Figure 36-21 Examples of transdiaphragmatic pressure monitoring. A, Simultaneous esophageal and gastric pressure waveforms during tidal breathing in a normal individual. Negative esophageal (and therefore pleural) pressure swings are accompanied by positive gastric pressure waves, indicating the development of transdiaphragmatic pressure during inspiration. B, The same waveforms in a patient with phrenic nerve palsy (and therefore absent diaphragmatic contraction). Negative intrathoracic pressure swings (arrowheads) are accompanied by gastric pressure swings in the same direction. Intrathoracic pressure changes are directly transmitted through a passive diaphragm. These changes can also be observed in the early postoperative period in patients who have had upper abdominal surgery. (From Brown KA, Hoffstein V, Byrick RJ: Bedside diagnosis of bilateral diaphragmatic paralysis in a ventilator-dependent patient after open-heart surgery. Anesth Analg 64:1208, 1985.)