This prospective observational study will include patients with atrial fibrillation that has indication for treatement with flecainide. Included patients are followed during a 12 month period. During the follow-up period they will have four clinical visits, during which clinical data, advanced echocardiographic data (strain and speckle tracking) and ECGs (Glasgow criteria) will be collected. These data will be analysed in relation to outcome parameters as: maintaining a normal sinus rhythm (arrythmia free health status), number of AF-free months, chances of successful electrical cardioversion, frequency of side effects, risk of pro-arrhythmias and mortality. The importance of these two analyses is to improve the use of flecainide. Hence, today patients with low benefit compared to risk of adverse events are inappropriately treated with flecainide with the "trial and error" approach currently used. On the other hand flecainide is currently underutilized, and patients denied the treatment that could improve their quality of life, prognosis and reduce their risk of cardiovascular adverse events. By investigating novel and promising parameters there is the potential of a better prediction of initiating safe and accurate anti-arrhythmic therapy for patients with atrial fibrillation.

Study synopsis for the Tambocor Prospective study

Inclusion: Patients with atrial fibrillation that admitted to the ward for prior to start of flecainide initiation.

Ethics: Informed Concent prior to inclusion.

Follow up after baseline: 4 visits during 12 month.

End points : Side effects that lead to discontinuation of flecainide. Persistent AF that lead to discontinuation of flecainide.

Evaluated parameters: 12 lead ECG, ECHO: LA, LV, HV strain , Dynamic Heart model: LV and RV.

The follow up visits:

Baseline: Normal ECHO after the first dose of flecainide, ECG and Questionnaire

4 Weeks: Normal ECHO + Protocol ECHO (LA, LV, HV strain , Dynamic Heart model: LV and RV) and Questionnaire.

6 month: Normal ECHO + Protocol ECHO (LA, LV, HV strain , Dynamic Heart model: LV and RV) and Questionnaire.

12 month: Normal ECHO + Protocol ECHO (LA, LV, HV strain , Dynamic Heart model: LV and RV) and Questionnaire.

Inclusion Criteria:

• All patients with written informed consent who are eligible for flecainide treatment for atrial fibrillation.
• The patients must be followed at the Skånes hospitals northwest and Skånes university hospitals.
• Age >18 years

Exclusion Criteria:

• Flecainide treatment with other indication than atrial fibrillation (including atrial flutter).
• No secure date for treatment start.
• Age <18 years

For patients with troublesome fibrillation burden, there are about as many ways to initiate and follow up treatment with Flecainide as there are Swedish hospitals. Although it was 30 years ago that the CAST study showed that Flecainide should not be given to patients with ischemic or structural heart disease, there is a lack of knowledge about which patient characteristics predict an effective and safe treatment. Today, the clinician is forced to use Flecainide according to the "trial-and-error" method, you simply have to see which patients benefit from the drug by trying. Regardless of the effect, patients are at risk of serious proarrhythmias and side effects.