Determination of ventricular vulnerable period and ventricular fibrillation threshold by use of T-wave shocks in patients undergoing implantation of cardioverter/defibrillators.

BACKGROUND

This study was designed to characterize the ventricular vulnerable period (VVP) and ventricular fibrillation (VF) threshold by use of T-wave shocks in patients undergoing implantation of cardioverter/defibrillators. A premature condensed shock applied during the VVP can induce VF. Most studies on the VVP and VF threshold have been conducted in animals rather than in humans.

METHODS AND RESULTS

Twenty-one patients undergoing implantation of Medtronic PCD Jewel 7219D cardioverter/defibrillators because of ventricular tachycardia and/or VF were enrolled. All had structural heart disease. Their ages ranged from 42 to 85 years (mean, 69 +/- 11.3 years). Seventeen (80.9%) had atherosclerotic coronary artery disease. The right ventricle (RV) was driven at a cycle length (S1) 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 were defined as the outer and inner limits of the VVP at an energy level of 0.6 J, respectively. To determine the VF threshold, a shock of 0.2 J was applied at the midpoint of the VVP at 0.2-J increments until sustained VF was induced. The lowest energy setting capable of inducing sustained VF was defined as the VF threshold. Of the 21 patients, the VVP at an energy level of 0.6 J averaged 53.8 +/- 26.0 ms. Characteristically, the VVP started from the ascending limb of the T wave and ended at or slightly beyond the peak of the T wave, occupying 12.2 +/- 5.8% of the QT interval. The midpoint of the VVP 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 threshold at < or = 0.2 J (estimated 57 V), 3 at 0.4 J (estimated 81 V), and 2 at 0.6 J (estimated 99 V).

CONCLUSIONS

The VF threshold is low (< or = 0.2 J) in the majority of patients requiring implantation of cardioverter/defibrillators. Further studies are needed to define clinical usefulness of the study technique relative to its potential role for risk stratification and for assessing antifibrillatory properties of antiarrhythmic drugs in this subset group of patients.