Smaller quadriceps size is associated with lower exercise capacity and greater disease severity in people with fibrotic interstitial lung disease

Purpose: Peripheral muscle dysfunction is common in people with fibrotic interstitial lung diseases (ILD). Smaller muscle size may contribute to exercise intolerance in this population, yet this is largely unknown. The purpose of this study is to investigate if relationships exist between quadriceps muscle size, exercise capacity, disease severity and dyspnoea in fibrotic ILD, and to characterise these relationships based on exertional oxyhaemoglobin desaturation. Methods: In this cross-sectional study, quadriceps cross-sectional area (CSA) and muscle thickness (MT) were measured by B-mode ultrasonography in 45 participants with fibrotic ILD. Relationships between quadriceps muscle size and six-minute walk distance (6MWD), peak workload, endurance time, forced vital capacity (FVC), transfer factor for carbon monoxide (TLCO), demographics and dyspnoea were assessed, with additional subgroup analyses for exertional oxyhaemoglobin saturation (SpO 2 ≤ 88%, SpO 2 > 88%). Results: Smaller CSA and MT were associated with lower 6MWD (both r = 0.49, P < 0.01) and peak workload (CSA r = 0.37, P = 0.028; MT r = 0.60; P < 0.001). Smaller CSA was associated with lower FVC (r = 0.47; P = 0.004). These relationships were stronger in participants who experienced exertional desaturation. There was no significant relationship between muscle size and age, TLCO or dyspnoea. FVC and MT were independently associated with both 6MWD and peak workload. Conclusions: In fibrotic ILD, those with smaller quadriceps muscle size have lower exercise capacity and worse lung function. This is more apparent in those who experience exertional desaturation. Identifying interventions that preserve quadriceps muscle size in fibrotic ILD would prove beneficial.