Transudative pleural effusions develop because the distribution of hyd
rostatic and oncotic pressure across the pleura is altered, so that th
e rate of pleural fluid formation exceeds that of its reabsorption, Th
ey are characterized by a low cell and protein content, Congestive hea
rt failure is the most common cause of transudative effusion, The flui
d that accumulates in a hepatic hydrothorax, urinothorax, during perit
oneal dialysis, and in many patients with nephrotic syndrome may also
have the characteristics of a transudate. The development of a transud
ative effusion indicates that the pleural membranes per se are intact,
so that if the underlying problem can be corrected, the effusion will
be reabsorbed.