My child can hold her breath until she passes out - what should I do?

Bite your lip! Ignore her--don't skip a beat. As counter-intuitive as it seems, don't say--or do--anything different from what you were already doing. Fainting from breath-holding isn't dangerous--but it sure is a great way to get mom's attention! the less effective it proves to be, the sooner she'll stop throwing tantrums. But if it does work, you can bet she'll cash in on it again in the near future!
Ignore it. This is a behavior designed to get your attention. Ignorring the behavior will help to extinguish it.
Nothing ! If a healthy child intentionally holds their breath, the brain has a very strong self preservation reflex to prevent any damage. The child passes out at which point they spontaneously begin breathing.Do not try to force them to breath because things like forcing their mouths open may trigger a reflex preventing them from breathing no matter what.Although scary, in this case , leave her alone!
Ignore it. As the other physicians describe this is a common tantrum/ attention seeking behavior of the pre-verbal & early verbal child. Most parents don't realize it, but kids in this age range do not understand peripheral vision. This means you can monitor their fit from a side view, intervene if needed to protect them, but if you don't lock eyes, they think you aren't watching, eventually this will pass.