Trepopnea /tre·pop·nea/ (tre″pop-ne´ah) is dyspnea (shortness of breath) that is sensed while lying on one side but not on the other (lateral recumbent position). It results from disease of one lung, one major bronchus, or chronic congestive heart failure. Patients with trepopnea from lung disease prefer to lie on the opposite side of the diseased lung, as the gravitation increases perfusion of the lower lung. Increased perfusion in diseased lung would increase shunting and hypoxemia, resulting in worsening shortness of breath. To maximize function of the healthier lung, the patient is best to lie on the side of the healthier lung, so that it receives adequate perfusion. Patients with chronic heart failure prefer to lie mostly on the right side, to enable a better blood return, whereby cardiac output is augmented.