Whilst trying to get a HR from your feline patients, it can be quite difficult to hear when they are purring away! A good tip I got from a vet student was to put a small amount of surgical spirit on a swab and rub it on there nose, works wonders and doesn't cause a huge amount of stress
Just place spirit swab under pussies nose, no need to dab the nose as this isn't feline friendly is it!!