Why is the normal reduction in vital capacity, which is associated with the supine position, greatly accentuated by diaphragmatic weakness?

A. Displacement of the diaphragm is enhanced by pressure from the abdominal contents.
B. The weight of the thorax cannot be opposed by the weakened diaphragm.
C. The accessory muscles of inspiration cause inward displacement of the diaphragm.
D. The diaphragm has a natural tendency to move cephalad in the supine position.

?ANS: A
A. When a patient is supine, the weight of the abdominal contents increases the inspiratory load on the already weakened diaphragm. With complete diaphragmatic paralysis, accessory inspiratory muscle contractions reduce the pleural pressure, displacing the diaphragm paradoxically into the chest during inspiration. In the upright position, this displacement is opposed by the weight of the abdomen. However, in the supine position, diaphragmatic displacement is enhanced by pressure from the abdominal contents. As a result, the normal reduction in vital capacity associated with the supine position is greatly accentuated by diaphragmatic weakness, with vital capacity decreasing by more than 25% when changing from upright to supine.
B. Incorrect response: See explanation A.
C. Incorrect response: See explanation A.
D. Incorrect response: See explanation A.