What is the condition of orthopnea characterized by?

Orthopnea is specifically characterized by severe dyspnea, or shortness of breath, that occurs when a person is lying down. This condition typically improves when the individual sits or stands up, as gravity can help facilitate better lung expansion and function. It's commonly associated with conditions such as congestive heart failure, where lying flat increases venous return to the heart and can exacerbate pulmonary congestion, leading to difficulty in breathing.

Understanding orthopnea is crucial for identifying and managing specific respiratory or cardiac conditions, allowing for more effective patient care. Other choices describe different types of breathing issues, but they do not capture the unique aspect of orthopnea, which is the positional aspect of dyspnea associated with lying down.