The Science Journal of the American Association for Respiratory Care
Introduction: Several devices are commercially available to provide nasal CPAP for the treatment of obstructive sleep apnea. All are based on centrifugal blowers with an internal method of pressure control. Ideally, airway pressure would be exactly constant throughout breathing, regardless of flow. In practice, pressure actually decreases slightly during inhalation and increases during exhalation, due to the mechanics of the blower. Previous studies have shown a high rate of non-compliance among CPAP users. The ability to maintain CPAP pressure may have an effect on patient work of breathing and hence, patient compliance. We compared the pressure regulation of seven commercially available CPAP systems under a variety of settings and breathing patterns, using a mechanical test lung model.
Methods: One side of a mechanical test lung (Michigan Instruments 1600), was driven by a linked ventilator (Puritan Bennett 7200ae). The second, ?breathing? side of the lung was connected via a 6 foot, 22 mmID patient tube to each CPAP system. An intentional leak (4 mm dia.), pressure transducer and pneumotachometer were interposed in the breathing limb, proximal to the lung. Two breathing patterns were simulated, condition 1 F=12/min, VT=500mL, Peak Inspiratory Flow (PIF)=28L/min and condition 2, F=12/min, VT=800mL, PIF=41L/min. A computer data acquisition system was used to monitor pressure and flow. The difference between the maximum and minimum pressures delivered by each device during the two breathing conditions was evaluated at CPAP settings of 5, 10 and 15 cmH2O.
Results: For condition 1, the mean (± S.D.) pressure range for all devices at all settings was 1.12 (±0.25) cmH20. For condition 2, it was 2.32 (±0.43). Changing CPAP settings had a small effect on the pressure range (The mean intra-device variance was 0.22 and 0.43 cmH2O for the two conditions). There was a large inter-device variance (see figure).
Conclusions: There is variance in the ability to maintain pressure between CPAP devices, which increases with increasing VT and Peak Flow. This may have an adverse affect on patient work of breathing and ultimately, patient compliance.
(See Original for Figure)