Electric shocks delivered to the heart are like a double-edged sword. Depending on the circumstances, they can either halt an arrhythmia or initiate one. Except for very large shocks that are so strong that their damaging effects cause immediate refibrillation, there is a range of shock strengths, below which shocks almost always fail to defibrillate and above which they usually successfully defibrillate. Throughout this range, the probability of successful defibrillation increases as the shock strength increases. The defibrillation threshold (DFT) is a single shock strength within this range whose mean value depends on the method used to estimate it.1 For example, one method gave a mean DFT value that was at the 71% probability of success point (DF71), meaning that this shock strength would be expected to succeed 71% of the time.2 There is a different range of shock strengths in which ventricular fibrillation (VF) is induced when the shock is given during cardiac repolarization (i.e., the vulnerable period). The lower limit of this range, the ventricular fibrillation threshold (VFT), is considerably lower than the range of shock strengths that defibrillate. However, the upper limit of this range, the upper limit of vulnerability (ULV), is typically within the range of shock strengths that successfully defibrillate.3,4 If the ULV did not exist, then it might not be possible to defibrillate with a shock of any strength because VF is so complex that some cardiac regions are probably in the vulnerable period at any time during VF. Thus, if the ULV did not exist, a shock larger than the VFT, even if it halted all the VF wavefronts, would immediately reinitiate VF in the regions in the vulnerable period. Just as for defibrillation, the ULV is also not a single value, but is a probability function in which the odds of not inducing VF increase with increasing shock strength.5 © 2009 Springer US.