Safety Pressure Effects in a Mechanical Demand Regulator
INTRODUCTION: Most U.S. Navy, but few U.S. Air Force, tactical jets use safety pressure (SP) regulators. SP effects have been studied only with confounding differences in regulator design. We compared a CRU-103 SP regulator to a CRU-103 with SP removed. The hypothesis was that SP does not alter breathing, only shifts pressure more positive. METHODS: Inspiratory flows and mask and hose pressures were measured in 24 subjects (29 for speech at rest, 31 for lung volumes) who breathed in counterbalanced order from both regulators while blind to SP condition. RESULTS: Both were easy to breathe. Neither was preferred overall. Between regulators, end-expiratory lung volume did not differ. SP stabilized hose pressure and favored inspiration: without speech, hose pressure swings were significantly lower (rest: 25%, exercise: 33%), as were inspiratory work of breathing at rest (33%) and peak inspiratory flow magnitude (rest: 14%; exercise: 11%). Waveforms showed interactions of mask valves and SP at the start and end of expiration. Mask leaks with SP activated the regulator during speech. DISCUSSION: SP as implemented in the CRU-103 causes mostly subtle differences in pressures and flows. The sensed difference during expiration may result from the initial large pressure gradient for expiratory flow. Shykoff BE, French DC, Warkander DE, Robinson FE. Safety pressure effects in a mechanical demand regulator. Aerosp Med Hum Perform. 2025; 96(7):547–555.
Expiratory reserve volume and inspiratory capacity, NSP and SP, 31 subjects. Dots indicate individual values. Lines connect the means for the two conditions. NSP = without safety pressure. SP = with safety pressure.
Maximum and minimum mask and hose pressures, 24 subjects, at rest and during exercise, without and with speech. Points indicate individual values. Maxima of mask pressures, ●; Minima of mask pressures, ○. Maxima of hose pressures, ■; Minima of hose pressures, □. Solid lines connect means of mask pressures; dotted lines connect means of hose pressures. Hose pressures are offset on the x-axis for clarity. The extreme maximum hose pressure during exercise with safety pressure occurred with other evidence of poor inspiratory valve closure.
Mask and hose pressure swings during breathing. Solid lines, black circles: mask; dashed lines, white circles: hose. Means are joined by lines.
Samples of inspiratory flow, mask, and hose pressure as functions of time during rest alone and during speech at rest, NSP and SP (Subject ID 22). Inspiratory flow is negative and inspiratory pressures are below setpoint. Note the offset in pressure scaling, SP to NSP. The difference in breathing frequency was not universal across subjects. NSP = without safety pressure. SP = with safety pressure.
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