These EADs did not induce triggered action potentials. At a
potassium concentration of 2 mM, EADs usually occurred at higher membrane
potentials (-50 to -70 mV) and longer coupling intervals (470 to 1360 msec)
and were manifested as a delay of phase 3 repolarization. These EADs
induced triggered action potentials. When the triggered rhythms became
sustained, premature stimuli either reset or terminated them, depending on
the maximum diastolic potential. Termination occurred more frequently as
the maximum diastolic potential increased. The extent of overdrive
suppression induced by pacing for 15 sec to 3 min also increased as the
maximum diastolic potential increased. In summary, cesium induced EADs at
low or high membrane potentials. The former did not induce triggered
activity, but the latter did. The triggered rhythms that occurred were
similar to abnormal automatic mechanisms in their response to overdrive
pacing and extrastimuli.
ARTICLES
Effects of pacing on triggered activity induced by early afterdepolarizations
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