Doctor so often changes stethoscope on body,
Then how can he\she identify heart beat rate?
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Probably the doctor isn't using the stethoscope to determine your heart beat rate. Mine seem to use it to listen to my lungs as I breathe, and I've seen them used to listen for valve sounds or other oddnesses in a baby's heartbeat. The actual rate can be determined other ways: the stethoscope is more for observing the sounds in your chest. They may move on quickly from a spot because they can't hear anything useful at all, or because they can hear that everything sounds fine.
Anecdotally, I can tell you that when I had growths in my lungs that caused an almost constant cough, doctors held their stethoscopes in place for quite a while at a time, and now that those growths are all gone, they listen for a shorter time in each place and then say "your lungs sound completely clear." As an adult, I've never been told anything about my heart after a stethoscope has been used.