Fast heart rhythms originating in the upper chambers of the heart are relatively common. Patients who suffer from such rhythms may feel unwell, manifested by symptoms such as shortness of breath or palpitations. Fortunately, such rhythms are rarely life-threatening if treated appropriately. In most cases, a simple Electrocardiogram test using skin electrodes can identify rhythm type. In other cases, however, differentiating fast heart rhythms with an identical pattern can be challenging, potentially causing a misdiagnosis or delayed diagnosis with subsequent patient deterioration. Ultrasound imaging of the heart (Echocardiography) is a non-invasive method of visualising the hearts structure and mechanical function. A specific measurement performed during this procedure has previously been shown to provide information regarding the heart rhythm of unborn babies. It is unclear, however, if this measurement can be used to differentiate the origin of rhythms within the heart. Fast heart rhythms may be treated using catheter ablation, a procedure which essentially ‘removes’ the responsible pathway in the heart. This study proposes one additional echocardiographic measurement during the procedure, to ascertain whether a relationship exists between this measurement and origin of the rhythm. Correct rhythm classification will ensure prompt and appropriate treatment, hence improving patient care.