Estimating the duration of ventricular fibrillation.

As the duration of time between the onset of ventricular fibrillation and the application of defibrillation (downtime) increases, the rate of successful resuscitation decreases. Results of recent animal studies suggest that the rate of successful resuscitation may be increased after a prolonged cardiorespiratory arrest when pharmacologic therapy is instituted before defibrillation. An accurate estimation of downtime could be critical in selecting the most appropriate therapeutic intervention. The purpose of our study was to determine whether changes in the frequency or amplitude of the ventricular fibrillation ECG signal during cardiac arrest could be used to estimate downtime. We characterized the dynamics of both total power and frequency distribution of the power in the ECG during ventricular fibrillation in 11 swine to determine whether enough information existed in either parameter to estimate downtime. The median frequency of the power spectrum was used to track power distribution. Both parameters followed a dynamic, repeatable pattern. However, median frequency showed less intersubject variability than did total power. A mathematical model of median frequency was developed and used with data obtained from ten additional swine to estimate downtime. The model estimated downtime to within 1.3 minutes of actual downtime between one and ten minutes of ventricular fibrillation. Our study has identified a new, potentially useful parameter for studying various management strategies in ventricular fibrillation as a function of downtime.