As I'm sure you are aware, the key here is to catch the symptoms when present. If there are no symptoms when any tests are performed, it is likely nothing will be spotted. I'm sure you have told your Doctor an approximation of how often they occur, and if you feel that 24hrs is insufficient, then you could ask for a longer period of time, or a different test. One such test which might induce your condition is a stress ultrasound. They chemically induce the heart to work harder while continually monitoring you and looking at images of your heart. However, this is also likely not to show anything if the symptoms don't appear. If your heart is stressed and no problems occur, then it wouldn't really leave many options for exploration though.
I think a good first step would be the portable heart monitor, and if it doesn't show anything, or you don't get symptoms, don't let them give up.
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