I have the Mia; when it is charging, the protective cap is off, making the entire vibe even shorter and with a cord attached,yet. Although the Mia is splash proof not waterproof, using it w/o the cap is not a good idea.
Take your time, recharge it and enjoy!
I'd be worried about the risk of electrocution. Also, the power draw of simultaneous use and charging might fry your USB port.
Even with a heart condition, the spec for USB 2.0 is too low current with too small of a distance between poles and too far from the heart to electrocute. The person would get half the charge of someone licking a 9v battery (old-school way of testing them), albeit in a much more sensitive area. Given that people used to *insert* violet wand probes (MUCH higher V and A), it's pretty unlikely to do much more than startle. Or give someone a new fetish.
(Note: Please do not put my theory to the test - 4.5v at 0.5A on the clit *shouldn't* cause permanent damage or death, but please don't test my theory).
I haven't tried to it use while charging myself yet, but I do know that my Precious metal jewels the princess – rabbit vibrator does turn on while connected via USB. There is a red light that indicates charging and a blue light that indicates it's on. When I turned it on while charging both lights came on.