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Medical Definition of Vulnerable period
1. Vulnerable period of heart, a brief time during the cardiac cycle when stimuli are particularly likely to induce repetitive activity like tachycardia, flutter, or fibrillation which persists after the stimulus has ceased; for the ventricle, it occurs during the latter part of systole, during the relative refractory period coincident with the inscription of the latter half of the T wave of the electrocardiogram. (05 Mar 2000)