Pressures are read from a baseline value. Normally the baseline pressure is zero (or atmospheric), which indicates that no additional pressure is applied at the airway opening during expiration and before inspiration.

 

Sometimes the baseline pressure is higher than zero, such as when the ventilator operator selects a higher pressure to be present during exhalation. This is called positive end-expiratory pressure, or PEEP. When PEEP is set, the ventilator prevents the patient from exhaling to zero (atmospheric pressure). PEEP increases the volume of gas left in the lungs at the end of a normal exhalation; that is, PEEP increases the functional residual capacity. PEEP applied by the operator is called extrinsic PEEP.

 

Auto-PEEP (or intrinsic PEEP), a complication of positive pressure ventilation, is air that is accidentally trapped in the lung. This usually occurs when a patient does not have enough time to exhale before the ventilator delivers another breath.