My niece is 2 years old she suffered from a cardiac arrest in the morning her heartbeat is now 200 pulse/min is this okay? oxygen is 99-100 with oxygen bed?

Arrest at that age usually follows a period of respiratory compromise. A heart rate of 200 at that age is tolerated by their heart, but does indicate a worrisome process. Without a full set of information about the case little more can be offered except the hope she is being monitored & treated in a hospital setting.

The rhythm should be evaluated and a cardiac echo may provide additional information regarding the cardiac function. Viral illness may result in myocarditis. Her oxygen saturation is within normal range. She should be well hydrated and closely monitored. Was there a chest image obtained?