If you've got one of these machines at home it seems you really want to run it a couple of times. Often I find the first reading I get is quite alarming but then if I sit for a bit longer and run it another 1-2 times I get a very normal reading.
[0] https://www.garmin.com/en-US/p/716808/ [1] https://www.withings.com/us/en/bpm-connect
This often isn't possible, but it's a reason doctor visits usually drop you in a room by yourself with a machine that will take multiple measurements with a lengthy delay between. Though that's where the white coat thing comes into play and people are stressed about a doctor coming in, etc.