Postural Influence on Ventilation Efficiency and Relationship With Oxygen‐Enhanced MRI in Cystic Fibrosis

Background Pulmonary function tests (PFTs) and medical imaging in cystic fibrosis (CF) occasionally show discrepancies in severity levels, potentially due to different postures during testing (seated PFTs and supine imaging). To assess this, we utilised multiple breath washout with Short extension (MBW ShX ) and oxygen‐enhanced MRI (OE‐MRI). MBW ShX is a more sensitive PFT and offers novel insights into global ventilation efficiency by incorporating signal from under‐ventilated lung units (UVLU). We hypothesised there would be a significant difference in MBW ShX parameters between seated and supine postures and aimed to compare the relationship between MBW ShX obtained in both posture and OE‐MRI. Methods Twenty three CF patients performed two seated and at least 1onesupine MBW, along with same day OE‐MRI. MBW ShX outcome measures included conventional lung clearance index (LCI 2.5 ), and two novel parameters: LCI ShX and UVLU. The primary OE‐MRI parameter was the ventilation defect percentage (VDP%). Results We demonstrated a significant increase in LCI ShX (median difference [IQR]:2.00[0.16; 2.61]; p < 0.01) and UVLU (0.66[0.11; 2.22]; p < 0.05) from seated to supine posture, but no significant change in LCI 2.5 (0.27 [−0.21; 0.89]; p = 0.06). Moreover, we showed strong correlation between MBW and VDP% of OE‐MRI parameters across postures ( ρ > 0.8). However, no differences were detected between seated and supine MBW correlations with OE‐MRI (LCI 2.5 , p = 0.77; LCI ShX , p = 0.90; UVLU, p = 0.29). Conclusion We found significant postural differences in MBW ShX parameters, with reduced global ventilation efficiency (increased LCI ShX ) and increased UVLU in supine posture. However, the relationship between MBW and OE‐MRI remained unaffected by posture. In conclusion, supine MBW may offer novel clinical insights but does not enhance sensitivity for comparisons with medical imaging.