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Heart palpitations could be due to anxiety, stress,anemia,gastric reflux
or may be ectopic beats,SVT.
In healthy individuals, it is mostly of anxiety or stress but if your mother has sleeplessness
,then she must evaluate for premature atrial or ventricular contractions,paroxysmal supra ventricular tachycardia
, atrial fibrillation
The above heart diseases have many other symptoms apart from palpitations but they some times start with this symptoms.
Her ECG,holter monitoring ( 24 hr heart activity) and 2D echo are normal then she should go for thyroid function tests
She must consult your physician if facing this problem since long time.
You can give her tab.lexapro,its good drug for anxiety.