Baseline Variable—Expiration

The variable controlled during the expiratory phase is known as the baseline variable, most commonly, pressure, typically expressed as EPAP or PEEP. It is necessary to have a positive baseline pressure in bi-level devices to assure CO2 washout. Bi-level devices have also been shown to be effective in managing upper airway collapse, in patients with obstructive sleep apnea and overlap (14). Einally, a positive baseline pressure has been shown to decrease the work of breathing associated with intrinsic PEEP and improve 

Only in patient circuits that use an exhalation valve can the baseline pressure return to ambient pressure, known as zero PEEP or ZEEP. The advantage of ZEEP requires study, although it is certainly desirable during daytime ventilation when the upper airway is stable and ventilator energy conservation to preserve battery function is more important than leak compensation. Devices using a valve circuit are indicated in the ventilator table. There are some machines that allow the use of both leak and valve circuits in which case both EPAP and ZEEP are indicated in the Tables 1 to 3. 

CO2 rebreathing has been reported in bi-level ventilators with a single gas delivery circuit and no true exhalation valve (18,19). A recent in vivo study showed that the continual presence of nonintentional leak during NIV provided enough additional CO2 washout to address this issue (20). However, clinician should remain concerned when using a full-face mask, as the patient is more dependent on the intentional leak rates, which vary significantly. 

---