X-ray chest PA view in heart failure, showing cardiomegaly with bilateral pleural effusion, which is more on the right side. The lung fields show pulmonary congestion. Pleural effusion in heart failure, if unilateral, is more likely to occur on the right side. The proposed mechanisms are the larger surface area of the right lung leading to transudation of more fluid and the preference of patients with heart failure to lie on the right side. The latter is thought to be due to the presence of an enlarged heart on the left side, producing more discomfort in the left lateral position. In one study, unilateral right-sided effusions were twice as common as left-sided effusions in congestive heart failure (CHF) (28% vs 15%) . This data also shows that bilateral pleural effusion is more common than unilateral pleural effusion in CHF.
Mitrouska I, Bouros D. The trans-exudative pleural effusion. Chest. 2002 Nov;122(5):1503-5.