Background Atrial Fibrillation (AF) is the most common cardiac arrhythmia, and has significant morbidity and mortality. Measures aiming at termination of chronic AF (CAF) face resistance to pharmacological and even external DC cardioversion, as well as a high recurrence rate afterwards. Objectives Our aim was to study the EP parameters in CAF patients in relation to the immediate and long term outcome after DC cardioversion (IC) of AF, and their value for risk stratification and management. Methods The study included 42 AF patients (21 males, 21 females), mean age 48.8 + 13.5 yrs (median 52 yrs); including 31 CAF patients – all with failed external DC cardioversion – and 11 with paroxysmal AF. CAF patients were subjected to internal IC. Mapping during AF was done to measure the mean AF cycle length, mean AF wave duration, and wave length index, followed by complete diagnostic testing after restoration of sinus rhythm including assessment of the parameters of atrial conduction (intra- and inter-atrial conduction times during sinus rhythm and upon PES), as well as those of refractoriness (ARP at HRA and CS at different pacing CL, dispersion of refractoriness, and ARP adaptation to rate). Follow-up was done to all cases.Results IC terminated CAF – after failed external DC shocks – in 78% of cases, utilizing low energy monophasic shocks (mean energy 35.2 + 10.9 J) delivered using large-surface catheters. Comparing paroxysmal and CAF patients, there were significant differences as regards age, left atrial size, LV dimensions, prevalence of structural heart disease, as well as the electrophysiological parameters of refractoriness (ARP at different CLs), and atrial conduction as measured upon PES. Out of the CAF patients, 76% maintained sinus rhythm after successful IC for a mean follow-up duration of 11.7 + 1.9 months. There was a significant relation between the electrophysiological parameters of conduction (as measured upon PES), refractoriness (ARP, but not its dispersion or relation to rate) and the AF wave, and the immediate outcome of IC, as well as the incidence of recurrence during long-term follow-up. CAF patients premedicated with amiodarone had significantly longer ARP as compared to those with no premedication. EP study during sinus rhythm unmasked associated atrial tachyarrhythmias in 41.7% of AF patients .Their prevalence was higher in lone AF patients (67%). Successful RF ablation was associated with maintenance of sinus rhythm during follow-up even after discontinuation of AAD. EP study during sinus rhythm also unmasked sinus node dysfunction in two CAF patients. On DDD pacing and amiodarone therapy, there was no AF recurrence during long-term follow-up. The electrophysiological and clinical parameters could predict the outcome of cardioversion and recurrence during long-term follow-up with high diagnostic accuracy (up to 96% when combined with the left atrial size).Conclusions:-1.Low energy IC is an effective and safe method of terminating long-standing CAF whenever external cardioversion fails. 2.Electrophysiological remodeling is a progressive process; being more in CAF than paroxysmal AF, and being correlated to AF duration. Prompt cardioversion of AF patients is advisable before electrophysiological remodeling progresses.3.The propensity of AF to persist and recur is related to the degree of electrophysiological remodeling.4.The type of associated heart disease and degree of functional impairment should be put in consideration when deciding the management approach for CAF patients.5.Amiodarone may have a role in reversing the electrophysiological remodeling as regards refractoriness; and thus improving outcome of cardioversion and rhythm control during long term follow-up.6.Defibrillation-guided EP study may unmask associated atrial arrhythmias, as well as sinus node dysfunction in CAF patients.7.EP-derived parameters are useful for risk stratification of CAF patients as regards outcome of defibrillation, and recurrence, and are also helpful for guiding subsequent management.