Atrial fibrillation (AF) is the most common cardiac arrhythmia in clinical practice. One frequent and very effective alternative to revert AF back to a normal sinus rhythm (NSR) is external electrical cardioversion (ECV). However, conventional ECV protocols do not consider effects of electrical shocks on the atrial activity (AA) and its evolution during the procedure. The present study analyzes the feasibility of using the fibrillatory wave amplitude (fWA) and the dominant atrial frequency (DAF), extracted from the AA, to noninvasively detect AF recurrence under successive attempts of ECV and to predict the effectiveness of each shock in the ECV protocols. Results show that the effects of the first ineffective shock are only notably found in the fWA. The percentage of patients who needed several attempts who were correctly classified into patients who relapsed to AF, maintained NSR, and had an ineffective procedure was 64.3% for the fWA analyzed before the first shock, and increased to 89.3% for the same analysis after the first shock. Additionally, by jointly considering the patients who needed only one and several shocks, the discriminant ability increased when the values after the first ineffective attempt were used for the prediction. For the fWA, the diagnostic accuracy increased from 79.4% to 85.7%, for the DAF, from 65.1% to 71.4%, and for their combination, from 82.5% to 87.3%. The effect of the first unsuccessful shock is crucial for determining whether NSR will be restored through successive cardioversion attempts and predicting AF recurrence in patients undergoing ECV protocols. fWA and DAF can thus provide valuable information to improve the effectiveness of conventional ECV protocols, in which the effect of unsuccessful shocks is not taken into account at present.