What is the name of the condition characterized by severe difficulty in breathing except when sitting or standing?

The condition characterized by severe difficulty in breathing except when sitting or standing is known as orthopnea. This term specifically refers to the sense of breathlessness that worsens when lying flat and improves when the individual sits up or stands. It is often associated with heart failure or other medical conditions that affect the respiratory system, as these conditions can lead to fluid accumulation or other factors that make it hard to breathe when in a horizontal position.

In contrast, dyspnea is a more general term for difficulty in breathing and does not specify the positional aspect. Apnea refers to a temporary cessation of breathing, and hyperpnea refers to an increased depth and rate of breathing, neither of which captures the positional specificity of orthopnea.