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The description is highly suggestive of Sick Sinus Syndrome
In view of the long history, she should be seen by an Electro-physiologist -
Electro Physiological Studies (EPS) 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 has a risk (though minimal) 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.
She may need a pace-maker...
Wishing speedy recovery