PRISm has been reported to relate to poorer prognosis in epidemiological studies. However, the underlying pathology behind PRISm is not so well understood. We studied how respiratory mechanics, assessed by impulse oscillometry (IOS), relate with PRISm.

A total of 9993 subjects (50-64 years) from Uppsala and Malmö centers of the Swedish CArdioPulmonary bioImage Study performed post-bronchodilatory spirometry and IOS. PRISm was defined as having FEV1/FVC-ratio?5th percentile and both FEV1 and FVC<5th percentile while obstruction was defined as having FEV1/FVC-ratio<5th percentile.

A total of 385 subjects had PRISm and these subjects did not differ with regard to age and gender from subjects with normal (n=8639) or obstructive (n=969) spirometry. The prevalence of current smoking increased from 9.4% in normal spirometry to 16.5% in PRISm and 27.5% in obstructive spirometry (p<0.001). Higher and similar prevalences of respiratory symptoms were found in PRISm and obstructive spirometry: 43.3 and 43.4%, compared to 26% in normal spirometry.

The prevalence of having abnormal resistance at 5 Hz, 20 Hz, reactance at 5 Hz, area under reactance and resonance frequency were higher in PRISm: 14.8%, 10.4%, 26.5%, 25.2%, 17.9%, respectively,  compared to 5.4%, 5.1%, 4.8%, 4.9%, 4.8%, respectively, in subjects with normal spirometry (all p-values<0.05). PRISm had similar prevalences of abnormal IOS indices as subjects with obstructive spirometry: 14.2%, 9.7%, 16.9%, 18.8% and 19.0.

In conclusion, PRISm was associated with impaired IOS findings, both regarding resistance and reactance, suggesting altered respiratory mechanics in PRISm and a similar burden of respiratory symptoms as obstructive spirometry.