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
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Also one to try, turning the tap on (not necessarily putting the cat near the sink) but the sound and sight of the running water sometimes stops them purring, and again minimal stress.