Orthopnea or orthopnoea[1] is shortness of breath (dyspnea) that occurs when lying flat,[2] causing the person to have to sleep propped up in bed or sitting in a chair.
[citation needed] Orthopnea is due to increased distribution of blood to the pulmonary circulation when a person lies flat or closer to a horizontal position.
Lying flat decreases the inhibitory effect that the gravity usually has on the blood when coming back to the heart from the lower extremities of the body.
In a normal person, this redistribution of blood has little effect on respiratory function as the left ventricle has the adequate capacity to suddenly increase its stroke volume (as a result of the Frank-Starling mechanism).
[5] This is different from the dyspnea experienced by someone with lung parenchymal pathology (both restrictive and obstructive) when lying down, which is sudden and instead related to an acute change in diaphragmatic/accessory respiratory muscle mechanical advantage lost when moving the body into a more horizontal position.