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Nasal Continuous Positive Airway Pressure Devices Do Not Maintain the Set Pressure Dynamically When Tested Under Simulated Clinical Conditions^*

^* From the Department of Medicine (Dr. Bacon), University of Utah Medical Center; Intermountain Sleep Disorders Center (Drs. Farney, Walker, and Cloward), LDS Hospital; and the Pulmonary Division (Dr. Jensen), Department of Medicine, LDS Hospital, Salt Lake City, UT.

Correspondence to: Robert J. Farney MD, FCCP, Intermountain Sleep Disorders Center, LDS Hospital, 325 8th Ave, Salt Lake City, UT 84143; e-mail: rjfmd{at}msn.com

Study objectives: Nasal continuous positive airway pressure (CPAP) is standard therapy for obstructive sleep apnea syndrome. The effective nasal mask pressure may be adversely affected by factors that increase system resistance (eg, long tubing and/or water condensation) and by dynamic variables (breathing frequency [f] and tidal volume [VT]). The present study was conducted in order to assess the performance of CPAP machines throughout a range of simulated clinical conditions.

Design: Four currently used CPAP machines were tested at settings of 5, 10, 15, and 20 cm H₂O using a pulmonary waveform generator to produce VTs of 0.4, 0.8, and 1.2 L at frequencies of 10, 20, and 30 breaths/min. Machines were tested under five conditions: 6-foot and 12-foot tubing, with and without an in-line humidifier, and 12-foot tubing with humidifier and water condensation.

Measurements: Maximum and minimum mask pressure measurements were obtained during five respiratory cycles for each dynamic variable under each of the five conditions and CPAP settings (180 experiments on each of four CPAP models).

Results: Using typical clinical parameters (VT, 0.4 L and 0.8 L; f, 10 breaths/min and 20 breaths/min; and CPAP, 5 to 15 cm H₂O), mask pressure consistently varied above and below the set point when additional tubing and/or a humidifier were added to the system (0.7 to 2.9 cm H₂O below and 0.5 to 1.0 cm H₂O above the set pressure). Water condensation caused large pressure deviations (inspiratory pressure ranged from 3.5 to 5.6 cm H₂O below set pressure, and expiratory pressure ranged from 0.7 to 3.5 cm H₂O above set pressure).

Conclusions: Therapy and compliance could be adversely affected because some CPAP machines in current use do not maintain constant continuous mask pressure when tested using simulated conditions, especially when water condenses in the tubing.

Key Words: compliance • humidifier • nasal continuous positive airway pressure • obstructive sleep apnea

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