This is dyspnoea on lying flat which is relieved by sitting up.
Orthopnoea occurs when right atrial and right ventricular function is relatively normal but there is impaired function on the left side of the heart.
When supine there is an increase in venous return to the right atrium and the right ventricle, and hence an increase in blood flow to the lungs. In the presence of conditions such as mitral stenosis or left ventricular failure, there is an increase in left atrial and pulmonary venous pressure. If the pulmonary venous pressure is raised above approximately 25 mm Hg (ie the oncotic pressure of the plasma proteins) there is transudation of fluid from the capillaries into the interstitial tissues of the lungs which become stiff. The result of the above process is the development of interstitial oedema and dyspnoea.
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