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Orthopnea and its mechanism

Orthopnea is dyspnea on lying flat
Mechanism of orthopnea
Increased venous return to the pulmonary vascular system on lying flat lead to pulmonary congestion decrease the  compliance of the lung.
Mechanical disadvantage of diaphragmatic movement due to viscera, in lying position.
The part of the lung lying below the level of heart is more in lying position, thus more  area of lung congestion.
These changes will be reversed in upright position; orthopnea is an important feature of Left heart failure and also observed in acute asthma, COPD and diaphragmatic paralysis.