En-route care in the air: Snapshot of mechanical ventilation at 37,000 feet

Objective: En-route care necessitates the evacuation of seriously wounded service members requiring mechanical ventilation in aircraft where low light, noise, vibration, and barometric pressure changes create a unique clinical environment. Our goal was to evaluate ventilatory requirements, oxygenation, and oxygen use in flight and assess the feasibility of a computer interface in this austere environment. Methods: A personal computer was integrated with the pulse oximeter and ventilator data port used in aeromedical evacuation from Iraq to Germany. Ventilator settings, inspired oxygen (FiO(2)), tidal volume (V-T), respiratory rate (RR), minute ventilation (V-E), monitored values, heart rate (HR), and oxygen saturation (SPO2), were recorded continuously. Oxygen use was determined using the equation ([FiO(2) - 21]/79) x (MVE). Additional data were obtained through the United States Air Force (USAF) Transcom Regulation and Command/Control Evacuation System (TRAC2ES) and the United States Army Institute of Surgical Research Joint Theater Trauma Registry databases. Results: During a 4 month time frame 117 hours of continuous recording was accomplished in 22 patients. Mean age was 27 +/- 9.83 and injury severity score military was 31.75 +/- 20.63 (range, 9-75). All patients survived transport. Mean values for ventilator settings were FiO(2) (24-100%) of 49% +/- 13%, positive end-expiratory pressure of 6 +/- 2.5 (range, 0-17 cm H2O), RR of 15 +/- 2.4 (range, 10-22 breaths/min), and V-T of 611 +/- 75 (range, 390-700 mL). Delivered V-T in mililiter per kilogram was 6.9 +/- 1.30 and V-E was 9.1 L/min +/- 1.4 L/min. Oxygen requirements for desired FiO(2) and V, resulted in a mean oxygen usage of 3.24 L/min +/- 1.87 L/min (range, 1.6-10.2 L/min). There were 32 changes to FiO(2),. 18 changes to PEEP, 26 changes to RR, and 20 changes to V-T during flight. Five patients underwent no recorded changes in flight. Three desaturation events (<90%) were recorded lasting 35, 115, and 280 seconds. Recorded ventilatory changes averaged less than 1 (0.82) per hour of recorded flight with FiO(2) being the most common. Conclusions: A computer interface is feasible in the austere aeromedical environment. Implications to military operations and civilian homeland defense include understanding casualty oxygen requirements for resource planning in support of aeromedical evacuation. Portable oxygen generation systems may be able to provide adequate oxygen flow for transport, reducing the need for compressed gas. Future studies of oxygen conservation systems including closed loop control of FiO(2) are warranted.