A patient is receiving mechanical ventilation with a fractional inspired oxygen (FIO2) of 0.85 and a positive end-expiratory pressure (PEEP) of 5 cm H2O

His arterial partial pressure of oxygen (PaO2) is 68 mm Hg, arterial oxygen saturation (SaO2) is 88%, and partial pressure of end-tidal carbon dioxide (PetCO2) is 32 mm Hg. Over the next few minutes his PEEP is titrated resulting in the following data:
Time FIO2 PEEP (cm H2O) SpO2 (%) PetCO2 (mm Hg)
0600 0.85 5 88 30
0630 0.85 8 88 30
0650 0.85 10 90 32
0720 0.80 12 93 34
0740 0.80 15 90 25
At 0740 the single-breath carbon dioxide (SBCO2) curve shifted to the right. What action should the respiratory therapist take at this time?
a. Increase the FIO2 to 0.90.
b. Reduce the set tidal volume.
c. Continue to increase the PEEP.
d. Reduce the PEEP to 12 cm H2O.

ANS: D
The increase in positive end-expiratory pressure (PEEP) to 15 cm H2O seems to have decreased pulmonary perfusion because of overinflation of the alveoli. This is evident by the decrease in the partial pressure of end-tidal carbon dioxide (PetCO2) to 25 mm Hg and the right shift in the sin-gle-breath carbon dioxide (SBCO2) curve. Increasing the fractional inspired oxygen (FIO2) will not address this problem. Reducing the set tidal volume will increase the PetCO2 but will not im-prove the pulmonary circulation. Continuing to increase the PEEP will further reduce pulmonary perfusion and cause more dead space. Reducing the PEEP back to 12 cm H2O will optimize the PEEP and reduce overinflation.