THE VENTRICULAR DEFIBRILLATION AND UPPER LIMIT OF VULNERABILITY DOSE-RESPONSE CURVES

Introduction: A stimulus delivered in the T wave of a paced cardiac cy cle can induce ventricular fibrillation (VF), If the stimulus strength is increased, the probability of inducing VF decreases, This study de termines an ideal mathematical model (a dose-response curve) for the r elationship between the shock strength and the probability of inducing VF or defibrillating. Methods and Results: Defibrillating electrodes were implanted in the right ventricle and superior vena cava in 16 pig s. The electrode in the vena cava was electrically connected to a cuta neous patch, The same electrodes were used for both VF induction and d efibrillation, T wave stimuli were given at the peak of the T wave acc ording to a modified up-down protocol (40 V up, 20 V down), When a T w ave stimulus induced VF, a defibrillation stimulus was delivered 10 se conds later, also according to the modified up-down protocol, Exponent ial, logistic, log-dose logistic, piecewise linear, and Box-Tiao dose- response curves were fit to the resulting data using the maximum likel ihood method, For the defibrillation data, it vas found that only the logistic and Box-Tiao curves fit all of the animals (P < 0.05). For VF induction, only the Box-Tiao curve fit all of the animals (P < 0.05), Extrapolating along a dose-response curve that did not fit to a shack strength with a very low probability of inducing VF or a very high pr obability of defibrillating yielded errors as great as 610 V. Conclusi on: The Box-Tiao dose-response curve is the best single choice for fit ting VF induction or defibrillation datasets.