In order to appreciate the effect which the aspiration of pleural effusions exerts on the blood pressure, it is well to bear in mind the influence which the collection of fluid in the pleural cavity has on the arterial circulation. Our knowledge of this subject has been obtained chiefly through experiments on animals.
If olive oil is allowed to flow slowly into the pleural cavity of a dog, the blood pressure undergoes a moderate initial rise and then remains almost constant. It is only after the entrance of a very large amount of fluid that the arterial pulse waves become slower and of greater amplitude and a marked fall in blood pressure takes place (Rosenbach). The respirations, however, are affected early, becoming more labored and less frequent until they cease entirely.
Pleural exudates during the expiratory act are often under a positive pressure of 5 to 26 mm. Hg. (Quincke,