Vulnerable period' during the cardiac excitation coincides with:

Peak or midpoion of T wave is the 'vulnerable period' in the hea. T-wave represents ventricular repolarization. Mid-poion of the T wave indicates that the ventricular repolrization is paly complete. Each cell will have different status of excitability, at this point of time. If a premature impulse arrives in the ventricles at this time, it may result in ventricular fibrillation. During this time, the excitability of the ventricular fibers varies spatially. Some fibers are completely repolarized (and hence have recovered their excitability, and are ready to conduct another impulse), whereas others are paially recovered, and still others are in their refractory periods. J-point or reference point. End of S-wave (of QRS) is sometimes called the reference point or J-point. It is an isoelectric point. It indicates the completion of ventricular depolarization. It means, all the ventricular cells are in a dequipotential (depolarized). U wave represent the repolarization of papillary muscle.