Objective: To determine whether a geographic information system (GIS) and historical transport data can be used to create a map that identifies locations (zones) from which either ambulance or helicopter transport will result in shorter out-of-hospital times.
Methods: A retrospective, cross-sectional review of a trauma registry was conducted in a two-county region served by a single trauma center. Data were gathered for all patients transported directly to the trauma center between 1993 and 1996. Incident locations and times from first 911 contact until arrival at the trauma center (out-of-hospital time) were extracted. A GIS was used to create a reference map with all incident locations plotted and given z-coordinates corresponding to out-of-hospital time. Two contour surfaces were interpolated: one for all helicopter transports and one for all ground transports. Areas where the helicopter surface was lower than the ambulance surface were designated air zones since helicopter transport resulted in shorter out-of-hospital times. The remaining area was designated a ground zone since ambulance transport resulted in shorter out-of-hospital times. The mean out-of-hospital times were calculated for each mode of transport in both zones and were compared using a two-tailed t-test.
Results: An air zone was identified beginning between 5 and 16 miles from the trauma center. Mean (+/-SD) out-of-hospital time from the air zone was 50 +/- 9 minutes for helicopter transport (n = 54) and 63 +/- 14 minutes for ambulance transport (n = 140). The difference between the means was 13 minutes (p < 0.000001; 95% CI = 8.95 to 17.05). Mean out-of-hospital time from the ground zone was 68 +/-16 minutes for helicopter transport (n = 122) and 32 +/- 14 minutes for ambulance transport (n = 2,047). The difference between the means was 36 minutes (p < 0.000001; 95% CI = 33.59 to 38.41).
Conclusions: A GIS and historical transport data can be used to create a map identifying locations from which either helicopter or ambulance transport will minimize out-of-hospital time. Inappropriate choice of transport mode is associated with increased out-of-hospital time.