vulnerable phase

[vŭlnər-ə-bəl]
n.
  1. A phase in the cardiac cycle during which an ectopic impulse may lead to repetitive activity, such as flutter or fibrillation, of the affected chamber.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.