a period in the cardiac cycle during which an ectopic impulse may lead to repetitive activity such as flutter or fibrillation of the affected chamber.
vul·ner·a·ble phase
(vŭl'nĕr-ă'bĕl fāz) A period in the cardiac cycle during which an ectopic impulse may lead to repetitive activity such as flutter or fibrillation of the affected chamber.