It is an open clinical problem to distinguish atrial flutter from atrial fibrillation. The discrimination is imperative, as atrial fibrillation is the most frequent arrhythmia in the adult (6 million individuals across Europe) and both types have to be treated differently in the interest of the patient. Atrial flutter is underrecognized in clinical practice. Especially in cases of irregular ventricular response it is often misinterpreted as atrial fibrillation. Caused by an increasing number of left atrial catheter ablations (pulmonary vein isolations), the number of left atrial flutter is increasing at an alarming rate, making the mentioned decoding task an ubiquitous one. Whereas atrial flutter is characterized by rapid regular electric impulses (constant frequency) in the atria, electrical activation exhibits high irregularity in atrial fibrillation (non-constant frequency). Electric impulses from the atria are conducted via the atrioventricular (AV) node to the ventricles. Using the electrocardiogram (ECG) it is easy to identify the electric activation of the ventricles, but in clinical practice often difficult to determine the exact electrical activation of the atria. We follow an optimization approach to identify the underlying rhythm. For the resulting non-standard optimization problems we present tailored solution strategies and encouraging results from a transfer to clinical practice.