Clinical indications that a patient has impending acute ventilatory failure include:

1. pH <7.35.
2. vital capacity (VC) <20 mL/kg.
3. PaCO2 >45 mm Hg.
4. Negative inspiratory force (NIF) <25 cm H2O.
a. 1, 3
b. 1, 2, 3
c. 2, 4
d. 1, 2, 3, 4

B
A vital capacity of less than 20 mL/kg, a PCO2 exceeding 45 mm Hg, and a pH below 7.35 all indicate impeding ventilatory failure.