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

Nasal continuous positive airwaypressure (CPAP) is standard therapy for obstructive sleep apneasyndrome. The effective nasal mask pressure may be adversely affectedby factors that increase system resistance (eg, longtubing and/or water condensation) and by dynamic variables (breathingfrequency [f] and tidal volume [Vt]).The present study was conducted in order to assess the performance ofCPAP machines throughout a range of simulated clinical conditions. Four currently used CPAP machines were testedat settings of 5, 10, 15, and 20 cm H2O using a pulmonarywaveform generator to produce Vts of 0.4, 0.8, and 1.2 L atfrequencies of 10, 20, and 30 breaths/min. Machines were tested underfive conditions: 6-foot and 12-foot tubing, with and without an in-linehumidifier, and 12-foot tubing with humidifier and watercondensation. Maximum and minimum maskpressure measurements were obtained during five respiratory cycles foreach dynamic variable under each of the five conditions and CPAPsettings (180 experiments on each of four CPAP models). 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 consistentlyvaried above and below the set point when additional tubing and/or ahumidifier were added to the system (0.7 to 2.9 cm H2Obelow and 0.5 to 1.0 cm H2O above the set pressure). Watercondensation caused large pressure deviations (inspiratory pressureranged from 3.5 to 5.6 cm H2O below set pressure, andexpiratory pressure ranged from 0.7 to 3.5 cm H2O above setpressure). Therapy and compliance couldbe adversely affected because some CPAP machines in current use do notmaintain constant continuous mask pressure when tested using simulatedconditions, especially when water condenses in thetubing.