Example of invasive measurement of respiratory pressures. Example of invasive measurement of respiratory pressures. Shown is the recording of a respiratory trace during spontaneous breathing with CPAP in a subject in whom an esophageal and a gastric balloon catheter are inserted. Pes = esophageal pressure (a proxy for pleural pressure); Pga = gastric pressure (a proxy for abdominal pressure); Paw = airway pressure; Pdi = transdiaphragmatic pressure. White areas represent inspiration, whereas gray shaded areas depict expiration. The horizontal line represents zero flow. Michele Umbrello, and Paolo Formenti Respir Care 2016;61:542-555 (c) 2012 by Daedalus Enterprises, Inc.