# Intercostal thickening fraction adds no value to diaphragm thickening fraction in healthy subjects undergoing noninvasive ventilation

## TL;DR

This study finds that measuring intercostal muscle thickening does not add value to assessing respiratory effort in healthy people using noninvasive ventilation.

## Contribution

The study demonstrates that intercostal thickening fraction does not correlate with respiratory effort metrics in healthy individuals during noninvasive ventilation.

## Key findings

- Diaphragm thickening fraction moderately correlates with tidal oesophageal pressure swing during exercise.
- Intercostal thickening fraction does not correlate with respiratory effort parameters.
- Diaphragm thickening fraction can differentiate exercise loads better than intercostal thickening fraction.

## Abstract

Assessing respiratory effort is essential for optimizing ventilatory support. While diaphragm thickening fraction (DTF) has been proposed as a bedside parameter, the role of intercostal muscle thickening fraction (ITF) remains unclear, especially during noninvasive ventilation. This study investigates the relationship between DTF, ITF and tidal oesophageal pressure swing (ΔPoes) as surrogate parameters for respiratory effort under different exercise loads. We conducted a physiological study in healthy volunteers with no contraindications to noninvasive ventilation. Participants completed three exercise sets on a semi-recumbent cycle ergometer. Each set consisted of five randomised phases with different ventilatory settings. ΔPoes was measured using a nasogastric balloon catheter. Diaphragmatic and parasternal intercostal muscle thickening fractions (DTF and ITF) were assessed by ultrasound in B-mode. Repeated measures correlation and Friedman’s test were used to analyse associations and differences between parameters. 38 individuals were included in the study. Repeated measures correlation analysis revealed a moderate correlation between DTF and ΔPoes (ρ = 0.419, p < 0.001), whereas ITF did not correlate with either parameter. Both DTF and ΔPoes increased with exercise load and allowed discrimination between different exercise loads, although ΔPoes provided a more accurate separation. In contrast, ITF did not discriminate between exercise phases. These findings suggest that DTF could be a useful indicator of respiratory effort during noninvasive ventilation. ITF may only be informative in the presence of diaphragmatic dysfunction. Further investigations in clinical populations are needed to test the hypotheses derived from this study.

The online version contains supplementary material available at 10.1038/s41598-026-40192-4.