Abstract
Because it is associated with mortality, hyponatremia is an important feature of pulmonary arterial hypertension. Its mechanisms remain unclear, although right heart failure resulting from pulmonary arterial hypertension may lead to systemic congestion and arterial underfilling. However, most patients with pulmonary arterial hypertension are clinically euvolemic and have no peripheral edema. Unlike patients with underlying heart disease, neurohumoral activation is not demonstrated in hyponatremic patients with pulmonary arterial hypertension, who show features of congestive heart failure only at later stages in their disease. Here, a case vignette is introduced, and the pathophysiology of hyponatremia in pulmonary arterial hypertension will be discussed. Syndrome of inappropriate antidiuresis (SIAD) appears to underlie hyponatremia in the initial phase of pulmonary arterial hypertension. The mechanisms by which various lung diseases can lead to SIAD remain an enigma.