Abstract: Objective To evaluate the effect of body position(right and left lateral decubitus positions)on arterial oxygen tension(PaO₂)and the relationship between this postural effect on gas exchange and lung function.Methods Self control design,significance test and linear correlation analysis were used.Results In 53 patients with unilateral pleural effusions,the mean PaO₂ obtained in lateral decubitus position with the normal lung down(PaO₂-N) was higher than that obtained in lateral decubitus position with the effusion down (PaO₂-E).There was a significant difference in the values of mean PaO₂ between the two positions(P<0.001).The values of mean PaO₂-N and PaO₂-E were 79.9±13.8mmHg and 67.8±15.3mmHg respectively.The differences in PaO₂ ranged from 2.3 to 26.4mmHg (mean 12.1±6.9mmHg), PaO₂-N was higher than PaO₂-E in 40 of 53 patients and lower in 13 patients.Conclusion Postural change took effect on gas exchange in these patients.Although gas exchange was improved in most patients in the lateral decubitus position with normal side down,no consistent relationship was found.The difference in PaO₂ between two laterals decubitus position was not correlated with the patients’pulmonary function such as F-EV₁ and FVC.

Key words: Pelural effusion, Body position, Gas exchange