To determine the frequency with which physician, on-line medical direction (OLMD) [direct medical control] of prehospital care results in orders, to describe the nature of these orders, and to measure OLMD time intervals.

Blinded, prospective study.

A university hospital base-station resource center.

Ten emergency physicians, 50 advanced life support providers.

Prehospital treatment was directed by both standing orders and OLMD physician orders. Independent observers recorded event times and the characteristics of OLMD.

Physician orders were given in 47 (19%) of the 245 study cases, and covered a variety of interventions, including many already authorized by standing orders. Mean OLMD radio time was four minutes (245 ± 216 seconds [sec]), and time from beginning of OLMD to hospital arrival averaged 12 minutes (718 ± 439 sec). Mean transport time in this system was 13 minutes.

Despite detailed standing orders, OLMD results in orders for clinical interventions in 19% of cases. On-line medical direction requires about four minutes of physician time per call. This constituted about one-third of the potential field treatment time interval in this system. Thus, OLMD appears to play an important role in providing quality prehospital care.