The dynamics of pleural liquid estimated through urea clearance

Various volumes of saline labelled with indocyanine green (ICG) and urea were injected into the pleural cavity of the dogs. The osmolarity of the saline was exactly adjusted to be equal to that of normally occurring pleural liquid. In all experiments ICG remained constant. This suggested that the injected saline were removed via the lymphatics. The lymphatic removal rate increased linearly with the increase in volume of injected saline. Based on the change in urea concentration in the pleural liquid, the turnover of the pleural liquid was studied. The fraction of exchange of pleural liquid is thought to be constant when the volume of the pleural liquid is small but when the volume is increased, this fraction correspondingly decreases. This method is also available in clinical cases.