DETERMINATION OF VENTRICULAR VULNERABLE PERIOD AND VENTRICULAR-FIBRILLATION THRESHOLD BY USE OF T-WAVE SHOCKS IN PATIENTS UNDERGOING IMPLANTATION OF CARDIOVERTER DEFIBRILIATORS/

Background This study was designed to characterize the ventricular vul nerable period (VVP) and ventricular fibrillation (VF) threshold by us e of T-wave shocks in patients undergoing implantation of cardioverter /defibrillators. A premature condensed shock applied during the VVP ca n induce VF. Most studies on the WP and VF threshold have been conduct ed in animals rather than in humans. Methods and Results Twenty-one pa tients undergoing implantation of Medtronic PCD Jewel 7219D cardiovert er/defibrillators because of ventricular tachycardia and/or VF were en rolled. All had structural heart disease. Their ages ranged from 42 to 85 years (mean, 69 +/- 11.3 years). Seventeen (80.9%) had atheroscler otic coronary artery disease. The right ventricle (RV) was driven at a cycle length (SI) of 400 ms, and monophasic shocks (S2) of 0.6 J were delivered through an RV apex lead (cathode) and a superior vena cava lead (anode) during: the T wave of each cardiac cycle. The longest and shortest S1-S2 intervals that were capable of inducing sustained VF w ere defined as the outer and inner limits of the VVP at an energy leve l of 0.6 J, respectively. To determine the VF threshold, a shock of 0. 2 J was applied at the midpoint of the WP at 0.2-J increments until su stained VF was induced The lowest energy setting capable of inducing s ustained VF was defined as the VF threshold. Of the 21 patients, the V VP at an energy level of 0.6 J averaged 53.8 +/- 26.0 ms. Characterist ically, the VVP started from the ascending limb of the T wave and ende d at or slightly beyond the peak of the T wave, occupying 12.2 +/- 5.8 % of the QT interval. The midpoint of the,WP used for determination of the VF threshold measured 0 to 90 ms (mean, 32.9 +/- 26.0 ms) before the peak of the T wave. Of the 21 patients, 16 (76.2%) had a VF thresh old at less than or equal to 0.2 J (estimated 57 V), 3 at 0.4 J (estim ated 81 V), and 2 at 0.6 1 (estimated 99 V). Conclusions The VF thresh old is low (less than or equal to 0.2 J) in the majority of patients r equiring implantation of cardioverter/defibrillators. Further, studies are needed to define clinical usefulness of the study technique relat ive to its potential role for risk stratification and for assessing an tifibrillatory properties of antiarrhythmic drugs in this subset group of patients.