RELATIONSHIP BETWEEN THE UPPER LIMIT OF VULNERABILITY DETERMINED IN NORMAL SINUS RHYTHM AND THE DEFIBRILLATION THRESHOLD IN PATIENTS WITH IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS

The upper limit of vulnerability is the strength above which Ventricul ar fibrillation is no longer inducible with a shock delivered during t he vulnerable phase of the cardiac cycle. It has been demonstrated tha t the upper limit of vulnerability correlates with the defibrillation threshold in a paced rhythm. The purpose of this study is to evaluate the correlation of the upper limit of vulnerability determined in norm al sinus rhythm with the defibrillation threshold using a simplified p rotocol in patients undergoing placement of an ICD. We studied 28 pati ents who underwent ICD implantation. CPI generators and Endotak leads were used in all patients. Device-based testing was used to determine the defibrillation threshold and the upper limit of vulnerability. The upper limit of vulnerability was tested with three shocks delivered a t 0, 20, and 40 ms before the peak of the T wave during normal sinus r hythm. The defibrillation threshold was determined by a simple step up -down protocol. The upper limit of vulnerability (9.0 +/- 4.5 J) did n ot significantly differ from the defibrillation threshold (9.9 +/- 4.0 J), P = NS. A close correlation was present, correlation coefficient = 0.75, P < 0.0001. The upper limit of vulnerability was within 5 J of the defibrillation threshold in 27 (96%) of the 28 patients. The uppe r limit of vulnerability underestimated the defibrillation threshold b y 10 J in one patient who had a defibrillation threshold of 15 J. The upper limit of vulnerability determined in normal sinus rhythm correla tes significantly with the defibrillation threshold in patients underg oing ICD implantation. The protocol is simple and easily implemented c linically.