Thank you Nick for the further information.
This is likely to be sinus bradycardia. The commonest cause for it is sick sinus syndrome. The following investigations may be advised if not yet done;
1) Fasting blood sugar and glycosylated hemoglobin
2) Thyroid function tests
3) Serum calcium
4) Serum magnesium
5) Adrenal function tests
6) Serum electrolytes
Since your cardiac work up is normal, a 24 hours Holter monitoring may be recommended to look for any abnormal beats. Drug treatment of sinus bradycardia is usually not indicated for asymptomatic patients. In symptomatic patients, underlying electrolyte or acid-base disorders or hormonal disorder should be corrected.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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