• Maria Sehlin, Helge Brändström, Ola Winsö, Michael Haney, Karin Wadell, and Fredrik Ohberg.
• Department of Surgical and Perioperative Sciences, Anesthesiology and Intensive Care Medicine, Umeå University, Umeå, Sweden.
• Aviat Space Envir Md. 2014 Nov 1; 85 (11): 1092-9.

IntroductionContinuous positive airway pressure (CPAP) is used in air ambulances to treat patients with impaired oxygenation. Differences in mechanical principles between CPAP devices may affect their performance at different ambient air pressures, as will occur in an air ambulance during flight.MethodsTwo different CPAP systems, a threshold resistor device and a flow resistor device, at settings of 5 and 10 cm H₂O were examined. Static pressure, static airflow, and pressure during simulated breathing were measured at ground level and at three different altitudes [2400 m (7874 ft), 3000 m (9843 ft), and 10,700 m (35,105 ft)].ResultsWhen altitude increased, the performance of the two CPAP systems differed during both static and simulated breathing pressure measurements. With the threshold resistor CPAP, measured pressure levels were close to the preset CPAP level. Static pressure decreased 0.71 ± 0.35 cm H₂O at CPAP 10 cm H₂O compared to ground level and 35,105 ft (10,700 m). With the flow resistor CPAP, as the altitude increased, CPAP produced pressure levels increased. At 35,105 ft (10,700 m), the increase was 5.13 ± 0.33 cm H₂O at CPAP 10 cm H₂O.DiscussionThe velocity of airflow through the flow resistor CPAP device is strongly influenced by reduced ambient air pressure, leading to a higher delivered CPAP effect than the preset CPAP level. Threshold resistor CPAP devices seem to have robust performance regardless of altitude. Thus, the threshold resistor CPAP device is probably more appropriate for CPAP treatment in an air ambulance cabin, where ambient pressure will vary during patient transport.

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