What is orthopnea defined as?

Orthopnea is defined as difficulty breathing that occurs when a person is lying flat but improves when they sit or stand up. This condition is often related to heart failure or other respiratory issues, where blood flow and pressure change positions, leading to increased lung congestion and difficulty in oxygen exchange when reclined. The characteristic of orthopnea is the alleviation of breathlessness when the individual assumes an upright position, making this definition particularly distinct.

The other choices do not accurately capture the specific nature of orthopnea. For instance, difficulty breathing only at night does not address the positional factor that is central to orthopnea. Similarly, shortness of breath after physical activity refers to exertional dyspnea, which is different from orthopnea since it does not encompass the positional aspect. Chronic difficulty breathing with exercise relates to ongoing issues but does not specifically link to the positional changes associated with orthopnea. Thus, the correct answer encapsulates the essence of orthopnea by highlighting its relationship with body position and breathlessness.