Ambulances in the United States must be staffed with a minimum of 2 personnel. The level of crew certification varies depending on the jurisdiction the ambulance is operating in. In most areas, the bare minimum is an EMT to provide patient care and an EMR to assist and drive the unit. This set-up would be classified as a Basic Life Support Unit (BLS) due to the fact that the highest ranking provider cannot perform Advanced Life Support (ALS) interventions. If patient condition warrants, an ALS provider may be summoned to assist and meet the ambulance en route to the hospital. Other staffing combinations include one EMT and one paramedic (the most common arrangement), or two Paramedics, which are classified in most areas as an Advanced Life Support Unit (ALS). Unlike in Europe, Emergency Physicians do not regularly practice in the field, and only crew ambulances for specialty situations, such as extreme-low-weight infant transports, ECMO or cardiac bypass transports, or unusual situations such as crush injuries necessitating field amputation, or mass casualty/disaster situations.