Which of the following statements are true about negative-pressure ventilation (NPV)?

1. NPV is similar to spontaneous breathing.
2. Airway (mouth) pressure during NPV is zero.
3. Expiration during NPV is by passive recoil.
4. NPV decreases pressure at the body surface.
a. 2 and 4 only
b. 1, 2, 3, and 4
c. 3 and 4 only
d. 1, 3, and 4 only

ANS: B
Mechanical NPV is similar to spontaneous breathing. NPV decreases pleural pressure (Ppl) during inspiration by exposing the chest to sub-atmospheric pressure. Negative pressure at the body sur-face (Pbs) is transmitted first to the pleural space and then to the alveoli (Palv). Because the airway opening remains exposed to atmospheric pressure during NPV, a transairway pressure gradient is created. Thus, gas flows from the relatively high pressure at the airway opening (zero) to the rela-tively low pressure in the alveoli (negative).