The diaphragm is the main muscle for inspiration and vital for ventilation. Multiple diagnostic modalities can be performed in the work-up of suspected diaphragm dysfunction. Fluoroscopy has traditionally been the method of choice in diagnosing diaphragm paralysis and is still widely used in clinical practice, while alternative non-invasive and accessible methods have been available. Superiority of ultrasound over fluoroscopy for the diagnosis of diaphragm dysfunction has been suggested.

The primary objective of this study is to investigate the construct validity of ultrasound in diaphragm paralysis.

To study the sensitivity of ultrasound in diaphragm dysfunction we propose a prospective, operator-blinded, two-center, observational study. Participants will be evaluated for diaphragm dysfunction with fluoroscopy and pulmonary function testing as in standard of care. Additionally, ultrasound will be performed.

Construct validity The primary objective of this study is to investigate the construct validity of ultrasound in diaphragm paralysis. Validity is defined as the degree to which an instrument truly measure the construct(s) it purports to measure. In general, three different types of validity can be distinguished: content validity, criterion validity, and construct validity. Since evaluation with gold standard testing is invasive, as explained in the introduction, criterion validity testing not a feasible option. Therefore, in this study, we use construct validity to provide evidence of validity. Construct validity is defined as the degree to which the scores of an instrument (here: ultrasound) are consistent with scores of other instruments (here: pulmonary function tests, fluoroscopy). Construct validation is often considered less powerful, however, with strong theories and specific and challenging hypothesis, it is possible to acquire substantial evidence. Therefore, specific definitions of 'diaphragm paresis' and 'diaphragm paralysis' regarding diagnostic modalities are established in this protocol before the start of the study

Inclusion Criteria:

• 18 years or older
• Suspicion of diaphragm dysfunction based on medical history and/or physical examination; as determined by the treating physician.

Exclusion Criteria:

• Inability for fluoroscopy (e.g. severely limited mobility, or unable to follow vocal instructions).
• Inability for diaphragm imaging (e.g. mechanical ventilation, or unable to follow vocal instructions).
• Those not able or unwilling to give written informed consent.
• Pregnant women

Our population consists of 18-years or older patients suspected of diaphragm dysfunction based on medical history and/or physical examination; as determined by the treating physician.