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
High-Yield Explanation
The prone positioning reduces the disparity in mechanics between the dependent and nondependent regions of the lungs. This reduces the collapsing of the alveoli in the dependent portions of the lungs and overdistention in the nondependent portions of the lungs. The prone position also has other effects - it allows a more normal curvature of the diaphragm and allows better function. It also stiffens the chest wall allowing a more even distribution of ventilation and reduction in overventilation of nondependent alveoli