Functional residual capacity equals end-expiratory lung volume in normal patients. Air trapping develops when air remains in alveolae at the end of expiration. In this case, alveolar pressure remains positive, a condition known as intrinsic PEEP or auto-PEEP.
Auto-PEEP can be due to either:
- flow restriction (as in chronic obstructive pulmonary disease)
- insufficient time for lungs to return to FRC.
In a flow restriction scenario, work of breathing is increased because more force is required for inspiration. The patient must generate negative intrathoracic pressure to open the collapsed distal airways. External PEEP which is greater than the pressure level in upstream airways will reduce work of breathing by opening these airways for the patient.
High minute ventilation, high lung compliance, high expiratory resistance (due to airway or circuit) all reduce expiration time and flow, resulting in auto-PEEP. In this situation, external PEEP will worsen the situation by making it more difficult for air to leave the lungs.