For the evacuation of pleural fluid by aspiration or siphonage a trocar, with lateral outlet, presents certain advantages over an ordinary needle. The needle may become plugged during its introduction into the chest or during the withdrawal of fluid, and must then be removed to dislodge the obstruction if the danger of artificial pneumothorax and possible infection of the pleura are to be avoided. Its unprotected point, moreover, may wound the expanding lung or the diaphragm. A trocar so constructed that air can not enter the chest on the withdrawal of the stilet not only guards against artificial pneumothorax and possible infection of the pleura, but also prevents the exit of pleural fluid from the body of the instrument before a stopcock can be turned.
Of the different trocars now in use Fraentzel's1 and Potain's2 are the most satisfactory. Potain's instrument is less cumbersome and more easily cleaned.