An emergency medicine physician is responsible for diagnosing and treating emergency medical conditions. The doctor is responsible if they do not diagnose an emergency medical condition, and therefore are responsible for medical malpractice.
An emergency medical condition means a condition that could cause death very soon, for example within the next 24 hours.  An example of something that is not an emergency would be, for example, if somebody has cancer.
Generally, that will not result in death within 24 hours, so the doctor would refer that patient for treatment back with the primary care physician. The responsibility of the emergency medicine doctor is to diagnose and treat conditions that are likely to result in serious harm or death in the next 24 to 48 hours. Medical malpractice can occur in a variety of ways in an emergency setting.