A 70-yeаr-оld femаle аdmitted fоr cоngestive heart failure is in respiratory distress and is being seen by the hospital’s medical emergency team in her regular room. The patient is in obvious respiratory distress and is immediately placed on a nonrebreather mask. Physical assessment reveals: pulse 120 and thready; respiratory rate 28 and labored; temperature 37° C; blood pressure 130/68. Breath sounds with coarse crackles on inspiration. EKG shows normal atrial fibrillation. She has a productive cough of a small amount of pink frothy secretions. The arterial blood gas on the nonrebreather mask is: pH 7.35; PCO2 45 mm Hg; PaO2 is 50 mm Hg; oxygen saturation is 86%; HCO3- is 24 mEq/L. The most appropriate ventilator mode to manage this patient initially is which of the following?