In heart failure (HF), pleural effusion results from increased interstitial fluid in the lung due to elevated pulmonary capillary pressure. Rarely, pleural effusions may occur in association with isolated right HF. HF-associated effusions are typically bilateral, but if unilateral, they are more commonly seen on the right side. The fluid typically meets the biochemical characteristics of a transudate, although in 25% of the cases it may fall into the exudative range. Testing for natriuretic peptides, such as NT-proBNP, significantly aids in diagnosing or excluding HF in patients with pleural effusion of unknown origin. The measurement of pleural fluid NT-proBNP is the best way to identify pleural effusions that meet the exudative criteria of Light but are due to HF. However, if natriuretic peptide assays are not available, calculation of the serum to pleural fluid albumin gradient represents a good substitute for making this distinction. Loop diuretics are the mainstay of therapy, although a therapeutic thoracentesis for very large effusions may occasionally be required.
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