A male patient (83-kg IBW) is intubated and ventilated with PC-CMV, f = 12/min, set PIP = 28 cm H2O, resulting in a VT of 430 mL

The ABG results on this setting are: pH 7.35, PaCO2 45 mm Hg, and HCO3? 23 mEq/L. For-ty-eight hours later on the same settings, the ABG results are: pH 7.54, PaCO2 27 mm Hg, and HCO3? 21 mEq/L with an exhaled VT of 800 mL. The most appropriate action at this time is which of the following?
a. Decrease PIP to 25 cm H2O.
b. Decrease PIP to 19 cm H2O.
c. Decrease f to 10/min.
d. Decrease f to 8/min.

ANS: B
At first the patient responded appropriately to the PC-CMV settings. At that point the Cs was 15 mL/cm H2O. After 48 hours, the patient's lungs improved and the same pressure, 28 cm H2O, resulted in a VT of 800 mL. The patient's Cs now is 28.5 mL/cm H2O, and the combination of Cs and PIP is resulting in respiratory alkalosis. The acceptable VT range for this patient is 415-664 mL (5-8 mL/kg IBW). Because the exhaled tidal volume now exceeds this range, the volume needs to be reduced. This is accomplished by reducing the set PIP using the following formulas:

Desired VT = and set PIP = VT/CS = 19 cm H2O.