Specialty Sections From CHEST® Physician

Critical Care Network


Mechanical Ventilation and Airways Section

Noninvasive ventilation

Noninvasive ventilation (NIV) is a ventilation modality that supports breathing by using mechanically assisted breaths without the need for intubation or a surgical airway. NIV is divided into two main types, negative-pressure ventilation (NPV) and noninvasive positive-pressure ventilation (NIPPV).


NPV periodically generates a negative (subatmospheric) pressure on the thorax wall, reflecting the natural breathing mechanism. As this negative pressure is transmitted into the thorax, normal atmospheric pressure air outside the thorax is pulled in for inhalation. Initiated by the negative pressure generator switching off, exhalation is passive due to elastic recoil of the lung and chest wall. The iron lung was a neck-to-toe horizontal cylinder used for NPV during the polio epidemic. New NPV devices are designed to fit the thorax only, using a cuirass (a torso-covering body armor molded shell).

For years, NPV use declined as NIPPV use increased. However, during the shortage of NIPPV devices during COVID and a recent recall of certain CPAP devices, NPV use has increased. NPV is an excellent alternative for those who cannot tolerate a facial mask due to facial deformity, claustrophobia, or excessive airway secretion (Corrado A et al. European Resp J. 2002;20[1]:187).


NIPPV is divided into several subtypes, including continuous positive airway pressure (CPAP), bilevel positive airway pressure (BPAP or BiPAP), and average volume-assured pressure support (AVAPS or VAPS). CPAP is defined as a single pressure delivered in inhalation (Pi) and exhalation (Pe). The increased mean airway pressure provides improved oxygenation (O2) but not ventilation (CO2). BPAP uses dual pressures with Pi higher than Pe. The increased mean airway pressure provides improved O2 while the difference between Pi minus Pe increases ventilation and decreases CO2.

AVAPS is a form of BPAP where Pi varies in an automated range to achieve the ordered tidal volume. In AVAPS, the generator adjusts Pi based on the average delivered tidal volume. If the average delivered tidal volume is less than the set tidal volume, Pi gradually increases while not exceeding Pi Max. Patients notice improved comfort of AVAPS with a variable Pi vs. BPAP with a fixed Pi (Frank A et al. Chest. 2018;154[4]:1060A).

Samantha Tauscher, DO, Resident-in-Training

Herbert Patrick, MD, MSEE, FCCP , Member-at-Large

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