regarding heart beat

Respected Sir/Madam
With due respect  i wish to state that my name is sachin vohra having age :- 31.5 years, Sir i have noted that occassionaly my heart beat reaches between 53-59beats/minute in resting postion? may i know the reason behind the same? also i want to know which lab test gives claer picture about your heart working? I have gone through T.M.T on Dec.,07 and it was Normal. may i know after what span a person has to go under T.M.T test and i have also undergone ECHO in 2005 which was also normal ?

Regards

Sachin Vohra