Relation between upper limit of vulnerability and defibrillation threshold in humans

Background. In the canine model, an upper limit of shock strength exists that can induce ventricular fibrillation during the vulnerable period of the cardiac cycle. This shock strength (the upper limit of vulnerability) closely correlates with the defibrillation threshold and supports the "upper limit of vulnerability" hypothesis of defibrillation. It is not known whether an upper limit of vulnerability exists in humans or whether this limit correlates with the defibrillation threshold. Methods and Results. In 13 patients undergoing implantable cardioverter-defibrillator implantation, the shock strengths associated with a 50% probability of reaching the upper limit of vulnerability (ULV₅₀) and a 50% probability of reaching the defibrillation threshold (DFT₅₀) were determined by the up-down algorithm. The ULV₅₀ was determined only for the mid-upslope of the positive T waves and for the mid-downslope of the negative T waves. No major complications occurred during surgery. An upper limit of vulnerability was demonstrated in each patient. The ULV₅₀ was 300±138 V or 6.8±5.8 J, which was significantly lower than the DFT₅₀ of 347±167 V (p=0.038) or 9.1+7.3 J (p=0.013). The correlation between the ULV₅₀ and the DFT₅₀ was significant (r=0.90, p<0.001 for voltage; r=0.93, p<0.001 for energy). Conclusions. An upper limit of vulnerability is present in humans. There is a significant correlation between the ULV₅₀ and the DFT₅₀, and the ULV₅₀ is significantly lower than the DFT₅₀.