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

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 (ΔP oes ) 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. ΔP oes 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 ΔP oes (ρ = 0.419, p < 0.001), whereas ITF did not correlate with either parameter. Both DTF and ΔP oes increased with exercise load and allowed discrimination between different exercise loads, although ΔP oes 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.