A 67-year-old man with severe ARDS is receiving pressure assisted control ventilation. He is requiring an FiO2 of 100% to maintain the following blood gas levels: pH=7.26, PCO2=60, PO2=58. You decide to put the patient in prone position. Fifteen minutes later, on the same vent settings, the patient's tidal volume is now decreased and his blood gas values are pH=7.09, PCO2=76, PO2=89. He is hemodynamically unchanged and his chest x-ray (CXR) is also unchanged. The most likely cause of his worsening respiratory acidosis in the prone position is