CPAP commonly is used to improve oxygenation, particularly in hospitalized patients. It also is used to treat obstructive sleep apnea,especially in the home. In addition, CPAP can be used to assist patients with chronic obstructive pulmonary disease (COPD) who have difficulty breathing. Application of low levels of continous airway pressure through a mask interface created a pneumatic splint that prevented airway collapse during sleep.
Air trapping can occur in a spontaneously breathing individual with increased airway resistance (Raw), such as a person with COPD or acute asthma; this can lead to an increase in the functional residual capacity (FRC). In the past, CPAP was considered contraindicated in patients with COPD, because these individuals already had an increased FRC. Some believed that external CPAP or positive endexpiratory pressure (PEEP) would further increase the FRC and would not benefit the patient . This can be true if CPAP is not applied appropriately.
Patients with COPD often have difficulty generating the pressure difference between the alveoli and the mouth to begin inspiratory gas flow. The air trapped in the lungs (called intrinsic PEEP or auto-PEEP) creates a positive alveolar pressure (Palv).
If the pressure in the lungs is positive at end-exhalation, the pressure must drop below the pressure at the mouth (atmospheric pressure) to start gas flowing into the lung for inspiration. The patient must work hard to accomplish this task. For example, if auto-PEEP is +5 cm H20, the patient must exert an effort of at least -5 cm H20 to drop the lung pressure below zero. Once alveolar pressure drop below zero, inspiratory gas flow can start.
Externally applied CPAP may reduce the pressure difference between the mouth and the alveoli, when flow limitation (increased Raw) is the cause of auto-PEEP. The patient therefore need not work as hard to drop the Palv so that inspiratory gas flow enters the lung. In other words, externally applied CPAP can lower inspiratory work. Mask CPAP set at 80% to 90% of the measured auto-PEEP (usually about 4to 10 cm H2O) reduces diaphragmatic work and dyspnea, improves gas exchange, and does not worsen hyperinflation.