CLINICAL PREDICTORS OF THE DEFIBRILLATION THRESHOLD WITH THE UNIPOLARIMPLANTABLE DEFIBRILLATION SYSTEM

Objectives. The purpose of this study was to determine the relation be tween clinical variables and the defibrillation threshold by using a s tandardized testing protocol and a uniform implantable defibrillator s ystem. Background. Past studies have not revealed useful correlations between clinical variables and the energy required to terminate ventri cular fibrillation. Most of these studies did not use a uniform implan table defibrillator system or a standardized protocol to measure the d efibrillation threshold and, thus, did not control for the influence o f these technical influences. We postulated that defibrillator and def ibrillation threshold measurement-based sources of variability oversha dowed important clinical predictors. Methods. The defibrillation thres hold was measured by using a standardized protocol in 101 consecutive patients. We used a transvenous unipolar pectoral defibrillation syste m that employed a single endocardial right ventricular defibrillation coil as the anode and the shell of an 80-cm(3) pulse generator as the cathode to deliver a 65% tilt biphasic pulse. Results. Several clinica l variables were found to be significantly associated with the defibri llation threshold: patient gender, height, weight, body surface area, heart rate at rest, QRS and corrected QT (QTc) intervals, left ventric ular mass and several measures of heart and chest size by chest roentg enogram. None of these variables had a correlation coefficient >0.45 w ith the defibrillation threshold. On multivariate analysis, left ventr icular mass and heart rate at rest were the only independent predictor s of the defibrillation threshold and explained only 25% of the observ ed variability. Conclusions. Despite the use of a uniform transvenous defibrillation system and a standardized protocol to measure the defib rillation threshold, no clinically relevant correlation was found betw een clinical variables and the defibrillation threshold. The defibrill ation threshold is probably a function of a complex interaction of ana tomic, physiologic and cellular variables that are not adequately repr esented by easily obtainable clinical information. It is probably not possible to predict defibrillation outcome from standard clinical vari ables.