Referral to electrophysiological testing is based on anamnesis and arrhythmic ECG recording. In this study, flexible thin diagnostic catheters are introduced into the patient's heart and by pacing the heart from the atria and ventricles the properties of the heart's intrinsic conducting pathways are determined, as well as whether the patient has potentially treatable structures (substrates) causing arrhythmias. The aim of the study is to initiate an arrhythmia similar to that having caused the patient to seek medical examination. The arrhythmia can often be simultaneously treated with catether ablation.