The esophageal balloon is the standard clinical method for measuring pleural pressure. It consists of a fine polyethylene tube whose open end is covered by a very thin-walled flaccid balloon.
It is passed via the nose into the lower third of the esophagus. Because the esophagus shares the pleural space with the lungs and heart, and is itself very flaccid (except when one swallows), no difference in pressure develops between its inside and its outside, and changes in intraesophageal pressure mimic very closely changes in Ppl.
Likewise, since very little air is put into the balloon, its walls stay quite flaccid, and no difference in pressure develops between the balloon pressure and Pes. Thus, changes in balloon pressure, which are measured via a pressure transducer outside the body, reflect changes in Ppl. The esophageal balloon technique gives an accurate determination of changes in Ppl, rather than Ppl itself, primarily because Ppl varies according to where it is measured, becoming greatest at the base of the lung (in an erect subject), and more subatmospheric as one moves toward the head.