The H-13 Sioux helicopter, made famous by the film and television versions of "M*A*S*H", transported 18,000 wounded soldiers during the conflict. The work of the Medical Air Evacuation Squadrons was a success and was repeated by U.S. forces in Vietnam. The use of helicopters for emergency medical evacuations extended to civilian practice by groups such as the Shock Trauma Air Rescue Society. After the Harrow and Wealdstone rail crash in 1952, ambulances in Britain were restructured to be a "mobile hospital", rather than just transporting patients, thus leading to modern ambulances. CPR was developed and accepted as the standard of care for out-of-hospital cardiac arrest; defibrillation, based in part on an increased understanding of heart arrhythmias, was introduced, as were new pharmaceuticals to be used in cardiac arrest situations; in Belfast, Northern Ireland, a mobile coronary care ambulance successfully resuscitated patients using these technologies; and well-developed studies demonstrated the need for overhauling ambulance services. These studies placed pressure on governments to improve emergency care in general, including the care provided by ambulance services. Part of the result was the creation of standards in ambulance construction concerning the internal height of the patient care area (to allow for an attendant to continue to care for the patient during transport), and in the equipment (and thus weight) that an ambulance had to carry. Few, or perhaps none of the then-available ambulances could meet these standards. Ambulance design therefore underwent major changes in the 1970s. High-topped car-based ambulances were developed, but car chassis proved unable to accept the weight and other demands of the new standards; van (and later, light truck) chassis would have to be used instead. The early van-based ambulances looked very similar to their civilian counterparts, having been given a limited amount of emergency vehicle equipment such as audible and visual warnings, and the internal fittings for carrying medical equipment, most notably a stretcher. As time went on, ambulances matured in parallel to the newly developed EMS, gaining the capacity to carry additional equipment (both portable and permanently installed) as EMTs and paramedics added this equipment to their arsenal. Ambulance design also evolved to reflect the ergonomics and other human factors of emergency medical care (for instance, raising the roof higher than typical for a van). Advances in the technology and understanding of emergency vehicle equipment also continued to influence ambulance design, just as it did for police and fire-suppression vehicles.