Static transdiaphragmatic pressures in normal subjects and in patients with chronic hyperinflation

Transdiaphragmatic pressure (Pdi) during static maximal inspiratory effort was measured in 11 normal subjects and in 4 patients with severe chronic hyperinflation (mean total lung capacity, 138% predicted). The Pdi varied widely among subjects, some production positive abdominal pressure (Pab) at all volumes and some producing Pab close to zero or negative. During modified maximal inspiratory efforts in which the abdomen was protruded, several subjects produced greater Pdi, but in none was pleural pressure (Ppl) more negative, possibly because of inhibition of diaphragmatic activation during spontaneous inspiratory efforts. In the hyperinflated patients, spontaneous maximal inspiratory inspiratory efforts produced smaller values of Pdi, but the diaphragm retained an inspiratory effect and Pdi could still be increased by abdominal protrusion during the effort. Although Pdi during such modified maneuvers probably gives a better index of the strength of the diaphragm, this may not be directly related to its effective capacity as an inspiratory muscle, which may be best assessed by comparing Pdi with Ppl during spontaneous maximal inspiratory efforts.