CPR prior to defibrillation improves survival from prolonged ventricular fibrillation (VF) by over 10%, based on recent studies. Waveform analysis could identify those patients with VF of over 5 to 7-min duration who should receive CPR first. A method based on the changes in the Fourier-derived frequency spectrum over time could improve the ability to identify prolonged VF.The Fourier frequency spectra were calculated on 5-s epochs of VF from 45 swine for 12.5min. The average power at each frequency shows a marked loss of frequencies above 8Hz occurring at 5min accompanied by an increase in the power in frequency spectra from 3 to 5Hz. A measure termed the frequency ratio is defined as the ratio of the power in the high frequency band from 8 to 24Hz compared to the power in the low frequency band from 3 to 5Hz. The frequency ratio is shown to detect 90% of epochs with VF less than 5min while allowing selection of 74% of those epochs over 5min. When the frequency ratio is set to detect 90% of episodes of VF under 7min, it is able to select 88% of those traces with VF over 7min. The receiver operating curve (ROC) for the frequency ratio has an area under the curve of 0.91 at 5min and 0.95 at 7min of VF duration.The frequency ratio is an improved frequency-based measure of VF duration. Waveform analysis techniques could determine which patients should receive shock first and which should receive a period of CPR prior to shock, thereby increasing probability of survival.