Background : Determining the pleural PH and PCo₂ could be helpful in differential diagnosis of pleural effusion. Usually pleural fluid PH and Pco₂ are measured immediately following the thoracocentesis and in case of any delay samples will be preserved on ice, meanwhile, PH could be remained unchanged under special situations in vivo and at room temperature. The present study was conducted with the aim of determining the changes in pleural PH, Po₂, PCo₂, and HCo₃ immediately following the thoracocentesis and one hour later at room temperature.

Materials and methods : For this descriptive study, 84 patients with pleural effusion referring to Shaheed Beheshti Hospital in Kashan were included. Pleural aspiration was carried out in sitting position, then samples were examined for pleural PH, Po₂, PCo₂, and HCo₃. To differentiate transudate and exudate blood samples were also obtained.

Results: There was no significant difference between PH and Hco₃ of samples immediately obtained following the thoracocentesis and one hour later, however this difference was observed in Po₂. Of 84 patients, 33 had transudative pleural effusion, whereas the remaining 51 had exudative pleural effusion. Among transudative pleural effusion PH, PCo₂, and Po₂ were significantly differed between T₀ and T₆₀, however this significant relation was not found in HCo₃. Among exudative patients we have found significant association between Po₂ T₀ and Po₂ T₆₀.

Conclusion : The pleural fluid PH, PCo₂, and HCo₃ of samples kept at room temperature does not change significantly during the first hour following the thoracocentesis. Po₂ is increased in transudative and exudative pleural effusion within the first hour following the thoracocentesis. Meanwhile, changes in PH, Po₂, and PCo₂ of transudate samples were more than exudate samples.