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The primary reason for impaired diffusion in a patient with pulmonary edema is due to increased thickness of the alveolar-capillary membrane. When pulmonary edema occurs, fluid accumulates in the interstitial spaces of the lungs and can infiltrate the alveoli. This accumulation of fluid increases the distance that gases must diffuse across the alveolar-capillary membrane. As a result, oxygen and carbon dioxide exchange is hindered, leading to impaired diffusion.

The accumulation of fluid creates a barrier that must be crossed for effective gas exchange, making it more difficult for oxygen to reach the bloodstream and for carbon dioxide to be expelled. This situation can lead to hypoxia and respiratory distress.

In contrast, retaining carbon dioxide, changes in body temperature, and decreased thickness of the alveolar-capillary membrane do not contribute significantly to the impaired diffusion seen in pulmonary edema. Retaining CO2 typically occurs due to other respiratory factors and does not directly impact the diffusion process related to the thickening caused by fluid accumulation.