The relationship between the diaphragm thickening fraction and the transdiaphragmatic pressure, the reference method to evaluate the diaphragm function, has not been clearly established. This study investigated the global and intraindividual relationship between the thickening fraction of the diaphragm and the transdiaphragmatic pressure. The authors hypothesized that the diaphragm thickening fraction would be positively and significantly correlated to the transdiaphragmatic pressure, in both healthy participants and ventilated patients.

Fourteen healthy individuals and 25 mechanically ventilated patients (enrolled in two previous physiologic investigations) participated in the current study. The zone of apposition of the right hemidiaphragm was imaged simultaneously to transdiaphragmatic pressure recording within different breathing conditions, i.e., external inspiratory threshold loading in healthy individuals and various pressure support settings in patients. A blinded offline breath-by-breath analysis synchronously computed the changes in transdiaphragmatic pressure, the diaphragm pressure-time product, and diaphragm thickening fraction. Global and intraindividual relationships between variables were assessed.


In healthy subjects, both changes in transdiaphragmatic pressure and diaphragm pressure-time product were moderately correlated to diaphragm thickening fraction (repeated measures correlation = 0.40, P < 0.0001; and repeated measures correlation = 0.38, P < 0.0001, respectively). In mechanically ventilated patients, changes in transdiaphragmatic pressure and thickening fraction were weakly correlated (repeated measures correlation = 0.11, P = 0.008), while diaphragm pressure-time product and thickening fraction were not (repeated measures correlation = 0.04, P = 0.396). Individually, changes in transdiaphragmatic pressure and thickening fraction were significantly correlated in 8 of 14 healthy subjects (ρ = 0.30 to 0.85, all P < 0.05) and in 2 of 25 mechanically ventilated patients (ρ = 0.47 to 0.64, all P < 0.05). Diaphragm pressure-time product and thickening fraction correlated in 8 of 14 healthy subjects (ρ = 0.41 to 0.82, all P < 0.02) and in 2 of 25 mechanically ventilated patients (ρ = 0.63 to 0.66, all P < 0.01).


Overall, diaphragm function as assessed with transdiaphragmatic pressure was weakly related to diaphragm thickening fraction. The diaphragm thickening fraction should not be used in healthy subjects or ventilated patients when changes in diaphragm function are evaluated.

Editor’s Perspective
What We Already Know about This Topic
  • Transdiaphragmatic pressure is proportional to the tension developed in the muscle fibers of the diaphragm.
  • In research studies, transdiaphragmatic pressure can be estimated as a surrogate of diaphragm function using invasive catheters to record gastric and esophageal pressure. A derivative (pressure-time index) can be calculated from real-time analysis of pressure tracings.
  • Interrogation of the diaphragm by ultrasound has recently become popular. The change in diaphragm thickness during inspiration (thickening fraction) has been proposed as an alternative approach to measure diaphragm function, although previous studies have reported wide variability in relationship to transdiaphragmatic pressure.
What This Article Tells Us That Is New
  • Diaphragm thickening fraction and transdiaphragmatic pressure were compared using ultrasound and catheter pressure measurements from two previously published studies of 14 healthy and 25 mechanically ventilated patients.
  • In healthy patients, moderate correlations between diaphragm thickening fraction with transdiaphragmatic pressure and pressure-time indices were observed.
  • In ventilated patients, weak correlations were observed with transdiaphragmatic pressure and none with pressure-time index.
  • Ultrasound use to assess diaphragm function should be done with caution.