Ohopnea is hea failure develops due to

When a normal individual lies down, the pulmonary blood volume increases by up to 400 mL When the person stands up this blood is discharged into the general circulation. This shift is the cause of the decrease in vital capacity in the supine position and is responsible for the occurrence of ohopnea in hea failure. Ref: Ganong's Review of Medical Physiology Twenty-Third Edition Page No:603 In normal individuals ,this has minimal effect but in patients whom the additional volume cannot be pumped out by the left ventricle because of disease there is a significant reduction in vital capacity and pulmonary compliance and leads to shoness of breath.