Detection of the defibrillation threshold using the upper limit of vulnerability following defibrillator implantation

This study was designed to test defibrillation threshold (DFT) with the least number of fibrillation inductions using upper limit of vulnerability (ULV) and to describe the most practical set of ICD during DFT following implantation.

Although the correlation between ULV and DFT has been well described, there has been no uniform DFT testing protocol taking the advantage of ULV after defibrillator (ICD) implantation.

A total of 26 patients undergoing a new ICD implantation had a DFT induced with scanned T wave shock. The hypothesis that ventricular fibrillation (VF) could be defibrillated with 5 J higher than the highest T wave shock needed to induce VF or with 10 J if the T wave shock needed to induce VF was less than 5 J, was tested and 20 patients fulfilled these criteria. The methodology is improved by detecting peak T wave with 12-lead ECG, applying biphasic T wave shock and scanning the T wave shock in a wider window.

Five patients in the first group (n = 15) and one patient in the second group (n = 11) did not fulfill the above hypothesis. The common features of six patients who did not fulfill the hypothesis were that T wave shock needed to induce VF was either under 5 J (5 patients) or high (1 patient).

This study revealed the importance of methodology in studies regarding ULV and DFT. Following ICD implantation, we propose the first biphasic T wave detected by 12-lead ECG and rescue shock set at 10 and 15 J, respectively. If any of the scanned T wave (40 ms before and 40 ms after the peak T wave with decrements and increments of 20 ms) shocks could not induce VF, then the T wave and the first rescue shock should be set at 5 and 10 J, respectively. If the induction of VF has been unsuccessful with T wave shock at 5 J, then a safe defibrillation with 10 J should be expected in majority.