Thanks for coming to Healthcare magic for your query.
When there is cardiac arrest
, there is cessation of blood supply to different organs in the body including brain. If the duration of cardiac arrest is sufficiently prolonged (>20 mins)- there is very high chance of brain injury
For shorter duration of cardiac arrest, outcome is difficult to predict. From the description given, she had cardiac arrest in ER (and thankfully not on the way to hospital). In-hospital cardiac arrest has better outcome than out-of-hospital cardiac arrest, as prompt treatment was given.
In your sister's case - the next step over next 48-72 hours will be try and get her off the ventilator and sedation and assess her brain function. Your doctor may ask for a MRI
Brain/EEG to get some idea about any neurological damage.
I hope I've able to answer most of your queries. Wish your sister good luck for complete recovery.
Dr Tejas Shah
Critical Care Specialist/Pediatric Intensivist