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You need further investigations
Standard ECG records the electrical activity of the heart, for a few seconds. It can detect the arrhythmia
occurring during that time only. If the episode occurs randomly, one needs Holter or event monitor. Holter records for days; Event monitor for weeks / Holter records continuously / Event monitor only at certain times.
If there are no clues and if the problem is still bothering to the extent of interfering with life style, you may need ElectroPhysiological Studies (EPS) > it resembles angio – a catheter is put inside the heart / electrical activity recorded / stimulation and suppression tests are carried out / suitable medicine tested and so on. Though the test is the gold standard, it is INVASIVE and is not generally done unless there are compelling indications. This super-speciality expert is called ELECTRO-PHYSIOLOGIST.
The treating doctor may suggest them depending on need, based on his assessment of the situation.
Wishing all well